Patent Application
20110294168
The present invention lies in the field of DNA polymerases and their use in various applications, including nucleic acid primer extension and amplification.
DNA polymerases are responsible for the replication and maintenance of the genome, a role that is central to accurately transmitting genetic information from generation to generation. DNA polymerases function in cells as the enzymes responsible for the synthesis of DNA. They polymerize deoxyribonucleoside triphosphates in the presence of a metal activator, such as Mg2+, in an order dictated by the DNA template or polynucleotide template that is copied. In vivo, DNA polymerases participate in a spectrum of DNA synthetic processes including DNA replication, DNA repair, recombination, and gene amplification. During each DNA synthetic process, the DNA template is copied once or at most a few times to produce identical replicas. In contrast, in vitro, DNA replication can be repeated many times such as, for example, during polymerase chain reaction (see, e.g., U.S. Pat. No. 4,683,202 to Mullis).
In the initial studies with polymerase chain reaction (PCR), the DNA polymerase was added at the start of each round of DNA replication (see U.S. Pat. No. 4,683,202, supra). Subsequently, it was determined that thermostable DNA polymerases could be obtained from bacteria that grow at elevated temperatures, and that these enzymes need to be added only once (see U.S. Pat. No. 4,889,818 to Gelfand and U.S. Pat. No. 4,965,188 to Mullis). At the elevated temperatures used during PCR, these enzymes are not irreversibly inactivated. As a result, one can carry out repetitive cycles of polymerase chain reactions without adding fresh enzymes at the start of each synthetic addition process. DNA polymerases, particularly thermostable polymerases, are the key to a large number of techniques in recombinant DNA studies and in medical diagnosis of disease. For diagnostic applications in particular, a target nucleic acid sequence may be only a small portion of the DNA or RNA in question, so it may be difficult to detect the presence of a target nucleic acid sequence without amplification. Due to the importance of DNA polymerases in biotechnology and medicine, it would be highly advantageous to generate DNA polymerase mutants having desired enzymatic properties such as, for example, improved primer extension rates, reverse transcription efficiency, or amplification ability.
The overall folding pattern of polymerases resembles the human right hand and contains three distinct subdomains of palm, fingers, and thumb. (See Beese et al., Science 260:352-355, 1993); Patel et al., Biochemistry 34:5351-5363, 1995). While the structure of the fingers and thumb subdomains vary greatly between polymerases that differ in size and in cellular functions, the catalytic palm subdomains are all superimposable. For example, motif A, which interacts with the incoming dNTP and stabilizes the transition state during chemical catalysis, is superimposable with a mean deviation of about one Å amongst mammalian pol α and prokaryotic pol I family DNA polymerases (Wang et al., Cell 89:1087-1099, 1997). Motif A begins structurally at an antiparallel β-strand containing predominantly hydrophobic residues and continues to an α-helix. The primary amino acid sequence of DNA polymerase active sites is exceptionally conserved. In the case of motif A, for example, the sequence DYSQIELR (SEQ ID NO:22) is retained in polymerases from organisms separated by many millions years of evolution, including, e.g., Thermus aquaticus, Chlamydia trachomatis, and Escherichia coli. Taken together, these observations indicate that polymerases function by similar catalytic mechanisms.
In addition to being well-conserved, the active site of DNA polymerases has also been shown to be relatively mutable, capable of accommodating certain amino acid substitutions without reducing DNA polymerase activity significantly. (See, e.g., U.S. Pat. No. 6,602,695 to Patel et al.) Such mutant DNA polymerases can offer various selective advantages in, e.g., diagnostic and research applications comprising nucleic acid synthesis reactions. Thus, there is a need in the art for identification of amino acid positions amenable to mutation to yield improved polymerase activity, including, for example, improved extension rates, reverse transcription efficiency, or amplification ability. The present invention, as set forth herein, meets these and other needs.
The present invention provides DNA polymerases having improved enzyme activity relative to the corresponding unmodified polymerase and which are useful in a variety of nucleic acid synthesis applications. In some embodiments, the polymerase comprises an amino acid sequence having at least one of the following motifs in the polymerase domain:
a) Xa1-Xa2-Xa3-Xa4-R-Xa6-Xa7-Xa8-K-L-Xa11-Xa12-T-Y-Xa15-Xa16 (SEQ ID NO:1);
wherein Xa1 is I or L;
Xa2 is L or Q;
Xa3 is Q, H or E;
Xa4 is Y, H or F;
Xa6 is E, Q or K;
Xa7 is I, L or Y;
Xa8 is an amino acid other than Q, T, M, G or L;
Xa11 is K or Q;
Xa12 is S or N;
Xa15 is I or V; and
Xa16 is E or D;
b) T-G-R-L-S-S-Xb7-Xb8-P-N-L-Q-N (SEQ ID NO:2); wherein
Xb7 is S or T; and
Xb8 is an amino acid other than D, E or N; and
c) Xc1-Xc2-Xc3-Xc4-Xc5-Xc6-Xc7-D-Y-S-Q-I-E-L-R (SEQ ID NO:3); wherein
Xc1 is G, N, or D;
Xc2 is W or H;
Xc3 is W, A, L, or V;
Xc4 is an amino acid other than I or L;
Xc5 is V, F or L;
Xc6 is an amino acid other than S, A, V, or G; and
Xc7 is A or L,
wherein the polymerase has an improved nucleic acid extension rate and/or an improved reverse transcription efficiency relative to an otherwise identical polymerase wherein Xa8 is an amino acid selected from Q, T, M, G or L; Xb8 is an amino acid selected from D, E or N and/or Xc6 is an amino acid selected from S, A, V, or G (i.e., a reference polymerase). In some embodiments of the reference polymerase (e.g., Z05 or CS5/CS6), Xa8 is Q, T, M, G or L, Xb8 is D, E or N, Xc4 is I or L, and Xc6 is S, A, V, or G (SEQ ID NOS:23 and 24). In some embodiments of the reference polymerase, Xb8 is D, E or N (SEQ ID NOS:25 and 26).
With respect to motif a) Xa1-Xa2-Xa3-Xa4-R-Xa6-Xa7-Xa8-K-L-Xa11-Xa12-T-Y-Xa15-Xa16 (SEQ ID NO:1), in some embodiments, Xa8 is a D- or L-amino acid selected from the group consisting of: A, C, D, E, F, H, I, K, N, P, R, S, V, W, Y (SEQ ID NO:27), and analogs thereof. In some embodiments, Xa8 is an amino acid selected from the group consisting of: R, K and N (SEQ ID NO:28). In some embodiments, Xa8 is Arginine (R) (SEQ ID NO:29).
With respect to motif b) T-G-R-L-S-S-Xb7-Xb8-P-N-L-Q-N (SEQ ID NO:2), in some embodiments, Xb8 is D- or L-amino acid selected from the group consisting of: A, C, F, G, H, I, K, L, M, P, Q, R, S, T, V, W, Y (SEQ ID NO:30), and analogs thereof. In some embodiments, Xb8 is an amino acid selected from the group consisting of: G, A, S, T, R, K, Q, L, V and I (SEQ ID NO:31). In some embodiments, Xb8 is an amino acid selected from the group consisting of: G, T, R, K and L (SEQ ID NO:32). In some embodiments, Xb8 is an amino acid selected from the group consisting of: G, K and R (SEQ ID NO:88).
With respect to motif c) Xc1-Xc2-Xc3-Xc4-Xc5-Xc6-Xc7-D-Y-S-Q-I-E-L-R (SEQ ID NO:3), in some embodiments, Xc4 is a D- or L-amino acid selected from the group consisting of: A, C, D, E, F, G, H, K, M, N, P, Q, R, S, T, V, W, Y (SEQ ID NO:33), and analogs thereof. In some embodiments, Xc4 is an amino acid selected from the group consisting of F and Y (SEQ ID NO:34). In some embodiments, Xc4 is phenylalanine (F) (SEQ ID NO:35). In some embodiments, Xc6 is an amino acid selected from the group consisting of C, D, E, F, H, I, K, L, M, N, P, Q, R, T, W and Y (SEQ ID NO:36). In some embodiments, Xc6 is an amino acid selected from the group consisting of F and Y (SEQ ID NO:37). In some embodiments, Xc6 is phenylalanine (F) (SEQ ID NO:38).
In some embodiments, the improved polymerases (e.g., Z05 or CS5/CS6) that comprise at least one of Arginine (R) at position Xa8; Glycine (G) at position Xb8; Phenylalanine (F) at position Xc4; and/or Phenylalanine (F) at position Xc6 (SEQ ID NOS:39-68).
In some embodiments, the DNA polymerases of the invention are modified versions of an unmodified polymerase. In its unmodified form, the polymerase includes an amino acid sequence having the following motifs in the polymerase domain:
Various DNA polymerases are amenable to mutation according to the present invention. Particularly suitable are thermostable polymerases, including wild-type or naturally occurring thermostable polymerases from various species of thermophilic bacteria, as well as thermostable polymerases derived from such wild-type or naturally occurring enzymes by amino acid substitution, insertion, or deletion, or other modification. Exemplary unmodified forms of polymerase include, e.g., CS5, CS6 or Z05 DNA polymerase, or a functional DNA polymerase having at least 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98% or 99% sequence identity thereto. Other unmodified polymerases include, e.g., DNA polymerases from any of the following species of thermophilic bacteria (or a functional DNA polymerase having at least 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98% or 99% sequence identity to such a polymerase): Thermotoga maritima; Thermus aquaticus; Thermus thermophilus; Thermus flavus; Thermus filiformis; Thermus sp. sps17; Thermus sp. Z05; Thermotoga neopolitana; Thermosipho africanus; Thermus caldophilus or Bacillus caldotenax. Suitable polymerases also include those having reverse transcriptase (RT) activity and/or the ability to incorporate unconventional nucleotides, such as ribonucleotides or other 2′-modified nucleotides.
In some embodiments, the unmodified form of the polymerase comprises a chimeric polymerase. In one embodiment, for example, the unmodified form of the chimeric polymerase is CS5 DNA polymerase (SEQ ID NO:18), CS6 DNA polymerase (SEQ ID NO:19), or a polymerase having at least 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98% or 99% sequence identity to the CS5 DNA polymerase or the CS6 DNA polymerase. In specific variations, the unmodified form of the chimeric polymerase includes one or more amino acid substitutions relative to SEQ ID NO:18 or SEQ ID NO:19 that are selected from G46E, L329A, and E678G. For example, the unmodified form of the mutant or improved polymerase can be G46E CS5; G46E L329A CS5; G46E E678G CS5; or G46E L329A E678G CS5. In exemplary embodiments, these unmodified forms are substituted to provide a mutant polymerase including one or more amino acid substitutions selected from S671F, D640G, Q601R, and I669F. For example, the mutant or improved DNA polymerase can be any one of the following: G46E S671F CS5; G46E D640G CS5; G46E Q601R CS5; G46E I669F CS5; G46E D640G S671F CS5; G46E L329A S671F CS5; G46E L329A D640G CS5; G46E L329A Q601R CS5; G46E L329A I669F CS5; G46E L329A D640G S671F CS5; G46E S671F E678G CS5; G46E D640G E678G CS5; G46E Q601R E678G CS5; G46E I669F E678G CS5; G46E L329A S671F E678G CS5; G46E L329A D640G E678G CS5; G46E L329A Q601R E678G CS5; G46E L329A Q601R D640G I669F S671F E678G CS5; G46E L329A I669F E678G CS5; or the like.
In some embodiments, the polymerase is a CS5 polymerase (SEQ ID NO:15), a CS6 polymerase (SEQ ID NO:16) or a Z05 polymerase (SEQ ID NO:6), wherein Xb8 is an amino acid selected from the group consisting of: G, T, R, K and L. For example, the CS5 or CS6 polymerase can be selected from the following: D640G, D640T, D640R, D640K and D640L. The Z05 polymerase can be selected from the group consisting of: D580G, D580T, D580R, D580K and D580L.
The mutant or improved polymerase can include other, non-substitutional modifications. One such modification is a thermally reversible covalent modification that inactivates the enzyme, but which is reversed to activate the enzyme upon incubation at an elevated temperature, such as a temperature typically used for primer extension. Exemplary reagents for such thermally reversible modifications are described in U.S. Pat. Nos. 5,773,258 and 5,677,152 to Birch et al., which are expressly incorporated by reference herein in their entirety. In one embodiment, the mutant or improved polymerase comprising the thermally reversible covalent modification is produced by a reaction, carried out at alkaline pH at a temperature that is less than about 25° C., of a mixture of a thermostable DNA polymerase and a dicarboxylic acid anhydride having one of the following formulas I or II:
wherein R1 and R2 are hydrogen or organic radicals, which may be linked; or
wherein R1 and R2 are organic radicals, which may linked, and the hydrogens are cis. In a specific variation of such an enzyme, the unmodified form of the polymerase is G64E CS5.
In some embodiments, the extension rate is determined using a single-stranded DNA as a template (e.g, M13 mp 18, HIV), primed with an appropriate primer (e.g., a polynucleotide of the nucleic acid sequence 5′-GGGAAGGGCGATCGGTGCGGGCCTCTTCGC-3′ (SEQ ID NO:72)), and detecting formation of double-stranded DNA by measuring the incorporation of a fluorophore at regular time intervals (e.g., every 5, 10, 15, 20, 30 or 60 seconds), as described herein. The extension rate of a polymerase of the invention can be compared to the extension rate of a reference polymerase (e.g., a naturally occurring or unmodified polymerase), over a preselected unit of time, as described herein.
In some embodiments, the extension rate and/or reverse transcription efficiency is determined using an RNA as a template (e.g., an HCV RNA), as described herein. The extension rate and/or reverse transcription efficiency of a polymerase of the invention can be compared to the extension rate and/or reverse transcription efficiency of a reference polymerase (e.g., a naturally occurring or unmodified polymerase), over a preselected unit of time or number of amplification denaturation and extension cycles, as described herein.
In various other aspects, the present invention provides a recombinant nucleic acid encoding a mutant or improved DNA polymerase as described herein, a vector comprising the recombinant nucleic acid, and a host cell transformed with the vector. In certain embodiments, the vector is an expression vector. Host cells comprising such expression vectors are useful in methods of the invention for producing the mutant or improved polymerase by culturing the host cells under conditions suitable for expression of the recombinant nucleic acid. The polymerases of the invention may be contained in reaction mixtures and/or kits. The embodiments of the recombinant nucleic acids, host cells, vectors, expression vectors, reaction mixtures and kits are as described above and herein.
In yet another aspect, a method for conducting primer extension is provided. The method generally includes contacting a mutant or improved DNA polymerase of the invention with a primer, a polynucleotide template, and free nucleotides under conditions suitable for extension of the primer, thereby producing an extended primer. The polynucleotide template can be, for example, an RNA or DNA template. In some embodiments, the polynucleotide template is isolated from blood. The free nucleotides can include nucleotide triphosphates or unconventional nucleotides such as, e.g., ribonucleotides and/or labeled nucleotides. Further, the primer and/or template can include one or more nucleotide analogs. In some variations, the primer extension method is a method for polynucleotide amplification that includes contacting the mutant or improved DNA polymerase with a primer pair, the polynucleotide template, and the free nucleotides under conditions suitable for amplification of the polynucleotide.
Optionally, the primer extension reaction comprises an actual or potential inhibitor of a reference or unmodified polymerase. The inhibitor can inhibit the nucleic acid extension rate and/or the reverse transcription efficiency of a reference or unmodified (control) polymerase. In some embodiments, the inhibitor is hemoglobin, or a degradation product thereof. For example, in some embodiments, the hemoglobin degradation product is a heme breakdown product, such as hemin, hematin, hematoporphyrin, or bilirubin. In some embodiments, the inhibitor is at a concentration of at least 0.5 μM hemin, for example at least about 0.5, 1.0, 2.0, 3.0, 4.0, 5.0, 6.0, 7.0, 8.0, 10.0, 15.0, or 20.0 μM or greater hemin. In some embodiments, the inhibitor is an iron-chelator or a purple pigment. In other embodiments, the inhibitor is heparin. In some embodiments, the inhibitor is at a concentration of at least 1.0 ng/μl heparin, for example, at least about 1.0, 2.0, 3.0, 4.0, 5.0, 10, 20, 30, 40, 50, 100, 200, 300 or 400 ng/μl or greater of heparin. In some embodiments, the inhibitor is melanin. In some embodiments, the inhibitor is at a concentration of at least 0.1 ng/μl melanin, for example, at least about 0.2, 0.3, 0.4, 0.5, 1.0, 2.0, 3.0, 4.0, 5.0, or 10.0 ng/μl or greater of melanin. In certain embodiments, the inhibitor is not an intercalating dye. In some embodiments, the conditions suitable for extension comprise Mg++. In some embodiments, the conditions suitable for extension comprise Mn++.
In other embodiments, the method for polynucleotide amplification includes contacting the mutant or improved DNA polymerase with a primer pair, the polynucleotide template, the free nucleotides, and an actual or potential inhibitor, as described above. The amplification method can comprise an amount of an inhibitor that is sufficient to produce a difference in the amout of template amplified by the mutant or improved DNA polymerase compared to a control or reference DNA polymerase. For example, the amount of template amplified by a mutant or improved polymerase can result in a difference in measured crossing point (Cp) (delta Cp) values of at least one when compared to the amount of template amplified by a control or reference polymerase. In some embodiments, the amount of inhibitor is sufficient to decrease the Cp value by at least one when the amplification method comprises a mutant or improved polymerase compared to a control polymerase.
The present invention also provides a kit useful in such a primer extension method. Generally, the kit includes at least one container providing a mutant or improved DNA polymerase as described herein. The kit can also include a blood collection tube, container, or unit that comprises heparin or a salt thereof, or releases heparin into solution. The blood collection unit can be a heparinized tube, examples of which are well known in the art. In certain embodiments, the kit further includes one or more additional containers providing one or more additional reagents. For example, in specific variations, the one or more additional containers provide free nucleotides; a buffer suitable for primer extension; and/or a primer hybridizable, under primer extension conditions, to a predetermined polynucleotide template.
Further provided are reaction mixtures comprising the polymerases of the invention. In some embodiments, the reaction mixtures comprise an iron chelator and/or a purple pigment. In certain embodiments, the reaction mixtures comprise hemoglobin, or a degradation product of hemoglobin. For example, in certain embodiments, the degradation products of hemoglobin include heme breakdown products such as hemin, hematin, hematoporphyrin, and bilirubin. In other embodiments, the reaction mixtures comprise heparin or a salt thereof. In some embodiments, the reaction mixtures comprise melanin. The reactions mixtures can also contain a template nucleic acid (DNA and/or RNA), one or more primer or probe polynucleotides, free nucleotides (including, e.g., deoxyribonucleotides, nucleotide triphosphates, ribonucleotides, labeled nucleotides, unconventional nucleotides), buffers, salts, labels (e.g., fluorophores). In certain embodiments, the reaction mixture contains a template nucleic acid that is isolated from blood. In other embodiments, the template nucleic acid is RNA and the reaction mixture comprises heparin or a salt thereof. In other embodiments, the template nucleic acid is RNA and the reaction mixture comprises melanin.
Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. Although essentially any methods and materials similar to those described herein can be used in the practice or testing of the present invention, only exemplary methods and materials are described. For purposes of the present invention, the following terms are defined below.
The terms “a,” “an,” and “the” include plural referents, unless the context clearly indicates otherwise.
An “amino acid” refers to any monomer unit that can be incorporated into a peptide, polypeptide, or protein. As used herein, the term “amino acid” includes the following twenty natural or genetically encoded alpha-amino acids: alanine (Ala or A), arginine (Arg or R), asparagine (Asn or N), aspartic acid (Asp or D), cysteine (Cys or C), glutamine (Gln or Q), glutamic acid (Glu or E), glycine (Gly or G), histidine (His or H), isoleucine (Ile or I), leucine (Leu or L), lysine (Lys or K), methionine (Met or M), phenylalanine (Phe or F), proline (Pro or P), serine (Ser or S), threonine (Thr or T), tryptophan (Trp or W), tyrosine (Tyr or Y), and valine (Val or V). The structures of these twenty natural amino acids are shown in, e.g., Stryer et al., Biochemistry, 5th ed., Freeman and Company (2002), which is incorporated by reference. Additional amino acids, such as selenocysteine and pyrrolysine, can also be genetically coded for (Stadtman (1996) “Selenocysteine,” Annu Rev Biochem. 65:83-100 and Ibba et al. (2002) “Genetic code: introducing pyrrolysine,” Curr Biol. 12(13):R464—R466, which are both incorporated by reference). The term “amino acid” also includes unnatural amino acids, modified amino acids (e.g., having modified side chains and/or backbones), and amino acid analogs. See, e.g., Zhang et al. (2004) “Selective incorporation of 5-hydroxytryptophan into proteins in mammalian cells,” Proc. Natl. Acad. Sci. U.S.A. 101(24):8882-8887, Anderson et al. (2004) “An expanded genetic code with a functional quadruplet codon” Proc. Natl. Acad. Sci. U.S.A. 101(20):7566-7571, Ikeda et al. (2003) “Synthesis of a novel histidine analogue and its efficient incorporation into a protein in vivo,” Protein Eng. Des. Sel. 16(9):699-706, Chin et al. (2003) “An Expanded Eukaryotic Genetic Code,” Science 301(5635):964-967, James et al. (2001) “Kinetic characterization of ribonuclease S mutants containing photoisomerizable phenylazophenylalanine residues,” Protein Eng. Des. Sel. 14(12):983-991, Kohrer et al. (2001) “Import of amber and ochre suppressor tRNAs into mammalian cells: A general approach to site-specific insertion of amino acid analogues into proteins,” Proc. Natl. Acad. Sci. U.S.A. 98(25):14310-14315, Bacher et al. (2001) “Selection and Characterization of Escherichia coli Variants Capable of Growth on an Otherwise Toxic Tryptophan Analogue,” J. Bacteriol. 183(18):5414-5425, Hamano-Takaku et al. (2000) “A Mutant Escherichia coli Tyrosyl-tRNA Synthetase Utilizes the Unnatural Amino Acid Azatyrosine More Efficiently than Tyrosine,” J. Biol. Chem. 275(51):40324-40328, and Budisa et al. (2001) “Proteins with {beta}-(thienopyrrolyl)alanines as alternative chromophores and pharmaceutically active amino acids,” Protein Sci. 10(7):1281-1292, which are each incorporated by reference.
To further illustrate, an amino acid is typically an organic acid that includes a substituted or unsubstituted amino group, a substituted or unsubstituted carboxy group, and one or more side chains or groups, or analogs of any of these groups. Exemplary side chains include, e.g., thiol, seleno, sulfonyl, alkyl, aryl, acyl, keto, azido, hydroxyl, hydrazine, cyano, halo, hydrazide, alkenyl, alkynl, ether, borate, boronate, phospho, phosphono, phosphine, heterocyclic, enone, imine, aldehyde, ester, thioacid, hydroxylamine, or any combination of these groups. Other representative amino acids include, but are not limited to, amino acids comprising photoactivatable cross-linkers, metal binding amino acids, spin-labeled amino acids, fluorescent amino acids, metal-containing amino acids, amino acids with novel functional groups, amino acids that covalently or noncovalently interact with other molecules, photocaged and/or photoisomerizable amino acids, radioactive amino acids, amino acids comprising biotin or a biotin analog, glycosylated amino acids, other carbohydrate modified amino acids, amino acids comprising polyethylene glycol or polyether, heavy atom substituted amino acids, chemically cleavable and/or photocleavable amino acids, carbon-linked sugar-containing amino acids, redox-active amino acids, amino thioacid containing amino acids, and amino acids comprising one or more toxic moieties.
The term “biological sample” encompasses a variety of sample types obtained from an organism and can be used in a diagnostic or monitoring assay. The term encompasses urine, urine sediment, blood, saliva, and other liquid samples of biological origin, solid tissue samples, such as a biopsy specimen or tissue cultures or cells derived therefrom and the progeny thereof. The term encompasses samples that have been manipulated in any way after their procurement, such as by treatment with reagents, solubilization, sedimentation, or enrichment for certain components. The term encompasses a clinical sample, and also includes cells in cell culture, cell supernatants, cell lysates, serum, plasma, biological fluids, and tissue samples.
The term “mutant,” in the context of DNA polymerases of the present invention, means a polypeptide, typically recombinant, that comprises one or more amino acid substitutions relative to a corresponding, naturally-occurring or unmodified DNA polymerase.
The term “unmodified form,” in the context of a mutant polymerase, is a term used herein for purposes of defining a mutant DNA polymerase of the present invention: the term “unmodified form” refers to a functional DNA polymerase that has the amino acid sequence of the mutant polymerase except at one or more amino acid position(s) specified as characterizing the mutant polymerase. Thus, reference to a mutant DNA polymerase in terms of (a) its unmodified form and (b) one or more specified amino acid substitutions means that, with the exception of the specified amino acid substitution(s), the mutant polymerase otherwise has an amino acid sequence identical to the unmodified form in the specified motif. The polymerase may contain additional mutations to provide desired functionality, e.g., improved incorporation of dideoxyribonucleotides, ribonucleotides, ribonucleotide analogs, dye-labeled nucleotides, modulating 5′-nuclease activity, modulating 3′-nuclease (or proofreading) activity, or the like. Accordingly, in carrying out the present invention as described herein, the unmodified form of a DNA polymerase is predetermined. The unmodified form of a DNA polymerase can be, for example, a wild-type and/or a naturally occurring DNA polymerase, or a DNA polymerase that has already been intentionally modified. An unmodified form of the polymerase is preferably a thermostable DNA polymerases, such as DNA polymerases from various thermophilic bacteria, as well as functional variants thereof having substantial sequence identity to a wild-type or naturally occurring thermostable polymerase Such variants can include, for example, chimeric DNA polymerases such as, for example, the chimeric DNA polymerases described in U.S. Pat. No. 6,228,628 and U.S. Application Publication No. 2004/0005599, which are incorporated by reference herein in their entirety. In certain embodiments, the unmodified form of a polymerase has reverse transcriptase (RT) activity.
The term “thermostable polymerase,” refers to an enzyme that is stable to heat, is heat resistant, and retains sufficient activity to effect subsequent primer extension reactions and does not become irreversibly denatured (inactivated) when subjected to the elevated temperatures for the time necessary to effect denaturation of double-stranded nucleic acids. The heating conditions necessary for nucleic acid denaturation are well known in the art and are exemplified in, e.g., U.S. Pat. Nos. 4,683,202, 4,683,195, and 4,965,188, which are incorporated herein by reference. As used herein, a thermostable polymerase is suitable for use in a temperature cycling reaction such as the polymerase chain reaction (“PCR”). Irreversible denaturation for purposes herein refers to permanent and complete loss of enzymatic activity. For a thermostable polymerase, enzymatic activity refers to the catalysis of the combination of the nucleotides in the proper manner to form primer extension products that are complementary to a template nucleic acid strand. Thermostable DNA polymerases from thermophilic bacteria include, e.g., DNA polymerases from Thermotoga maritima, Thermus aquaticus, Thermus thermophilus, Thermus flavus, Thermus filiformis, Thermus species sps17, Thermus species Z05, Thermus caldophilus, Bacillus caldotenax, Thermotoga neopolitana, and Thermosipho africanus.
As used herein, a “chimeric” protein refers to a protein whose amino acid sequence represents a fusion product of subsequences of the amino acid sequences from at least two distinct proteins. A chimeric protein typically is not produced by direct manipulation of amino acid sequences, but, rather, is expressed from a “chimeric” gene that encodes the chimeric amino acid sequence. In certain embodiments, for example, an unmodified form of a mutant DNA polymerase of the present invention is a chimeric protein that consists of an amino-terminal (N-terminal) region derived from a Thermus species DNA polymerase and a carboxy-terminal (C-terminal) region derived from Tma DNA polymerase. The N-terminal region refers to a region extending from the N-terminus (amino acid position 1) to an internal amino acid. Similarly, the C-terminal region refers to a region extending from an internal amino acid to the C-terminus.
In the context of mutant DNA polymerases, “correspondence” to another sequence (e.g., regions, fragments, nucleotide or amino acid positions, or the like) is based on the convention of numbering according to nucleotide or amino acid position number and then aligning the sequences in a manner that maximizes the percentage of sequence identity. Because not all positions within a given “corresponding region” need be identical, non-matching positions within a corresponding region may be regarded as “corresponding positions.” Accordingly, as used herein, referral to an “amino acid position corresponding to amino acid position [X]” of a specified DNA polymerase represents referral to a collection of equivalent positions in other recognized DNA polymerases and structural homologues and families. In typical embodiments of the present invention, “correspondence” of amino acid positions are determined with respect to a region of the polymerase comprising one or more motifs of SEQ ID NO:1, SEQ ID NO:2, and SEQ ID NO:3, as discussed further herein.
“Recombinant,” as used herein, refers to an amino acid sequence or a nucleotide sequence that has been intentionally modified by recombinant methods. By the term “recombinant nucleic acid” herein is meant a nucleic acid, originally formed in vitro, in general, by the manipulation of a nucleic acid by endonucleases, in a form not normally found in nature. Thus an isolated, mutant DNA polymerase nucleic acid, in a linear form, or an expression vector formed in vitro by ligating DNA molecules that are not normally joined, are both considered recombinant for the purposes of this invention. It is understood that once a recombinant nucleic acid is made and reintroduced into a host cell, it will replicate non-recombinantly, i.e., using the in vivo cellular machinery of the host cell rather than in vitro manipulations; however, such nucleic acids, once produced recombinantly, although subsequently replicated non-recombinantly, are still considered recombinant for the purposes of the invention. A “recombinant protein” is a protein made using recombinant techniques, i.e., through the expression of a recombinant nucleic acid as depicted above. A recombinant protein is typically distinguished from naturally occurring protein by at least one or more characteristics.
A nucleic acid is “operably linked” when it is placed into a functional relationship with another nucleic acid sequence. For example, a promoter or enhancer is operably linked to a coding sequence if it affects the transcription of the sequence; or a ribosome binding site is operably linked to a coding sequence if it is positioned so as to facilitate translation.
The term “host cell” refers to both single-cellular prokaryote and eukaryote organisms (e.g., bacteria, yeast, and actinomycetes) and single cells from higher order plants or animals when being grown in cell culture.
The term “vector” refers to a piece of DNA, typically double-stranded, which may have inserted into it a piece of foreign DNA. The vector may be, for example, of plasmid origin. Vectors contain “replicon” polynucleotide sequences that facilitate the autonomous replication of the vector in a host cell. Foreign DNA is defined as heterologous DNA, which is DNA not naturally found in the host cell, which, for example, replicates the vector molecule, encodes a selectable or screenable marker, or encodes a transgene. The vector is used to transport the foreign or heterologous DNA into a suitable host cell. Once in the host cell, the vector can replicate independently of or coincidental with the host chromosomal DNA, and several copies of the vector and its inserted DNA can be generated. In addition, the vector can also contain the necessary elements that permit transcription of the inserted DNA into an mRNA molecule or otherwise cause replication of the inserted DNA into multiple copies of RNA. Some expression vectors additionally contain sequence elements adjacent to the inserted DNA that increase the half-life of the expressed mRNA and/or allow translation of the mRNA into a protein molecule. Many molecules of mRNA and polypeptide encoded by the inserted DNA can thus be rapidly synthesized.
The term “nucleotide,” in addition to referring to the naturally occurring ribonucleotide or deoxyribonucleotide monomers, shall herein be understood to refer to related structural variants thereof, including derivatives and analogs, that are functionally equivalent with respect to the particular context in which the nucleotide is being used (e.g., hybridization to a complementary base), unless the context clearly indicates otherwise. The term “free nucleotides” also includes nucleotide triphosphates.
The term “nucleic acid” or “polynucleotide” refers to a polymer that can be corresponded to a ribose nucleic acid (RNA) or deoxyribose nucleic acid (DNA) polymer, or an analog thereof. This includes polymers of nucleotides such as RNA and DNA, as well as synthetic forms, modified (e.g., chemically or biochemically modified) forms thereof, and mixed polymers (e.g., including both RNA and DNA subunits). Exemplary modifications include methylation, substitution of one or more of the naturally occurring nucleotides with an analog, internucleotide modifications such as uncharged linkages (e.g., methyl phosphonates, phosphotriesters, phosphoamidates, carbamates, and the like), pendent moieties (e.g., polypeptides), intercalators (e.g., acridine, psoralen, and the like), chelators, alkylators, and modified linkages (e.g., alpha anomeric nucleic acids and the like). Also included are synthetic molecules that mimic polynucleotides in their ability to bind to a designated sequence via hydrogen bonding and other chemical interactions. Typically, the nucleotide monomers are linked via phosphodiester bonds, although synthetic forms of nucleic acids can comprise other linkages (e.g., peptide nucleic acids as described in Nielsen et al. (Science 254:1497-1500, 1991). A nucleic acid can be or can include, e.g., a chromosome or chromosomal segment, a vector (e.g., an expression vector), an expression cassette, a naked DNA or RNA polymer, the product of a polymerase chain reaction (PCR), an oligonucleotide, a probe, and a primer. A nucleic acid can be, e.g., single-stranded, double-stranded, or triple-stranded and is not limited to any particular length. Unless otherwise indicated, a particular nucleic acid sequence optionally comprises or encodes complementary sequences, in addition to any sequence explicitly indicated.
The term “oligonucleotide” refers to a nucleic acid that includes at least two nucleic acid monomer units (e.g., nucleotides). An oligonucleotide typically includes from about six to about 175 nucleic acid monomer units, more typically from about eight to about 100 nucleic acid monomer units, and still more typically from about 10 to about 50 nucleic acid monomer units (e.g., about 15, about 20, about 25, about 30, about 35, or more nucleic acid monomer units). The exact size of an oligonucleotide will depend on many factors, including the ultimate function or use of the oligonucleotide. Oligonucleotides are optionally prepared by any suitable method, including, but not limited to, isolation of an existing or natural sequence, DNA replication or amplification, reverse transcription, cloning and restriction digestion of appropriate sequences, or direct chemical synthesis by a method such as the phosphotriester method of Narang et al. (Meth. Enzymol. 68:90-99, 1979); the phosphodiester method of Brown et al. (Meth. Enzymol. 68:109-151, 1979); the diethylphosphoramidite method of Beaucage et al. (Tetrahedron Lett. 22:1859-1862, 1981); the triester method of Matteucci et al. (J. Am. Chem. Soc. 103:3185-3191, 1981); automated synthesis methods; or the solid support method of U.S. Pat. No. 4,458,066, entitled “PROCESS FOR PREPARING POLYNUCLEOTIDES,” issued Jul. 3, 1984 to Caruthers et al., or other methods known to those skilled in the art. All of these references are incorporated by reference.
The term “primer” as used herein refers to a polynucleotide capable of acting as a point of initiation of template-directed nucleic acid synthesis when placed under conditions in which primer extension is initiated (e.g., under conditions comprising the presence of requisite nucleoside triphosphates (as dictated by the template that is copied) and a polymerase in an appropriate buffer and at a suitable temperature or cycle(s) of temperatures (e.g., as in a polymerase chain reaction)). To further illustrate, primers can also be used in a variety of other oligonuceotide-mediated synthesis processes, including as initiators of de novo RNA synthesis and in vitro transcription-related processes (e.g., nucleic acid sequence-based amplification (NASBA), transcription mediated amplification (TMA), etc.). A primer is typically a single-stranded oligonucleotide (e.g., oligodeoxyribonucleotide). The appropriate length of a primer depends on the intended use of the primer but typically ranges from 6 to 40 nucleotides, more typically from 15 to 35 nucleotides. Short primer molecules generally require cooler temperatures to form sufficiently stable hybrid complexes with the template. A primer need not reflect the exact sequence of the template but must be sufficiently complementary to hybridize with a template for primer elongation to occur. In certain embodiments, the term “primer pair” means a set of primers including a 5′ sense primer (sometimes called “forward”) that hybridizes with the complement of the 5′ end of the nucleic acid sequence to be amplified and a 3′ antisense primer (sometimes called “reverse”) that hybridizes with the 3′ end of the sequence to be amplified (e.g., if the target sequence is expressed as RNA or is an RNA). A primer can be labeled, if desired, by incorporating a label detectable by spectroscopic, photochemical, biochemical, immunochemical, or chemical means. For example, useful labels include 32P, fluorescent dyes, electron-dense reagents, enzymes (as commonly used in ELISA assays), biotin, or haptens and proteins for which antisera or monoclonal antibodies are available.
The term “conventional” or “natural” when referring to nucleic acid bases, nucleoside triphosphates, or nucleotides refers to those which occur naturally in the polynucleotide being described (i.e., for DNA these are dATP, dGTP, dCTP and dTTP). Additionally, dITP, and 7-deaza-dGTP are frequently utilized in place of dGTP and 7-deaza-dATP can be utilized in place of dATP in in vitro DNA synthesis reactions, such as sequencing. Collectively, these may be referred to as dNTPs.
The term “unconventional” or “modified” when referring to a nucleic acid base, nucleoside, or nucleotide includes modification, derivations, or analogues of conventional bases, nucleosides, or nucleotides that naturally occur in a particular polynucleotide. Certain unconventional nucleotides are modified at the 2′ position of the ribose sugar in comparison to conventional dNTPs. Thus, although for RNA the naturally occurring nucleotides are ribonucleotides (i.e., ATP, GTP, CTP, UTP, collectively rNTPs), because these nucleotides have a hydroxyl group at the 2′ position of the sugar, which, by comparison is absent in dNTPs, as used herein, ribonucleotides are unconventional nucleotides as substrates for DNA polymerases. As used herein, unconventional nucleotides include, but are not limited to, compounds used as terminators for nucleic acid sequencing. Exemplary terminator compounds include but are not limited to those compounds that have a 2′,3′ dideoxy structure and are referred to as dideoxynucleoside triphosphates. The dideoxynucleoside triphosphates ddATP, ddTTP, ddCTP and ddGTP are referred to collectively as ddNTPs. Additional examples of terminator compounds include 2′-PO4 analogs of ribonucleotides (see, e.g., U.S. Application Publication Nos. 2005/0037991 and 2005/0037398, which are both incorporated by reference). Other unconventional nucleotides include phosphorothioate dNTPs ([[α]-S]dNTPs), 5′-[α]-borano-dNTPs, [α]-methyl-phosphonate dNTPs, and ribonucleoside triphosphates (rNTPs). Unconventional bases may be labeled with radioactive isotopes such as 32P, 33P, or 35S; fluorescent labels; chemiluminescent labels; bioluminescent labels; hapten labels such as biotin; or enzyme labels such as streptavidin or avidin. Fluorescent labels may include dyes that are negatively charged, such as dyes of the fluorescein family, or dyes that are neutral in charge, such as dyes of the rhodamine family, or dyes that are positively charged, such as dyes of the cyanine family. Dyes of the fluorescein family include, e.g., FAM, HEX, TET, JOE, NAN and ZOE. Dyes of the rhodamine family include Texas Red, ROX, R110, R6G, and TAMRA. Various dyes or nucleotides labeled with FAM, HEX, TET, JOE, NAN, ZOE, ROX, R110, R6G, Texas Red and TAMRA are marketed by Perkin-Elmer (Boston, Mass.), Applied Biosystems (Foster City, Calif.), or Invitrogen/Molecular Probes (Eugene, Oreg.). Dyes of the cyanine family include Cy2, Cy3, Cy5, and Cy7 and are marketed by GE Healthcare UK Limited (Amersham Place, Little Chalfont, Buckinghamshire, England).
As used herein, “percentage of sequence identity” is determined by comparing two optimally aligned sequences over a comparison window, wherein the portion of the sequence in the comparison window can comprise additions or deletions (i.e., gaps) as compared to the reference sequence (which does not comprise additions or deletions) for optimal alignment of the two sequences. The percentage is calculated by determining the number of positions at which the identical nucleic acid base or amino acid residue occurs in both sequences to yield the number of matched positions, dividing the number of matched positions by the total number of positions in the window of comparison and multiplying the result by 100 to yield the percentage of sequence identity.
The terms “identical” or percent “identity,” in the context of two or more nucleic acids or polypeptide sequences, refer to two or more sequences or subsequences that are the same or have a specified percentage of nucleotides or amino acid residues that are the same (e.g., 60% identity, optionally 65%, 70%, 75%, 80%, 85%, 90%, or 95% identity over a specified region), when compared and aligned for maximum correspondence over a comparison window, or designated region as measured using one of the following sequence comparison algorithms or by manual alignment and visual inspection. Sequences are “substantially identical” to each other if they are at least 20%, at least 25%, at least 30%, at least 35%, at least 40%, at least 45%, at least 50%, or at least 55% identical. These definitions also refer to the complement of a test sequence. Optionally, the identity exists over a region that is at least about 50 nucleotides in length, or more typically over a region that is 100 to 500 or 1000 or more nucleotides in length.
The terms “similarity” or “percent similarity,” in the context of two or more polypeptide sequences, refer to two or more sequences or subsequences that have a specified percentage of amino acid residues that are either the same or similar as defined by a conservative amino acid substitutions (e.g., 60% similarity, optionally 65%, 70%, 75%, 80%, 85%, 90%, or 95% similar over a specified region), when compared and aligned for maximum correspondence over a comparison window, or designated region as measured using one of the following sequence comparison algorithms or by manual alignment and visual inspection. Sequences are “substantially similar” to each other if they are at least 20%, at least 25%, at least 30%, at least 35%, at least 40%, at least 45%, at least 50%, or at least 55% similar to each other. Optionally, this similarly exists over a region that is at least about 50 amino acids in length, or more typically over a region that is at least about 100 to 500 or 1000 or more amino acids in length.
For sequence comparison, typically one sequence acts as a reference sequence, to which test sequences are compared. When using a sequence comparison algorithm, test and reference sequences are entered into a computer, subsequence coordinates are designated, if necessary, and sequence algorithm program parameters are designated. Default program parameters are commonly used, or alternative parameters can be designated. The sequence comparison algorithm then calculates the percent sequence identities or similarities for the test sequences relative to the reference sequence, based on the program parameters.
A “comparison window,” as used herein, includes reference to a segment of any one of the number of contiguous positions selected from the group consisting of from 20 to 600, usually about 50 to about 200, more usually about 100 to about 150 in which a sequence may be compared to a reference sequence of the same number of contiguous positions after the two sequences are optimally aligned. Methods of alignment of sequences for comparison are well known in the art. Optimal alignment of sequences for comparison can be conducted, for example, by the local homology algorithm of Smith and Waterman (Adv. Appl. Math. 2:482, 1970), by the homology alignment algorithm of Needleman and Wunsch (J. Mol. Biol. 48:443, 1970), by the search for similarity method of Pearson and Lipman (Proc. Natl. Acad. Sci. USA 85:2444, 1988), by computerized implementations of these algorithms (e.g., GAP, BESTFIT, FASTA, and TFASTA in the Wisconsin Genetics Software Package, Genetics Computer Group, 575 Science Dr., Madison, Wis.), or by manual alignment and visual inspection (see, e.g., Ausubel et al., Current Protocols in Molecular Biology (1995 supplement)).
One example of an algorithm that is suitable for determining percent sequence identity and sequence similarity are the BLAST and BLAST 2.0 algorithms, which are described in Altschul et al. (Nuc. Acids Res. 25:3389-402, 1977), and Altschul et al. (J. Mol. Biol. 215:403-10, 1990), respectively. Software for performing BLAST analyses is publicly available through the National Center for Biotechnology Information (http://www.ncbi.nlm.nih.gov/). This algorithm involves first identifying high scoring sequence pairs (HSPs) by identifying short words of length W in the query sequence, which either match or satisfy some positive-valued threshold score T when aligned with a word of the same length in a database sequence. T is referred to as the neighborhood word score threshold (Altschul et al., supra). These initial neighborhood word hits act as seeds for initiating searches to find longer HSPs containing them. The word hits are extended in both directions along each sequence for as far as the cumulative alignment score can be increased. Cumulative scores are calculated using, for nucleotide sequences, the parameters M (reward score for a pair of matching residues; always >0) and N (penalty score for mismatching residues; always <0). For amino acid sequences, a scoring matrix is used to calculate the cumulative score. Extension of the word hits in each direction are halted when: the cumulative alignment score falls off by the quantity X from its maximum achieved value; the cumulative score goes to zero or below, due to the accumulation of one or more negative-scoring residue alignments; or the end of either sequence is reached. The BLAST algorithm parameters W, T, and X determine the sensitivity and speed of the alignment. The BLASTN program (for nucleotide sequences) uses as defaults a wordlength (W) of 11, an expectation (E) or 10, M=5, N=−4 and a comparison of both strands. For amino acid sequences, the BLASTP program uses as defaults a wordlength of 3, and expectation (E) of 10, and the BLOSUM62 scoring matrix (see Henikoff and Henikoff, Proc. Natl. Acad. Sci. USA 89:10915, 1989) alignments (B) of 50, expectation (E) of 10, M=5, N=−4, and a comparison of both strands.
The BLAST algorithm also performs a statistical analysis of the similarity between two sequences (see, e.g., Karlin and Altschul, Proc. Natl. Acad. Sci. USA 90:5873-87, 1993). One measure of similarity provided by the BLAST algorithm is the smallest sum probability (P(N)), which provides an indication of the probability by which a match between two nucleotide or amino acid sequences would occur by chance. For example, a nucleic acid is considered similar to a reference sequence if the smallest sum probability in a comparison of the test nucleic acid to the reference nucleic acid is less than about 0.2, typically less than about 0.01, and more typically less than about 0.001.
The term “nucleic acid extension rate” refers the rate at which a biocatalyst (e.g., an enzyme, such as a polymerase, ligase, or the like) extends a nucleic acid (e.g., a primer or other oligonucleotide) in a template-dependent or template-independent manner by attaching (e.g., covalently) one or more nucleotides to the nucleic acid. To illustrate, certain mutant DNA polymerases described herein have improved nucleic acid extension rates relative to unmodified forms of these DNA polymerases, such that they can extend primers at higher rates than these unmodified forms under a given set of reaction conditions. Thus, as used herein, the term “improved” can refer to increased nucleic acid extension rates.
The term “reverse transcription efficiency” refers to the fraction of RNA molecules that are reverse transcribed as cDNA in a given reverse transcription reaction. In certain embodiments, the mutant DNA polymerases of the invention have improved reverse transcription efficiencies relative to unmodified forms of these DNA polymerases. That is, these mutant DNA polymerases reverse transcribe a higher fraction of RNA templates than their unmodified forms under a particular set of reaction conditions. Thus, as used herein, the term “improved” can refer to increased reverse transcription efficiency.
The term “Cp value” or “crossing point” value refers to a value that allows quantification of input target nucleic acids. The Cp value can be determined according to the second-derivative maximum method (Van Luu-The, et al., “Improved real-time RT-PCR method for high-throughput measurements using second derivative calculation and double correction,” BioTechniques, Vol. 38, No. 2, February 2005, pp. 287-293). In the second derivative method, a Cp corresponds to the first peak of a second derivative curve. This peak corresponds to the beginning of a log-linear phase. The second derivative method calculates a second derivative value of the real-time fluorescence intensity curve, and only one value is obtained. The original Cp method is based on a locally defined, differentiable approximation of the intensity values, e.g., by a polynomial function. Then the third derivative is computed. The Cp value is the smallest root of the third derivative. The Cp can also be determined using the fit point method, in which the Cp is determined by the intersection of a parallel to the threshold line in the log-linear region (Van Luu-The, et al., BioTechniques, Vol. 38, No. 2, February 2005, pp. 287-293).
The term “PCR efficiency” refers to an indication of cycle to cycle amplification efficiency. PCR efficiency is calculated for each condition using the equation: % PCR efficiency=(10(−slope)−1)×100, wherein the slope was calculated by linear regression with the log copy number plotted on the y-axis and Cp plotted on the x-axis.
The present invention provides novel mutant DNA polymerases in which one or more amino acids in the polymerase domain have been mutated relative to a functional naturally occurring or unmodified DNA polymerase. The mutant DNA polymerases of the invention are active enzymes having improved rates of nucleotide incorporation relative to the unmodified form of the polymerase and, in certain embodiments, concomitant increases in reverse transcriptase activity and/or amplification ability. The mutant DNA polymerases may be used at lower concentrations for superior or equivalent performance as the parent enzymes. In certain embodiments, the mutant DNA polymerases described herein have improved thermostability relative to parent enzymes. In certain embodiments, the mutant DNA polymerases described herein have improved catalytic efficiency relative to the unmodified form of the polymerase. In certain embodiments, the mutant DNA polymerase described herein can be used in combination with an intercalating dye. In certain embodiments, the mutant DNA polymerase described herein can be used in the presence of an amount of hemoglobin, a degradation product of hemoglobin, an iron-chelator, and/or a purple pigment, that would inhibit the activity of an unmodified form of the polymerase. In some embodiments, the iron-chelator and/or purple pigment is derived from hemoglobin or a degradation product of hemoglobin. In certain embodiments, the mutant DNA polymerase described herein is used in the presence of heparin. In some embodiments, the mutant DNA polymerase described herein is used in the presence of melanin. In certain embodiments, the mutant DNA polymerase described herein at least partially overcomes the inhibitory effect of an intercalating dye, hemoglobin, a degradation product of hemoglobin, heparin, or melanin on primer extension. The mutant DNA polymerases are therefore useful in a variety of applications involving primer extension as well as reverse transcription or amplification of polynucleotide templates, including, for example, applications in recombinant DNA studies and medical diagnosis of disease.
Unmodified forms of DNA polymerases amenable to mutation in accordance with the present invention are those having a functional polymerase domain comprising the following amino acid motifs:
These motifs are present within a region of about 100 amino acids in the active site of many Family A type DNA-dependent DNA polymerases, particularly thermostable DNA polymerases from thermophilic bacteria. For example,
Accordingly, in some embodiments, the unmodified form of the DNA polymerase is a wild-type or a naturally occurring DNA polymerase, such as, for example, a polymerase from any of the species of thermophilic bacteria listed above. In one variation, the unmodified polymerase is from a species of the genus Thermus. In other embodiments of the invention, the unmodified polymerase is from a thermophilic species other than Thermus. The full nucleic acid and amino acid sequence for numerous thermostable DNA polymerases are available. The sequences each of Thermus aquaticus (Taq) (SEQ ID NO:78), Thermus thermophilus (Tth) (SEQ ID NO:79), Thermus species Z05 (SEQ ID NO:82), Thermus species sps17 (SEQ ID NO:81), Thermotoga maritima (Tma) (SEQ ID NO:77), and Thermosipho africanus (Taf) (SEQ ID NO:83) polymerase have been published in PCT International Patent Publication No. WO 92/06200, which is incorporated herein by reference. The sequence for the DNA polymerase from Thermus flavus (Tfo) (SEQ ID NO:80) has been published in Akhmetzjanov and Vakhitov (Nucleic Acids Research 20:5839, 1992), which is incorporated herein by reference. The sequence of the thermostable DNA polymerase from Thermus caldophilus (Tca) (SEQ ID NO:84) is found in EMBL/GenBank Accession No. U62584. The sequence of the thermostable DNA polymerase from Thermus filiformis (Tfi) (SEQ ID NO:86) can be recovered from ATCC Deposit No. 42380 using, e.g., the methods provided in U.S. Pat. No. 4,889,818, as well as the sequence information provided in Table 1. The sequence of the Thermotoga neapolitana (Tne) DNA polymerase (SEQ ID NO:85) is from GeneSeq Patent Data Base Accession No. R98144 and PCT WO 97/09451, each incorporated herein by reference. The sequence of the thermostable DNA polymerase from Bacillus caldotenax (Bca) (SEQ ID NO:87) is described in, e.g., Uemori et al. (J Biochem (Tokyo) 113(3):401-410, 1993; see also, Swiss-Prot database Accession No. Q04957 and GenBank Accession Nos. D12982 and BAA02361), which are each incorporated by reference. Examples of unmodified forms of DNA polymerases that can be modified as described herein are also described in, e.g., U.S. Pat. Nos. 6,228,628, entitled “Mutant chimeric DNA polymerase” issued May 8, 2001 to Gelfand et al.; 6,346,379, entitled “Thermostable DNA polymerases incorporating nucleoside triphosphates labeled with fluorescein family dyes” issued Feb. 12, 2002 to Gelfand et al.; 7,030,220, entitled “Thermostable enzyme promoting the fidelity of thermostable DNA polymerases-for improvement of nucleic acid synthesis and amplification in vitro” issued Apr. 18, 2006 to Ankenbauer et al.; 6,881,559, entitled “Mutant B-type DNA polymerases exhibiting improved performance in PCR” issued Apr. 19, 2005 to Sobek et al.; 6,794,177, entitled “Modified DNA-polymerase from carboxydothermus hydrogenoformans and its use for coupled reverse transcription and polymerase chain reaction” issued Sep. 21, 2004 to Markau et al.; 6,468,775, entitled “Thermostable DNA polymerase from carboxydothermus hydrogenoformans” issued Oct. 22, 2002 to Ankenbauer et al.; and U.S. Pat. Appl. Nos. 20040005599, entitled “Thermostable or thermoactive DNA polymerase molecules with attenuated 3′-5′ exonuclease activity” filed Mar. 26, 2003 by Schoenbrunner et al.; 20020012970, entitled “High temperature reverse transcription using mutant DNA polymerases” filed Mar. 30, 2001 by Smith et al.; 20060078928, entitled “Thermostable enzyme promoting the fidelity of thermostable DNA polymerases-for improvement of nucleic acid synthesis and amplification in vitro” filed Sep. 29, 2005 by Ankenbauer et al.; 20040115639, entitled “Reversibly modified thermostable enzymes for DNA synthesis and amplification in vitro” filed Dec. 11, 2002 by Sobek et al., which are each incorporated by reference. The following Table lists the sequence identifiers for the full length amino acid sequences of the unmodified forms of the DNA polymerases described herein.
Thermotoga maritima (Tma)
Thermus aquaticus (Taq)
Thermus thermophilus (Tth)
Thermus flavus (Tfl)
Thermus filiformis (Tfi)
Thermus species spsl7 (Spsl7)
Thermus species Z05 (Z05)
Thermotoga neapolitana (Tne)
Thermosipho africanus (Taf)
Thermus caldophilus (Tca)
Bacillus caldotenax (Bca)
Also amenable to the mutations described herein are functional DNA polymerases that have been previously modified (e.g., by amino acid substitution, addition, or deletion), provided that the previously modified polymerase retains the amino acid motifs of SEQ ID NOS:1, 2, and 3. Thus, suitable unmodified DNA polymerases also include functional variants of wild-type or naturally occurring polymerases. Such variants typically will have substantial sequence identity or similarity to the wild-type or naturally occurring polymerase, typically at least 80% sequence identity and more typically at least 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98% or 99% sequence identity. In certain embodiments, the unmodified DNA polymerase has reverse transcriptase (RT) activity and/or the ability to incorporate ribonucleotides or other 2′-modified nucleotides.
Suitable polymerases also include, for example, certain chimeric DNA polymerases comprising polypeptide regions from two or more enzymes. Examples of such chimeric DNA polymerases are described in, e.g., U.S. Pat. No. 6,228,628, which is incorporated by reference herein in its entirety. Particularly suitable are chimeric CS-family DNA polymerases, which include the CS5 (SEQ ID NO:18) and CS6 (SEQ ID NO:19) polymerases and variants thereof having substantial sequence identity or similarity to SEQ ID NO:18 or SEQ ID NO:19 (typically at least 80% sequence identity and more typically at least 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98% or 99% sequence identity). The CS5 and CS6 DNA polymerases are chimeric enzymes derived from Thermus sp. Z05 (SEQ ID NO:82) and Thermotoga maritima (Tma) (SEQ ID NO:77) DNA polymerases. They comprise the N-terminal 5′-nuclease domain of the Thermus enzyme and the C-terminal 3′-5′ exonuclease and the polymerase domains of the Tma enzyme. These enzymes have efficient reverse transcriptase activity, can extend nucleotide analog-containing primers, and can incorporate alpha-phosphorothioate dNTPs, dUTP, dITP, and also fluorescein- and cyanine-dye family labeled dNTPs. The CS5 and CS6 polymerases are also efficient Mg2+-activated PCR enzymes. Nucleic acid sequences encoding CS5 and CS6 polymerases are provided in
In some embodiments, the unmodified form of the DNA polymerase is a polymerase that has been previously modified, typically by recombinant means, to confer some selective advantage. Such modifications include, for example, the amino acid substitutions G46E, L329A, and/or E678G in CS5 DNA polymerase, CS6 DNA polymerase, or corresponding mutation(s) in other polymerases. Accordingly, in specific variations, the unmodified form of the DNA polymerase is one of the following (each having the amino acid sequence of SEQ ID NO:18 or SEQ ID NO:19 except for the designated substitution(s)): G46E; G46E L329A; G46E E678G; or G46E L329A E678G. The E678G substitution, for example, allows for the incorporation of ribonucleotides and other 2′-modified nucleotides, but this mutation also appears to result in an impaired ability to extend primed templates. In certain embodiments, the mutations according to the present invention, which result in a faster extension rate of the mutant polymerase, ameliorate this particular feature of the E678G mutation.
The mutant DNA polymerases of the present invention comprise one or more amino acid substitutions in the active site relative to the unmodified polymerase. In some embodiments, the amino acid substitution(s) are at least one of the following amino acid positions:
Amino acid substitution at one or more of these positions confers improved nucleotide-incorporating activity, yielding a mutant DNA polymerase with an improved (faster) nucleic acid extension rate relative to the unmodified polymerase. In addition, amino acid substitution at one or more of these positions confers increased 3′-5′ exonuclease (proofreading) activity relative to the unmodified polymerase. While not intending to be limited to any particular theory, the present inventors believe that the improved nucleic acid extension rate of the mutant polymerases of the invention is a consequence of tighter binding to a template, i.e., less frequent dissociation from the template, resulting in a higher “processivity” enzyme. These features permit using lower concentrations of the mutant polymerase in, e.g., primer extension reactions relative to reactions involving the unmodified DNA polymerase. Thus, at a sufficiently high enzyme concentration, the extension rate of the unmodified polymerase (i.e., lacking the specific mutations that are the subject of the invention) could conceivably approach that of the mutant enzyme. The mutant polymerases also appear to perform much better than the unmodified forms at high ionic strength. However, at a sufficiently high enzyme concentration, the performance of the unmodified polymerase at low ionic strength would approach that of the mutant polymerase.
Because the unmodified forms of DNA polymerase are unique, the amino acid position corresponding to each of Xa8, Xb8, Xc4, and Xc6 is typically distinct for each mutant polymerase. Amino acid and nucleic acid sequence alignment programs are readily available (see, e.g., those referred to supra) and, given the particular motifs identified herein, serve to assist in the identification of the exact amino acids (and corresponding codons) for modification in accordance with the present invention. The positions corresponding to each of Xa8, Xb8, Xc4, and Xc6 are shown in Table 1 for representative chimeric thermostable DNA polymerases and thermostable DNA polymerases from exemplary thermophilic species.
T. thermophilus
T. caldophilus
T. sp. Z05
T. aquaticus
T. flavus
T. filiformis
T. sp. sps17
T. maritima
T. neapolitana
T. africanus
B. caldotenax
In some embodiments, the amino acid substitutions are single amino acid substitutions. The mutant polymerase can, e.g., comprise any one of the amino acid substitutions at position Xa8, Xb8, Xc4, or Xc6 separately. Alternatively, the mutant polymerase comprises any one of various combinations of substitutions at two, three, or all four of these positions. For example, in one embodiment, the mutant DNA polymerase of the invention comprises amino acid substitutions at each of positions Xb8 and Xc6. Typically, the amino acid at position Xa8, Xb8, Xc4, or Xc6 is substituted with an amino acid that does not correspond to the respective motif as set forth in SEQ ID NO:1, SEQ ID NO:2, or SEQ ID NO:3. Thus, typically, the amino acid at position Xa8, if substituted, is not Q, T, M, G or L; the amino acid at position Xb8, if substituted, is not D, E or N; the amino acid at position Xc4, if substituted, is not I or L; and/or the amino acid at position Xc6, if substituted, is not S, A, V or G. In certain embodiments, amino acid substitutions include Arginine (R) at position Xa8, Glycine (G) at position Xb8, Phenylalanine (F) at position Xc4, and/or Phenylalanine (F) at position Xc6. Other suitable amino acid substitution(s) at one or more of the identified sites can be determined using, e.g., known methods of site-directed mutagenesis and determination of primer extension performance in assays described further herein or otherwise known to persons of skill in the art.
As previously discussed, in some embodiments, the mutant DNA polymerase of the present invention is derived from CS5 DNA polymerase (SEQ ID NO:18), CS6 DNA polymerase (SEQ ID NO:19), or a variant of those polymerases (e.g., G46E; G46E L329A; G46E E678G; G46E L329A E678G; or the like). As referred to above, in CS5 DNA polymerase or CS6 DNA polymerase, position Xa8 corresponds to Glutamine (Q) at position 601; position Xb8 corresponds to Aspartate (D) at position 640; position Xc4 corresponds to Isoleucine (I) at position 669; and position Xc6 corresponds to Serine (S) at position 671. Thus, in certain variations of the invention, the mutant polymerase comprises at least one amino acid substitution, relative to a CS5 DNA polymerase or a CS6 DNA polymerase, at 5671, D640, Q601, and/or I669. Exemplary CS5 DNA polymerase and CS6 DNA polymerase mutants include those comprising the amino acid substitution(s) S671F, D640G, Q601R, and/or I669F. In some embodiments, the mutant CS5 polymerase or mutant CS6 polymerase comprises, e.g., amino acid substitutions at both D640 and 5671 (e.g., D640G and S671F). Other, exemplary CS5 DNA polymerase and CS6 DNA polymerase mutants include the following (each having the amino acid sequence of SEQ ID NO:18 or SEQ ID NO:19 except for the designated substitutions):
In addition to mutation of the motifs of SEQ ID NOS:1, 2, and/or 3 as described herein, the mutant DNA polymerases of the present invention can also include other, non-substitutional modification(s). Such modifications can include, for example, covalent modifications known in the art to confer an additional advantage in applications comprising primer extension. For example, in certain embodiments, the mutant DNA polymerase further includes a thermally reversible covalent modification. In these embodiments, a modifier group is covalently attached to the protein, resulting in a loss of all, or nearly all, of the enzyme activity. The modifier group is chosen so that the modification is reversed by incubation at an elevated temperature. DNA polymerases comprising such thermally reversible modifications are particularly suitable for hot-start applications, such as, e.g., various hot-start PCR techniques. Thermally reversible modifier reagents amenable to use in accordance with the mutant DNA polymerases of the present invention are described in, for example, U.S. Pat. No. 5,773,258 to Birch et al., which is incorporated by reference herein. Exemplary modifications include, e.g., reversible blocking of lysine residues by chemical modification of the ε-amino group of lysine residues (see Birch et al., supra). In certain variations, the thermally reversible covalent modification includes covalent attachment, to the ε-amino group of lysine residues, of a dicarboxylic anhydride as described in Birch et al., supra.
For example, particularly suitable mutant polymerases comprising a thermally reversible covalent modification are produced by a reaction, carried out at alkaline pH at a temperature which is less than about 25° C., of a mixture of a thermostable enzyme and a dicarboxylic acid anhydride having a general formula as set forth in the following formula I:
where R1 and R2 are hydrogen or organic radicals, which may be linked; or having the following formula II:
where R1 and R2 are organic radicals, which may linked, and the hydrogens are cis, essentially as described in Birch et al, supra. In specific embodiments comprising a thermally reversible covalent modification, the unmodified form of the polymerase is G64E CS5 DNA polymerase.
The mutant DNA polymerases of the present invention can be constructed by mutating the DNA sequences that encode the corresponding unmodified polymerase (e.g., a wild-type polymerase or a corresponding variant from which the mutant polymerase of the invention is derived), such as by using techniques commonly referred to as site-directed mutagenesis. Nucleic acid molecules encoding the unmodified form of the polymerase can be mutated by a variety of polymerase chain reaction (PCR) techniques well-known to one of ordinary skill in the art. (See, e.g., PCR Strategies (M. A. Innis, D. H. Gelfand, and J. J. Sninsky eds., 1995, Academic Press, San Diego, Calif.) at Chapter 14; PCR Protocols: A Guide to Methods and Applications (M. A. Innis, D. H. Gelfand, J. J. Sninsky, and T. J. White eds., Academic Press, NY, 1990).
By way of non-limiting example, the two primer system, utilized in the Transformer Site-Directed Mutagenesis kit from Clontech, may be employed for introducing site-directed mutants into a polynucleotide encoding an unmodified form of the polymerase. Following denaturation of the target plasmid in this system, two primers are simultaneously annealed to the plasmid; one of these primers contains the desired site-directed mutation, the other contains a mutation at another point in the plasmid resulting in elimination of a restriction site. Second strand synthesis is then carried out, tightly linking these two mutations, and the resulting plasmids are transformed into a mutS strain of E. coli. Plasmid DNA is isolated from the transformed bacteria, restricted with the relevant restriction enzyme (thereby linearizing the unmutated plasmids), and then retransformed into E. coli. This system allows for generation of mutations directly in an expression plasmid, without the necessity of subcloning or generation of single-stranded phagemids. The tight linkage of the two mutations and the subsequent linearization of unmutated plasmids result in high mutation efficiency and allow minimal screening. Following synthesis of the initial restriction site primer, this method requires the use of only one new primer type per mutation site. Rather than prepare each positional mutant separately, a set of “designed degenerate” oligonucleotide primers can be synthesized in order to introduce all of the desired mutations at a given site simultaneously. Transformants can be screened by sequencing the plasmid DNA through the mutagenized region to identify and sort mutant clones. Each mutant DNA can then be restricted and analyzed by electrophoresis, such as for example, on a Mutation Detection Enhancement gel (Mallinckrodt Baker, Inc., Phillipsburg, N.J.) to confirm that no other alterations in the sequence have occurred (by band shift comparison to the unmutagenized control). Alternatively, the entire DNA region can be sequenced to confirm that no additional mutational events have occurred outside of the targeted region.
Verified mutant duplexes in pET (or other) overexpression vectors can be employed to transform E. coli such as, e.g., strain E. coli BL21 (DE3) pLysS, for high level production of the mutant protein, and purification by standard protocols. The method of FAB-MS mapping, for example, can be employed to rapidly check the fidelity of mutant expression. This technique provides for sequencing segments throughout the whole protein and provides the necessary confidence in the sequence assignment. In a mapping experiment of this type, protein is digested with a protease (the choice will depend on the specific region to be modified since this segment is of prime interest and the remaining map should be identical to the map of unmutagenized protein). The set of cleavage fragments is fractionated by, for example, microbore HPLC (reversed phase or ion exchange, again depending on the specific region to be modified) to provide several peptides in each fraction, and the molecular weights of the peptides are determined by standard methods, such as FAB-MS. The determined mass of each fragment are then compared to the molecular weights of peptides expected from the digestion of the predicted sequence, and the correctness of the sequence quickly ascertained. Since this mutagenesis approach to protein modification is directed, sequencing of the altered peptide should not be necessary if the MS data agrees with prediction. If necessary to verify a changed residue, CAD-tandem MS/MS can be employed to sequence the peptides of the mixture in question, or the target peptide can be purified for subtractive Edman degradation or carboxypeptidase Y digestion depending on the location of the modification.
Mutant DNA polymerases with more than one amino acid substituted can be generated in various ways. In the case of amino acids located close together in the polypeptide chain (as with amino acids Xc4 and Xc6 of the motif set forth in SEQ ID NO:3), they may be mutated simultaneously using one oligonucleotide that codes for all of the desired amino acid substitutions. If however, the amino acids are located some distance from each other (separated by more than ten amino acids, for example) it is more difficult to generate a single oligonucleotide that encodes all of the desired changes. Instead, one of two alternative methods may be employed. In the first method, a separate oligonucleotide is generated for each amino acid to be substituted. The oligonucleotides are then annealed to the single-stranded template DNA simultaneously, and the second strand of DNA that is synthesized from the template will encode all of the desired amino acid substitutions. An alternative method involves two or more rounds of mutagenesis to produce the desired mutant. The first round is as described for the single mutants: DNA encoding the unmodified polymerase is used for the template, an oligonucleotide encoding the first desired amino acid substitution(s) is annealed to this template, and the heteroduplex DNA molecule is then generated. The second round of mutagenesis utilizes the mutated DNA produced in the first round of mutagenesis as the template. Thus, this template already contains one or more mutations. The oligonucleotide encoding the additional desired amino acid substitution(s) is then annealed to this template, and the resulting strand of DNA now encodes mutations from both the first and second rounds of mutagenesis. This resultant DNA can be used as a template in a third round of mutagenesis, and so on. Alternatively, the multi-site mutagenesis method of Seyfang & Jin (Anal. Biochem. 324:285-291. 2004) may be utilized.
Accordingly, also provided are recombinant nucleic acids encoding any of the mutant DNA polymerases of the present invention. Using a nucleic acid of the present invention, encoding a mutant DNA polymerase, a variety of vectors can be made. Any vector containing replicon and control sequences that are derived from a species compatible with the host cell can be used in the practice of the invention. Generally, expression vectors include transcriptional and translational regulatory nucleic acid regions operably linked to the nucleic acid encoding the mutant DNA polymerase. The term “control sequences” refers to DNA sequences necessary for the expression of an operably linked coding sequence in a particular host organism. The control sequences that are suitable for prokaryotes, for example, include a promoter, optionally an operator sequence, and a ribosome binding site. In addition, the vector may contain a Positive Retroregulatory Element (PRE) to enhance the half-life of the transcribed mRNA (see Gelfand et al. U.S. Pat. No. 4,666,848). The transcriptional and translational regulatory nucleic acid regions will generally be appropriate to the host cell used to express the polymerase. Numerous types of appropriate expression vectors, and suitable regulatory sequences are known in the art for a variety of host cells. In general, the transcriptional and translational regulatory sequences may include, e.g., promoter sequences, ribosomal binding sites, transcriptional start and stop sequences, translational start and stop sequences, and enhancer or activator sequences. In typical embodiments, the regulatory sequences include a promoter and transcriptional start and stop sequences. Vectors also typically include a polylinker region containing several restriction sites for insertion of foreign DNA. In certain embodiments, “fusion flags” are used to facilitate purification and, if desired, subsequent removal of tag/flag sequence, e.g., “His-Tag”. However, these are generally unnecessary when purifying an thermoactive and/or thermostable protein from a mesophilic host (e.g., E. coli) where a “heat-step” may be employed. The construction of suitable vectors containing DNA encoding replication sequences, regulatory sequences, phenotypic selection genes, and the mutant polymerase of interest are prepared using standard recombinant DNA procedures. Isolated plasmids, viral vectors, and DNA fragments are cleaved, tailored, and ligated together in a specific order to generate the desired vectors, as is well-known in the art (see, e.g., Sambrook et al., Molecular Cloning: A Laboratory Manual (Cold Spring Harbor Laboratory Press, New York, N.Y., 2nd ed. 1989)).
In certain embodiments, the expression vector contains a selectable marker gene to allow the selection of transformed host cells. Selection genes are well known in the art and will vary with the host cell used. Suitable selection genes can include, for example, genes coding for ampicillin and/or tetracycline resistance, which enables cells transformed with these vectors to grow in the presence of these antibiotics.
In one aspect of the present invention, a nucleic acid encoding a mutant DNA polymerase is introduced into a cell, either alone or in combination with a vector. By “introduced into” or grammatical equivalents herein is meant that the nucleic acids enter the cells in a manner suitable for subsequent integration, amplification, and/or expression of the nucleic acid. The method of introduction is largely dictated by the targeted cell type. Exemplary methods include CaPO4 precipitation, liposome fusion, LIPOFECTIN®, electroporation, viral infection, and the like.
Prokaryotes are typically used as host cells for the initial cloning steps of the present invention. They are particularly useful for rapid production of large amounts of DNA, for production of single-stranded DNA templates used for site-directed mutagenesis, for screening many mutants simultaneously, and for DNA sequencing of the mutants generated. Suitable prokaryotic host cells include E. coli K12 strain 94 (ATCC No. 31,446), E. coli strain W3110 (ATCC No. 27,325), E. coli K12 strain DG116 (ATCC No. 53,606), E. coli X1776 (ATCC No. 31,537), and E. coli B; however many other strains of E. coli, such as HB101, JM101, NM522, NM538, NM539, and many other species and genera of prokaryotes including bacilli such as Bacillus subtilis, other enterobacteriaceae such as Salmonella typhimurium or Serratia marcesans, and various Pseudomonas species can all be used as hosts. Prokaryotic host cells or other host cells with rigid cell walls are typically transformed using the calcium chloride method as described in section 1.82 of Sambrook et al., supra. Alternatively, electroporation can be used for transformation of these cells. Prokaryote transformation techniques are set forth in, for example Dower, in Genetic Engineering, Principles and Methods 12:275-296 (Plenum Publishing Corp., 1990); Hanahan et al., Meth. Enzymol., 204:63, 1991. Plasmids typically used for transformation of E. coli include pBR322, pUCI8, pUCI9, pUCI18, pUC119, and Bluescript M13, all of which are described in sections 1.12-1.20 of Sambrook et al., supra. However, many other suitable vectors are available as well.
The mutant DNA polymerases of the present invention are typically produced by culturing a host cell transformed with an expression vector containing a nucleic acid encoding the mutant DNA polymerase, under the appropriate conditions to induce or cause expression of the mutant DNA polymerase. Methods of culturing transformed host cells under conditions suitable for protein expression are well-known in the art (see, e.g., Sambrook et al., supra). Suitable host cells for production of the mutant polymerases from lambda pL promotor-containing plasmid vectors include E. coli strain DG116 (ATCC No. 53606) (see U.S. Pat. No. 5,079,352 and Lawyer, F. C. et al., PCR Methods and Applications 2:275-87, 1993, which are both incorporated herein by reference). Following expression, the mutant polymerase can be harvested and isolated. Methods for purifying the thermostable DNA polymerase are described in, for example, Lawyer et al., supra.
Once purified, the ability of the mutant DNA polymerases to extend primed templates can be tested in any of various known assays for measuring nucleotide incorporation. For example, in the presence of primed template molecules (e.g., M13 DNA, etc.), an appropriate buffer, a complete set of dNTPs (e.g., dATP, dCTP, dGTP, and dTTP), and metal ion, DNA polymerases will extend the primers, converting single-stranded DNA (ssDNA) to double-stranded DNA (dsDNA). This conversion can be detected and quantified by, e.g., adding a dsDNA-binding dye (e.g., an intercalating dye) such as SYBR® Green I. Using a kinetic thermocycler (see, Watson, et al. Anal. Biochem. 329:58-67, 2004, and also available from, e.g., Applied Biosystems, Stratagene, and BioRad), digital images of reaction plates can be taken (e.g., at 10-30 second intervals), thereby allowing the progress of the reactions to be followed. The amount of fluorescence detected can be readily converted to extension rates. Using such routine assays, extension rates of the mutants relative to the unmodified forms of polymerase can be determined.
Intercalating dyes are generally soluble dyes whose signal changes depend on whether the dye is intercalated between a double-stranded nucleic acid or not. In some embodiments, intercalating dyes are planar, aromatic, ring-shaped chromophore molecules that bind to nucleic acids in a reversible, non-covalent fashion, by insertion between the base pairs of the double helix. Exemplary intercalating dyes in the present invention include fluorescent dyes. Numerous intercalating dyes are known in the art. Some non-limiting examples include Pico Green® (P-7581, Molecular Probes; [24N-bis-(3-dimethylaminopropyl)-amino]-4-[2,3-dihydro-3-methyl-(benzo-1,3-thiazol-2-yl)-methylidene]-1-phenyl-quinolinium]+), EB (E-8751, Sigma), propidium iodide (P-4170, Sigma), Acridine orange (A-6014, Sigma), 7-aminoactinomycin D (A-1310, Molecular Probes), cyanine dyes (e.g., TOTO, YOYO, BOBO, and POPO), SYTO, SYBR® Green I ([2-[N-(3-dimethylaminopropyl)-N-propylamino]-4-[2,3-dihydro-3-methyl-(benzo-1,3-thiazol-2-yl)-methylidene]-1-phenyl-quinolinium]+), SYBR® Green II, SYBR DX, OliGreen, CyQuant GR, SYTOX Green, SYTO9, SYTO10, SYTO17, SYBR14, FUN-1, DEAD Red, Hexidium Iodide, Dihydroethidium, Ethidium Homodimer, 9-Amino-6-Chloro-2-Methoxyacridine, DAPI, DIPI, Indole dye, Imidazole dye, Actinomycin D, Hydroxystilbamidine, and LDS 751 (U.S. Pat. No. 6,210,885). In some embodiments of the invention, the intercalating dye is not SYBR® Green.
The mutant DNA polymerases of the present invention can be used to extend templates in the presence of polynucleotide templates isolated from blood. For example, the mutant DNA polymerases of the present invention can be used to extend templates in the presence of hemoglobin, a major component of blood, or in the presence of a hemoglobin degradation product. Hemoglobin can be degraded to various heme breakdown products, such as hemin, hematin, hematoporphyrin, and bilirubin. Thus, in certain embodiments, the mutant DNA polymerases of the present invention can be used to extend templates in the presence of hemoglobin degradation products, including but not limited to, hemin, hematin, hematoporphyrin, and bilirubin. In certain embodiments, the hemoglobin degradation product is hemin. In some embodiments, the mutant DNA polymerases of the present invention can be used to extend templates in the presence of about 0.5 to 20.0 μM, about 0.5 to 10.0 μM, about 0.5 to 5.0 μM, about 1.0 to 10.0 μM, about 1.0 to 5.0 μM, about 2.0 to 5.0 μM, or about 2.0 to 3.0 μM hemin. In other embodiments, the mutant DNA polymerases of the present invention can be used to extend templates in the presence of at least about 0.5, 1.0, 1.5, 2.0, 2.5, 3.0, 4.0, 5.0, 10.0, 20.0, or greater than 20 μM hemin. The breakdown products of hemoglobin include iron-chelators and purple pigments. Thus, in some embodiments, the mutant DNA polymerases of the present invention can be used to extend templates in the presence of iron-chelators and/or purple pigments. In other embodiments, the mutant DNA polymerases of the present invention can be used to extend templates in the presence of amounts of hemoglobin degradation products that would inhibit extension of the same template by a reference or control DNA polymerase (i.e., having a native amino acid at position Xa8, Xb8, Xc4, and/or Xc6).
The mutant DNA polymerases of the present invention can be used to extend templates in the presence of heparin. Heparin is commonly present as an anticoagulant in samples isolated from blood. In some embodiments, the mutant DNA polymerases of the present invention can be used to extend templates in the presence of about 1.0 to 400 ng/μl, 1.0 to 300 ng/μl, 1.0 to 200 ng/μl, 5.0 to 400 ng/μl, 5.0 to 300 ng/μl, 5.0 to 200 ng/μl, 10.0 to 400 ng/μl, 10.0 to 300 ng/μl, or 10.0 to 200 ng/μl heparin. In some embodiments, the mutant DNA polymerases of the present invention can be used to extend templates in the presence of at least about 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 20, 30, 40, 50, 100, 150, 200, 250, 300, 350, 400 ng/μl, or greater than 400 ng/μl of heparin. In other embodiments, the mutant DNA polymerases of the present invention can be used to extend templates in the presence of amounts of heparin that would inhibit extension of the same template by a reference or control DNA polymerase. In some embodiments, the mutant DNA polymerases of the present invention can be used to extend templates in the presence of about 10-15 fold more heparin than the amount of heparin that would prevent extension of the template by a reference or control DNA polymerase. In certain embodiments, the polynucleotide template to be extended comprises RNA.
The mutant DNA polymerases of the present invention can also be used to extend templates in the presence of melanin, which has been described as a polymerase inhibitor. See, e.g, Ekhardt, et al., Biochem Biophys Res Commun. 271(3):726-30 (2000). In some embodiments, the mutant DNA polymerases of the present invention can be used to extend templates in the presence of about 0.1 ng/μl to about 10.0 ng/μl of melanin. In other embodiments, the mutant DNA polymerases of the present invention can be used to extend templates in the presence of at least about 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1.0, 2.0, 3.0, 4.0, 5.0, 6.0, 7.0, 8.0, 9.0, 10.0 ng/μl, or greater than 10.0 ng/μl of melanin. In other embodiments, the mutant DNA polymerases of the present invention can be used to extend templates in the presence of amounts of melanin that would inhibit extension of the same template by a reference or control DNA polymerase. In some embodiments, the mutant DNA polymerases of the present invention can be used to extend templates in the presence of about 2-10 fold more melanin than the amount of melanin that would prevent extension of the template by a reference or control DNA polymerase. In certain embodiments, the polynucleotide template to be extended in the presence of melanin comprises RNA.
The mutant DNA polymerases of the present invention can also be used to amplify a polynucleotide template. Amplification can be detected, for example, in real time by use of TaqMan® probes. Ability of a polymerase to amplify a polynucleotide template in the presence of potential inhibitors such as hemoglobin, hemoglobin degradation products, heparin, or melanin can be estimated by comparing the Cp values of the reactions with and without the potential inhibitor. “Delta Cp values” refers to the difference in value between the Cp associated with amplification of the template in the presence of the potential inhibitor minus the Cp of the amplified template in the absence of the potential inhibitor (see, e.g., the Examples). In some embodiments, the improved polymerases of the invention have a delta Cp value of less than 10, 9, 8, 7, 6, 5, 4, 3, 2, or 1 in the presence of a potential inhibitor compared to the Cp value in the absence of a potential inhibitor. In some embodiments, the improved polymerases of the invention have a delta Cp value of at least 1, 2, 3,4, 5, 6, 7, 8, 9 or more in the presence or absence of a potential inhibitor compared to a control polymerase. In other embodiments, the improved polymerases of the invention have a delta Cp value of at least 1, 2, 3, 4, 5, 6, 7, 8, 9 or more in the presence or absence of a potential inhibitor compared to an otherwise identical control polymerase having a native amino acid (e.g., D, E, or N) at position Xb8 of SEQ ID NOs:25 and 26. In some embodiments, this determination is made with the precise materials and conditions set forth in the Examples.
The mutant DNA polymerases of the present invention can be used for any purpose in which such enzyme activity is necessary or desired. Accordingly, in another aspect of the invention, methods of primer extension using the mutant polymerases are provided. Conditions suitable for primer extension are known in the art. (See, e.g., Sambrook et al., supra. See also Ausubel et al., Short Protocols in Molecular Biology (4th ed., John Wiley & Sons 1999). Generally, a primer is annealed, i.e., hybridized, to a target nucleic acid to form a primer-template complex. The primer-template complex is contacted with the mutant DNA polymerase and free nucleotides (e.g. nucleotide triphosphate) in a suitable environment to permit the addition of one or more nucleotides to the 3′ end of the primer, thereby producing an extended primer complementary to the target nucleic acid. The primer can include, e.g., one or more nucleotide analog(s). In addition, the free nucleotides can be conventional nucleotides, unconventional nucleotides (e.g., ribonucleotides or labeled nucleotides), or a mixture thereof. In some embodiments, primer extension is carried out in the presence of an intercalating dye. In some embodiments, the intercalating dye is, for example, SYBR® Green, SYBR® Green I, SYBR® Green II, or Pico Green® (P-7581, Molecular Probes), or other intercalating dyes described herein. In some embodiments, the intercalating dye is not SYBR® Green. In some variations, the primer extension reaction comprises amplification of a target nucleic acid. Conditions suitable for nucleic acid amplification using a DNA polymerase and a primer pair are also known in the art (e.g., PCR amplification methods). (See, e.g., Sambrook et al., supra; Ausubel et al., supra; PCR Applications: Protocols for Functional Genomics (Innis et al. eds., Academic Press 1999). In other, non-mutually exclusive embodiments, the primer extension reaction comprises reverse transcription of an RNA template (e.g., RT-PCR). In some embodiments, reverse transcription of an RNA template is carried out in the presence of an intercalating dye. In some embodiments, the intercalating dye is, for example, SYBR® Green, SYBR® Green I. SYBR® Green II, or Pico Green®, or other intercalating dyes described herein. In some embodiments, the intercalating dye is not SYBR® Green. Use of the present mutant polymerases, which provide an improved extension rate, allow for, e.g., the ability to perform such primer extension reactions with relatively short incubation times, decreased enzyme concentrations, and/or increased product yield, and/or decreased inhibitory effects from an intercalating dye.
In other embodiments, reverse transcription of an RNA template is carried out in the presence of an inhibitor that is not, or is in addition to, an intercalating dye. For example, reverse transcription can be carried out in the presence of hemoglobin, or a degradation product thereof, such as a heme breakdown product, including but not limited to hemin, hematin, hematoporphyrin, and bilirubin; an iron chelator; a purple pigment; heparin; or melanin.
In yet other embodiments, the mutant polymerases are used for primer extension in the context of DNA sequencing, DNA labeling, or labeling of primer extension products. For example, DNA sequencing by the Sanger dideoxynucleotide method (Sanger et al., Proc. Natl. Acad. Sci. USA 74: 5463, 1977) is improved by the present invention for polymerases capable of incorporating unconventional, chain-terminating nucleotides. Advances in the basic Sanger et al. method have provided novel vectors (Yanisch-Perron et al., Gene 33:103-119, 1985) and base analogues (Mills et al., Proc. Natl. Acad. Sci. USA 76:2232-2235, 1979; and Barr et al., Biotechniques 4:428-432, 1986). In general, DNA sequencing requires template-dependent primer extension in the presence of chain-terminating base analogs, resulting in a distribution of partial fragments that are subsequently separated by size. The basic dideoxy sequencing procedure involves (i) annealing an oligonucleotide primer, optionally labeled, to a template; (ii) extending the primer with DNA polymerase in four separate reactions, each containing a mixture of unlabeled dNTPs and a limiting amount of one chain terminating agent such as a ddNTP, optionally labeled; and (iii) resolving the four sets of reaction products on a high-resolution denaturing polyacrylamide/urea gel. The reaction products can be detected in the gel by autoradiography or by fluorescence detection, depending on the label used, and the image can be examined to infer the nucleotide sequence. These methods utilize DNA polymerase such as the Klenow fragment of E. coli Pol I or a modified T7 DNA polymerase.
The availability of thermostable polymerases, such as Taq DNA polymerase, resulted in improved methods for sequencing with thermostable DNA polymerase (see Innis et al., Proc. Natl. Acad. Sci. USA 85:9436, 1988) and modifications thereof referred to as “cycle sequencing” (Murray, Nuc Acids Res. 17:8889, 1989). Accordingly, mutant thermostable polymerases of the present invention can be used in conjunction with such methods. As an alternative to basic dideoxy sequencing, cycle sequencing is a linear, asymmetric amplification of target sequences complementary to the template sequence in the presence of chain terminators. A single cycle produces a family of extension products of all possible lengths. Following denaturation of the extension reaction product from the DNA template, multiple cycles of primer annealing and primer extension occur in the presence of terminators such as ddNTPs. Cycle sequencing requires less template DNA than conventional chain-termination sequencing. Thermostable DNA polymerases have several advantages in cycle sequencing; they tolerate the stringent annealing temperatures which are required for specific hybridization of primer to nucleic acid targets as well as tolerating the multiple cycles of high temperature denaturation which occur in each cycle, e.g., 90-95° C. For this reason, AMPLITAQ® DNA Polymerase and its derivatives and descendants, e.g., AmpliTaq CS DNA Polymerase and AmpliTaq FS DNA Polymerase have been included in Taq cycle sequencing kits commercialized by companies such as Perkin-Elmer (Norwalk, Conn.) and Applied Biosystems (Foster City, Calif.).
Variations of chain termination sequencing methods include dye-primer sequencing and dye-terminator sequencing. In dye-primer sequencing, the ddNTP terminators are unlabeled, and a labeled primer is utilized to detect extension products (Smith et al., Nature 32:674-679, 1986). In dye-terminator DNA sequencing, a DNA polymerase is used to incorporate dNTPs and fluorescently labeled ddNTPs onto the end of a DNA primer (Lee et al., Nuc. Acids. Res. 20:2471, 1992). This process offers the advantage of not having to synthesize dye labeled primers. Furthermore, dye-terminator reactions are more convenient in that all four reactions can be performed in the same tube.
Both dye-primer and dye-terminator methods can be automated using an automated sequencing instrument produced by Applied Biosystems (Foster City, Calif.) (U.S. Pat. No. 5,171,534, which is herein incorporated by reference). When using the instrument, the completed sequencing reaction mixture is fractionated on a denaturing polyacrylamide gel or capillaries mounted in the instrument. A laser at the bottom of the instrument detects the fluorescent products as they are electrophoresed according to size through the gel.
Two types of fluorescent dyes are commonly used to label the terminators used for dye-terminator sequencing-negatively charged and zwitterionic fluorescent dyes. Negatively charged fluorescent dyes include those of the fluorescein and BODIPY families. BODIPY dyes (4,4-difluoro-4-bora-3a,4a-diaza-s-indacene) are described in International Patent Publication WO 97/00967, which is incorporated herein by reference. Zwitterionic fluorescent dyes include those of the rhodamine family. Commercially available cycle sequencing kits use terminators labeled with rhodamine derivatives. However, the rhodamine-labeled terminators are rather costly and the product must be separated from unincorporated dye-ddNTPs before loading on the gel since they co-migrate with the sequencing products. Rhodamine dye family terminators seem to stabilize hairpin structures in GC-rich regions, which causes the products to migrate anomalously. This requires the use of dITP, which relaxes the secondary structure but also affects the efficiency of incorporation of terminator.
In contrast, fluorescein-labeled terminators eliminate the separation step prior to gel loading since they have a greater net negative charge and migrate faster than the sequencing products. In addition, fluorescein-labeled sequencing products have better electrophoretic migration than sequencing products labeled with rhodamine. Although wild-type Taq DNA polymerase does not efficiently incorporate terminators labeled with fluorescein family dyes, this can now be accomplished efficiently by use of the modified enzymes as described in U.S. Patent Application Publication No. 2002/0142333, which is incorporated by reference herein in its entirety. Accordingly, modifications as described in US 2002/0142333 can be used in the context of the present invention to produce fluorescein-family-dye-incorporating thermostable polymerases having improved primer extension rates. For example, in certain embodiments, the unmodified DNA polymerase in accordance with the present invention is a modified thermostable polymerase as described in US 2002/0142333 and having the motifs set forth in SEQ ID NOS:1, 2, and 3.
Other exemplary nucleic acid sequencing formats in which the mutant DNA polymerases of the invention can be used include those involving terminator compounds that include 2′-PO4 analogs of ribonucleotides (see, e.g., U.S. Application Publication Nos. 2005/0037991 and 2005/0037398, and U.S. patent application Ser. No. 12/174,488, which are each incorporated by reference). The mutant DNA polymerases described herein generally improve these sequencing methods, e.g., by reducing the time necessary for the cycled extension reactions and/or by reducing the amount or concentration of enzyme that is utilized for satisfactory performance.
In another aspect of the present invention, kits are provided for use in primer extension methods described herein. Typically, the kit is compartmentalized for ease of use and contains at least one container providing a mutant DNA polymerase in accordance with the present invention. One or more additional containers providing additional reagent(s) can also be included. Such additional containers can include any reagents or other elements recognized by the skilled artisan for use in primer extension procedures in accordance with the methods described above, including reagents for use in, e.g., nucleic acid amplification procedures (e.g., PCR, RT-PCR), DNA sequencing procedures, or DNA labeling procedures. For example, in certain embodiments, the kit further includes a container providing a 5′ sense primer hybridizable, under primer extension conditions, to a predetermined polynucleotide template, or a primer pair comprising the 5′ sense primer and a corresponding 3′ antisense primer. In other, non-mutually exclusive variations, the kit includes one or more containers providing free nucleotides (conventional and/or unconventional). In specific embodiments, the kit includes alpha-phosphorothioate dNTPs, dUTP, dITP, and/or labeled dNTPs such as, e.g., fluorescein- or cyanin-dye family dNTPs. In still other, non-mutually exclusive embodiments, the kit includes one or more containers providing a buffer suitable for a primer extension reaction. In some embodiments of the invention, the kit comprises a container containing an intercalating dye and a container providing a mutant DNA polymerase in accordance with the present invention. In some embodiments, the kit includes a container providing SYBR® Green, such as SYBR® Green I and SYBR® Green II. In some embodiments, the kit includes a container providing Pico Green®. In some embodiments, the kit includes a container providing other intercalating dyes described herein. In some embodiments, the kit includes a container providing an intercalating dye that is not SYBR® Green.
In other embodiments, the kit includes a blood collection container or tube that contains or releases heparin or a salt thereof. Commonly used heparin salts include ammonium, lithium and sodium. Such blood collection containers are commercially available and well known in the art (for example, BD Vacutainer® Blood Collection Tubes, BD (Becton, Dickinson and Company)). The typical concentration range of heparin in evacuated tubes is 10 to 30 USP units of heparin/mL of blood.
In yet another aspect of the invention, reactions mixtures are provided for use in the methods described herein. In some embodiments, the reaction mixture contains a mutant DNA polymerase in accordance with the present invention. In some embodiments, the reaction mixture further contains one or more other agents for use in the methods described herein. In some embodiments, the reaction mixture contains a polynucleotide template. In some embodiments, the reaction mixture further contains at least one primer. In some embodiments, the reaction mixture further contains free nucleotides (e.g. nucleotide triphosphates). In some embodiments, the reaction mixture further contains an intercalating dye. In some embodiments, the reaction mixture contains SYBR® Green, such as SYBR® Green I and SYBR® Green II. In some embodiments, the reaction mixture contains Pico Green®. In some embodiments, the reaction mixture contains other intercalating dyes described herein. In some embodiments, the reaction mixture contains an intercalating dye that is not SYBR® Green.
In some embodiments, the reaction mixtures comprise hemoglobin, and/or a degradation product of hemoglobin. For example, in certain embodiments, the degradation products of hemoglobin include heme breakdown products such as hemin, hematin, hematoporphyrin, and bilirubin. In other embodiments, the reaction mixtures comprise an iron chelator and/or a purple pigment. The iron chelator or purple pigment can be derived from hemoglobin or degradation products thereof. In other embodiments, the reaction mixtures comprise heparin or a salt thereof. In certain embodiments, the reaction mixtures comprise melanin.
In yet other embodiments, the reaction mixtures comprise a polynucleotide template isolated from blood or a blood sample. In some embodiments, the reaction mixtures comprise a DNA template. In other embodiments, the reaction mixtures comprise an RNA template. In certain embodiments, the reaction mixtures comprise an RNA template and heparin. In other embodiments, the reaction mixtures comprise an RNA template and melanin.
It is understood that the examples and embodiments described herein are for illustrative purposes only and are not intended to limit the scope of the claimed invention. It is also understood that various modifications or changes in light the examples and embodiments described herein will be suggested to persons skilled in the art and are to be included within the spirit and purview of this application and scope of the appended claims.
Mutations in CS family polymerases were identified that provide, e.g., improved ability to extend primed DNA templates in the presence of free nucleotides. In brief, the steps in this screening process included library generation, expression and partial purification of the mutant enzymes, screening of the enzymes for the desired property, DNA sequencing, clonal purification, and further characterization of selected candidate mutants, and generation, purification, and characterization of combinations of the mutations from the selected mutants. Each of these steps is described further below.
The mutations identified by this process include S671F, D640G, Q601R, and I669F, either separately or in various combinations. These mutations were placed in several CS-family polymerases, including G46E CS5, G46E L329A CS5, G46E E678G CS5, and G46E L329A E678G CS5. Some of these mutant polymerases are listed in Table 2. Other exemplary mutant polymerases that have been made include CS6 G46E Q601R D640G
S671F E678G DNA polymerase and certain Thermus sp. Z05 DNA polymerase mutants. The resulting mutant polymerases were characterized by analyzing their performance in a series of Kinetic Thermal Cycling (KTC) experiments.
The identified mutations, S671F, D640G, Q601R, and I669F, resulted in, e.g., an improved ability to extend primed templates. In the particular context of the E678G mutation, which allows for the incorporation of ribonucleotides and other 2′-modified nucleotides, but which also results in an impaired ability to extend primed templates, the S671F, D640G, Q601R, and I669F mutations ameliorated this property of impaired primer extension ability. The identified mutations, particularly S671F alone and S671F plus D640G, also showed improved efficiency of reverse transcription when placed in G46E CS5 and G46E L329A CS5 DNA polymerases. Additional features of the mutant DNA polymerases of the invention are described further below.
Clonal Library generation: A nucleic acid encoding the polymerase domain of CS5 E678G DNA polymerase was subjected to error-prone (mutagenic) PCR between Bgl II and Hind III restriction sites of a plasmid including this nucleic acid sequence. PCR was performed using a range of Mg+2 concentrations from 1.8-3.5 mM, in order to generate libraries with a corresponding range of mutation rates. Buffer conditions were: 50 mM Bicine pH 8.2, 115 mM KOAc, 8% w/v glycerol, 0.2 mM each dNTPs, and 0.2×SYBR® Green I. A GeneAmp® AccuRT Hot Start PCR enzyme was used at 0.15 U/μl. Starting with 5×105 copies of linearized CS5 E678G plasmid DNA/reaction volume of 50 μl, 30 cycles of amplification were performed, using an annealing temperature of 60° C. for 15 seconds, an extension temperature of 72° C. for 45 seconds, and a denaturation temperature of 95° C. for 15 seconds.
The resulting amplicon was purified over a Qiaquick spin column (Qiagen, Inc., Valencia, Calif., USA) and cut with Bgl II and Hind III, then re-purified. A vector plasmid, a modification of G46E L329A CS5 carrying a large deletion in the polymerase domain between the BglII and HindIII sites, was prepared by cutting with the same two restriction enzymes and treating with calf intestinal phosphatase (CIP). The cut vector and the mutated insert were mixed at different ratios and treated with T4 ligase overnight at 15° C. The ligations were purified and transformed into an E. coli host strain by electroporation.
Aliquots of the expressed cultures were plated on ampicillin-selective medium in order to determine the number of unique transformants in each transformation. Transformations with the most unique transformants at each mutagenesis rate were stored at −70 to −80° C. in the presence of glycerol as a cryo-protectant.
Each library was then spread on large format ampicillin-selective agar plates. Individual colonies were transferred to 384-well plates containing 2× Luria broth with ampicillin and 10% w/v glycerol using an automated colony picker (QPix2, Genetix Ltd). These plates were incubated overnight at 30° C. to allow the cultures to grow, then stored at −70 to −80° C. The glycerol added to the 2× Luria broth was low enough to permit culture growth and yet high enough to provide cryo-protection. Several thousand colonies at several mutagenesis (Mg+2) levels were prepared in this way for later use.
Extract library preparation Part 1—Fermentation: From the clonal libraries described above, a corresponding library of partially purified extracts suitable for screening purposes was prepared. The first step of this process was to make small-scale expression cultures of each clone. These cultures were grown in 96-well format; therefore there were 4 expression culture plates for each 384-well library plate. One μl was transferred from each well of the clonal library plate to a well of a 96 well seed plate, containing 150 μl of Medium A (see Table 3 below). This seed plate was shaken overnight at 1150 rpm at 30° C., in an iEMS plate incubater/shaker (ThermoElectron). These seed cultures were then used to inoculate the same medium, this time inoculating 10 μl into 300 μl Medium A in large format 96 well plates (Nunc # 267334). These plates were incubated overnight at 37° C. The expression plasmid contained transcriptional control elements, which allow for expression at 37° C. but not at 30° C. After overnight incubation, the cultures expressed the clone protein at typically 1-10% of total cell protein. The cells from these cultures were harvested by centrifugation. These cells were either frozen (−20° C.) or processed immediately, as described below.
Extract library preparation Part 2—Extraction: Cell pellets from the fermentation step were resuspended in 30 μl Lysis buffer (Table 4 below) and transferred to 384-well thermocycler plates. Note that the buffer contained lysozyme to assist in cell lysis, and two nucleases to remove both RNA and DNA from the extract. The plates were subjected to three rounds of freeze-thaw (−70° C. freeze, 37° C. thaw, not less than 15 minutes per step) to lyse the cells. Ammonium sulfate was added (5 μl of a 0.75M solution) and the plates incubated at 75° C. for 15 minutes in order to precipitate and inactivate contaminating proteins, including the exogenously added nucleases. The plates were centrifuged at 3000×g for 15 minutes and the supernatants transferred to a fresh 384-well thermocycler plate. These extract plates were frozen at −20° C. for later use in screens. Each well contained about 0.5-3 μM of the mutant library polymerase enzyme.
Screening Extract Libraries for improved extension rate: M13 mp18 single-stranded DNA (M13; GenBank Accession No. X02513), primed with an oligonucleotide having the following sequence:
was used as the template molecule in the extension assay screen. 0.5-1.0 μl of extract was added to 10-20 μl reaction master mix containing 0.5-1 nM primed M13 template in 384-well PCR plates. Extension of the primed template was monitored every 10-30 seconds in a modified kinetic thermal cycler using a CCD camera (see, Watson, supra). A typical reaction master mix is listed below. Master mixes invariably included metal ion, usually magnesium at 1-4 mM, a mixture of all four dNTPs or dNTP analogs, buffer components to control the pH and the ionic strength, typically 25 mM Tricine pH 8.3/35 mM KOAc, and SYBR® Green I at 0.6× (Molecular Probes), which allowed for the fluorescent detection of primer strand extension. In order to distinguish extension-derived fluorescence from background fluorescence, parallel wells were included in the experiment in which primer strand extension was prevented, for example, by adding a metal chelator such as EDTA, or leaving out the nucleotides from the reaction master mix.
In order to find mutant enzymes that have improved nucleic acid extension rates in the presence of ribonucleotides, extension reactions were run in the presence and absence of ribonucleotides and the resulting rates of extension were compared, using the methods described above. Adding a high level of ribonucleotide (for example, a 50:50 mix of rATP and dATP) reduced the rate of extension of the parental enzyme, G46E L329A E678G CS5. Mutant extracts that exhibited a reduced level of inhibition by ribonucleotides were identified in this screen. Primary screening was done on the scale of thousands of extracts. The top several percent of these were chosen for re-screening. Culture wells corresponding to the top extracts were sampled to fresh growth medium and re-grown to produce a new culture plate containing all of the top producers, as well as a number of parental cultures to be used for comparison. These culture plates were then fed into the same screening process, to get more data on the candidate mutants. Following this secondary screening round, a relatively small number of extracts still appeared to consistently display improved extension rate relative to the parental clone. These clones were chosen for further testing. They were first streaked on selective agar plates to ensure clonal purity, then the DNA sequence of the mutated region of the polymerase gene was sequenced to determine the mutation(s) that were present in any single clone. In parallel with this work, enough mutant enzyme was produced in shake flask culture for the concentration to be determined by gel-based densitometry, after partial purification in a manner similar to that used to prepare the primary extracts. These quantified extracts were compared to parental enzyme in the conditions used in the screen, but at equal protein concentration. This final screen ensured that the differences observed were not simply protein concentration effects.
Following this final round of screening, four clones still appeared to have improved extension rates in the presence of ribonucleotides. The sequences of these four clones were determined to code for the following amino acid changes relative to the parental strain:
clone 1: S553T D640G D664G E830A
clone 2: S671F
clone 3: F557L I669F
clone 4: Q601R Y739C V749A
In the case of clone 2, it was clear that the S671F mutation must have been responsible for the observed phenotype, since it was the only amino acid mutation in the clone. For the other three clones, it was initially impossible to tell which mutation, or combination of mutations, was responsible for the observed phenotype. Therefore, the individual mutations were separated from one another, by combining DNA from the mutant plasmid with the parental plasmid using restriction fragment swaps. This is easily effected in cases where a vector-unique restriction site exists between mutations to be separated. For clone 1, such sites exist between all four of the mutations, accordingly it was possible to prepare plasmids containing each mutation individually as well as other plasmids carrying any 2 or 3 of the 4 original mutations. For clone 4, there was no such site between Y739C and V749A, but there was a site between Q601R and Y739C. Therefore it was possible to prepare plasmid DNA encoding a polymerase carrying just the Q601R mutation, and another plasmid carrying the Y739C/V749A combination.
These new plasmids were transformed into the E. coli host, and polymerase protein was expressed, purified to homogeneity, and quantified. These resulting new mutant enzymes were compared to the parental types and to the original mutant enzymes under the conditions of the original screen. It was clear from this data that the mutation D640G was solely responsible for the improved phenotype of mutant clone 1, that the mutation I669F was responsible for the improvements in mutant clone 3, and that the mutation Q601R was responsible for the improvements in mutant clone 4.
These active mutations were then combined with one another, and moved into different CS-type backbones (see, e.g., Table 2, above), again using restriction fragment swaps to create the desired expression plasmid, then transforming the plasmid into the E. coli host, and finally expressing, purifying to homogeneity, and quantifying the mutant polymerase, as described above. These new combination mutants were tested for the ability to extend primed M13 DNA in the presence of ribonucleotides. Interestingly, it was found that combining the mutations D640G, S671F, and Q601R resulted in an increase in extension rate relative to clones carrying only a single mutation. The double combination mutants tested, including D640G S671F and Q601R S671F, also showed improved extension rates relative to strains carrying only a single mutation. Moreover, the combination mutants also demonstrated improved rates of extension on primed M13 DNA when only dNTPs were present, when compared to the parental type, and furthermore it was observed that the degree of improvement relative to the parental type was greatest when the extension rate experiment was performed at low enzyme concentration or relatively high salt concentration. These observations were repeated when the combination mutations were moved into a genetic backbone that did not include the riboincorporating mutation E678G. Surprisingly, even in the E678 background, the individual mutant enzymes and the combination mutant enzymes were even “faster” than their corresponding E678 “parents.” These and other characteristics of the mutant polymerases of the invention are further illustrated in the examples provided below.
The nucleic acid extension rates of various mutants of G46E L329A E678G CS5 DNA polymerase were determined in the presence of 90% riboadenosine triphosphate (ribo ATP or rATP). The reaction mixture contained 25 mM Tricine pH 8.3, 20 mM (
As indicated above,
The nucleic acid extension rate of various mutants of G46E L329A CS5 DNA polymerase, as well as Thermus sp. Z05 DNA polymerase and its truncate, delta Z05 DNA polymerase (see, e.g., U.S. Pat. No. 5,455,170, entitled “MUTATED THERMOSTABLE NUCLEIC ACID POLYMERASE ENZYME FROM THERMUS SPECIES Z05” issued Oct. 3, 1995 to Abramson et al. and U.S. Pat. No. 5,674,738, entitled “DNA ENCODING THERMOSTABLE NUCLEIC ACID POLYMERASE ENZYME FROM THERMUS SPECIES Z05” issued Oct. 7, 1997 to Abramson et al., which are both incorporated by reference), was determined. The reaction mixture contained 25 mM Tricine pH 8.3, 0 mM (
The data shown in
The nucleic acid extension rate of various mutants of G46E L329A CS5 DNA polymerase, as well as Thermus sp. Z05 DNA polymerase and its truncate, delta Z05 DNA polymerase, was determined. The reaction mixture contained 25 mM Tricine pH 8.3, 0-100 mM KOAc, 3 mM MgCl2, 2.5% v/v Storage Buffer (50% v/v glycerol, 100 mM KCl, 20 mM Tris pH 8.0, 0.1 mM EDTA, 1 mM DTT, 0.5% Tween 20), 1% DMSO, 1×SYBR® Green I, 0.5 nM primed M13, and 25 nM (
The data shown in
Mg2+-based RT: The mutations Q601R, D640G, and S671F, separately and in combination, were evaluated for their effect on PCR and RT-PCR efficiency in the presence of Mg+2. The reactions all contained the following components: 50 mM Tricine pH 8.0, 2.5 mM Mg(OAc)2, 6% v/v Storage Buffer (50% v/v glycerol, 100 mM KCl, 20 mM Tris pH 8.0, 0.1 mM EDTA, 1 mM DTT, 0.2% Tween 20), 0.2×SYBR® Green I, 0.02 units/μl UNG, 0.2 mM each dATP, dCTP, and dGTP, 0.3 mM dUTP, 0.03 mM dTTP, and 200 nM of each primer, wherein the primers comprise a 2′-amino-C at the 3′-end at the 3′-end.
Enzymes were used at their pre-determined concentration and KOAc optima. These are given in Table 5.
Each enzyme was tested with both 106 copies/50 μl reaction DNA template (pAW109 plasmid DNA) and 106 copies/50 μl reaction RNA template (pAW109 transcript). Reactions were run in a kinetic thermocycler (ABI 5700 thermalcycler). The thermocycling parameters were: 50° C. for 2 minutes; 65° C. for 45 minutes; 93° C. for 1 minute; then 40 cycles of: 93° C. for 15 seconds; and 65° C. for 30 seconds. Fluorescence data was analyzed to determine Ct values (emergence of fluorescence over baseline) (
Mg2+-based RT with reduced RT time: The mutations Q601R, D640G, and
S671F, separately and in combination, were evaluated for their effect on RT-PCR efficiency in the presence of Mg+2, using either 45 minute or 5 minute RT time. The reactions all contained the following components: 50 mM Tricine pH 8.0, 2.5 mM Mg(OAc)2, 6% v/v Storage Buffer (50% v/v glycerol, 100 mM KCl, 20 mM Tris pH 8.0, 0.1 mM EDTA, 1 mM DTT, 0.2% Tween 20), 1% DMSO, 0.2×SYBR® Green I, 0.02 units/μl UNG, 0.2 mM each dATP, dCTP, and dGTP, 0.3 mM dUTP, 0.03 mM dTTP, and 200 nM of each primer, wherein the primers comprise a 2′-amino-C at the 3′-end.
Enzymes were used at their pre-determined concentration and KOAc optima. These are given in the following Tables 6 and 7:
Each enzyme was tested with 106 copies/50 μl reaction RNA template (pAW109 transcript). Reactions were run in a kinetic thermocycler (ABI5700). The thermocycling parameters were: 50° C. for 2 minutes; 65° C. for 5 minutes or 45 minutes; 93° C. for 1 minute; then 40 cycles of: 93° C. for 15 seconds; and 65° C. for 30 seconds. Fluorescence data was analyzed to determine Ct values (emergence of fluorescence over baseline) (
Mn2+-based RT with reduced RT time: The mutations Q601R, D640G, and S671F, separately and in combination, were evaluated for their effect on RT-PCR efficiency in the presence of Mn+2, using either 45 minute or 5 minute RT time. The reactions all contained the following components: 50 mM Tricine pH 8.0, 1 mM Mn(OAc)2, 6% v/v Storage Buffer (50% v/v glycerol, 100 mM KCl, 20 mM Tris pH 8.0, 0.1 mM EDTA, 1 mM DTT, 0.2% Tween 20), 1% DMSO, 0.2×SYBR® Green I, 0.02 units/μl UNG, 0.2 mM each dATP, dCTP, and dGTP, 0.3 mM dUTP, 0.03 mM dTTP, and 200 nM of each primer, wherein the primers comprise a 2′-amino-C at the 3′-end.
Enzymes were used at their pre-determined concentration/KOAc optima. These are given in the following Tables 8 and 9:
Each enzyme was tested with 105 copies/50 μl reaction RNA template (pAW109 transcript). Reactions were run in a kinetic thermocycler (ABI5700). The thermocycling parameters were: 50° C. for 2 minutes; 65° C. for 5 minutes or 45 minutes; 93° C. for 1 minute; then 40 cycles of: 93° C. for 15 seconds; and 65° C. for 30 seconds. Fluorescence data was analyzed to determine Ct values (emergence of fluorescence over baseline) (
It is sometimes useful to fragment a PCR product, for example when analyzing the product in a hybridization-based assay. Fragmentation can be easily accomplished by treating with alkali and heat, if ribonucleotides have been incorporated into the PCR product. For such applications relatively low level ribo-substitution will suffice to achieve fragments of optimal length. The ability of various mutant DNA polymerases to generate ribo-substituted PCR product of length 1 kb was demonstrated in the following example.
The reaction mixture was composed of 100 mM Tricine pH 8.3, 75 mM KOAc, 5% v/v glycerol, 2.5 mM Mg(OAc)2, 50 nM enzyme, 0.1% v/v DMSO, and 2.5% v/v enzyme storage buffer (50% v/v glycerol, 100 mM KCl, 20 mM Tris pH 8.0, 0.1 mM EDTA, 1 mM DTT, 0.5% Tween 20). Various mixtures of dNTPs and rNTPS were tested. In all cases, the sum of rATP and dATP was 200 μM, as was the sum of dCTP and rCTP, and dGTP and rGTP. The sum of dTTP and rTTP was 40 μM and the sum of dUTP and rUTP was 360 μM. In this analysis either all four rNTPS were added together, up to 10% of the total (see, “rNTP Series” in
This reaction mix included primers used to generate a 1 kb product from an M13 template. The primers were used at 200 nM each, wherein the primers comprise a 2′-amino-C at the 3′-end. M13 DNA was added to 106 copies per 100 μl reaction.
Reactions were run in an ABI 9700 thermocycler. The thermocycling parameters were: 50° C. for 15 seconds; 92° C. for 1 minute; then 30 cycles of: 92° C. for 15 seconds; followed by an extension step of 62° C. for 4 minutes. The ability to make full length amplicon under the various conditions tested was determined by agarose gel electrophoresis, loading 5 μl of each reaction per lane on a 2% egel-48 (Invitrogen) (
These amplicons were then fragmented as follows: 2 μl amplicon was diluted 27.5× in 0.3N NaOH and 20 mM EDTA, then heated at 98° C. for 10 minutes. The fragmented amplicon was neutralized by adding 2.5 μl 6 N HCl. To determine the degree of fragmentation achieved, the copy number of an internal fragment of the amplicon was compared before and after fragmentation, using quantitative PCR without UNG. The cycle delay observed due to fragmentation is an indication of the degree of fragmentation (and of ribonucleotide incorporation). Increased ribonucleotide incorporation leads to increased Ct delay. For this amplification, the reaction mixture was composed of 100 mM Tricine pH 8.3, 50 mM KOAc, 5% v/v glycerol, 2.5 mM Mg(OAc)2, 20 nM GQDS, 0.5% DMSO, 0.1×SYBR® Green I, 2.5% v/v enzyme storage buffer (50% v/v glycerol, 100 mM KCl, 20 mM Tris pH 8.0, 0.1 mM EDTA, 1 mM DTT, 0.5% Tween 20), 200 μM each dCTP, dGTP, and dATP, 360 μM dUTP, and 40 μM dTTP. This reaction mix was used to generate a 340 bp product from the fragmented and unfragmented amplicons, diluting these templates a further 10,000-fold from the dilution used for fragmentation. The primer sequences were used at 200 nM each, wherein the primers comprise a 2′-amino-C at the 3′-end.
Reactions were run in 384-well plates, 20 μl per reaction in a kinetic thermocycler. The thermocycling parameters were: 50° C. for 15 seconds; 92° C. for 1 minute; then 46 cycles of: 92° C. for 15 seconds; followed by an extension step of 62° C. for 1 minute. Threshold Cts were determined and corresponding fragmented and unfragmented Cts were compared, thus generating a delta Ct for each enzyme/rNTP condition tested. In this example, the greater the amount of incorporated NTP (reflecting an improved ability to incorporate NTPs in the presence of dNTPs), the greater will be the delta Ct or Ct delay after alkali-induced fragmentation. These are shown in
Hybridization assays frequently involve attaching biotin to the molecule being detected. It is therefore useful to incorporate biotin into PCR product. If biotin is attached to ribonucleotide, each fragment (except the 3′ most distal fragment, which is usually complementary to the other primer and therefore uninformative) will carry a single biotin moiety, which will result in equal signal generation by each fragment.
The ability of various enzymes to incorporate ribo-nucleotides linked to a biotin into PCR product was determined, as described below. The reaction mixture was composed of 100 mM Tricine pH 8.3, 75 mM KOAc, 5% v/v glycerol, 2.5 mM Mg(OAc)2, 50 nM enzyme, 0.1% DMSO, 2.5% v/v enzyme storage buffer (50% v/v glycerol, 100 mM KCl, 20 mM Tris pH 8.0, 0.1 mM EDTA, 1 mM DTT, 0.5% Tween 20), 200 μM each dCTP+analogs, dGTP, and dATP, 360 μM dUTP, and 40 μM dTTP. rCTP up to 40% of the total or biotin-LC-rCTP up to 50% of the total were tested. Enzymes (CS5 polymerases) tested were GE, GQDSE, GDSE, and a 4:1 blend of GL and GLE (G=G46E, L=L329A, Q=Q601R, D=D640G, S=S671F, and E=E678G).
This reaction mix was used to generate a 1 kb product from an M13 template, using primer sequences comprising a 2′-amino-C at 200 nM each. M13 DNA was added to 5×105 copies per 50 μl reaction. Reactions were run in an ABI 9700 thermocycler. The thermocycling parameters were: 50° C. for 15 seconds; 92° C. for 1 minute; then 30 cycles of: 92° C. for 15 seconds; followed by an extension step of 62° C. for 4 minutes. The ability to make full length amplicon under the various conditions tested was determined by agarose gel electrophoresis, loading 5 μl of each reaction per lane on a 2% egel-48 (Invitrogen) (
These amplicons were then fragmented as follows: 2 μl amplicon was diluted 27.5× in 0.3N NaOH and 20 mM EDTA, then heated at 98° C. for 10 minutes. The fragmented amplicon was neutralized by adding 2.5 μl 6 N HCl. To determine the degree of fragmentation achieved, the copy number of an internal fragment of the amplicon was compared before and after fragmentation, using quantitative PCR without UNG. The cycle delay observed due to fragmentation is an indication of the degree of fragmentation (and of ribonucleotide incorporation). Thus, increased ribonucleotide incorporation leads to an increased Ct delay. For this amplification, the reaction mixture was composed of 100 mM Tricine pH 8.3, 50 mM KOAc, 5% v/v glycerol, 2.5 mM Mg(OAc)2, 20 nM GQDS, 0.5% DMSO, 0.1×SYBR® Green I, 2.5% v/v enzyme storage buffer (50% v/v glycerol, 100 mM KCl, 20 mM Tris pH 8.0, 0.1 mM EDTA, 1 mM DTT, 0.5% Tween 20), 200 μM each dCTP, dGTP, and dATP, 360 μM dUTP, and 40 μM dTTP. This reaction mix was used to generate a 340 bp product from the fragmented and unfragmented amplicons, diluting these templates a further 10,000-fold from the dilution used for fragmentation. The primer sequences were used at 200 nM each, wherein each primer comprised a 2′-amino-C.
Reactions were run in 384-well plates, 20 μl per reaction in a kinetic thermocycler. The thermocycling parameters were: 50° C. for 15 seconds; 92° C. for 1 minute; then 46 cycles of: 92° C. for 15 seconds; followed by an extension step of 62° C. for 1 minute. Threshold Cts were determined and corresponding fragmented and unfragmented Cts were compared, generating a delta Ct for each enzyme/rNTP condition tested. These are shown in
The abilities of G46E L329A E678G CS5 DNA polymerase and G46E L329A D640 S671F E678G CS5 DNA polymerase to perform pyrophosphorolysis activated polymerization (“PAP”) were compared. The reaction buffer was comprised of 100 mM Tricine pH 8.0, 2.5-50 mM G46E L329A E678G CS5 DNA polymerase or 2.5-50 mM G46E L329A D640G S671F E678G CS5 DNA polymerase, 50 nM KOAc, 10% v/v glycerol, 0.04 U/μl UNG, 4 mM Mg(OAc)2, 1% DMSO, 0.2×SYBR® Green I, 2.5% v/v enzyme storage buffer (50% v/v glycerol, 100 mM KCl, 20 mM Tris pH 8.0, 0.1 mM EDTA, 1 mM DTT, 0.5% Tween 20), 0.2 mM each dATP, dCTP, and dGTP, and 0.4 mM dUTP, and 100 μM pyrophosphate. M13 template and enzyme were cross-titrated. M13 concentrations used were 0, 104, 105, and 106 copies per 20 μl reaction. Enzyme concentrations used were 2.5 nM, 5 nM, 10 nM, 15 nM, 20 nM, 25 nM, 35 nM, and 50 nM. Reactions were set up in triplicate in a 384-well thermocycler, using the following cycling parameters: 50° C. for 2 minutes; 90° C. for 1 minute; then 46 cycles of: 90° C. for 15 seconds followed by an extension temperature of 62° C. for 60 seconds.
One of the primers comprised a 2′-amino-C at the 3′-end and the other primer comprised a 2′-PO4-A (i.e., a 2′-terminator nucleotide) at the 3′-end. These primers, added to the reaction mix at 0.1 μM each, will result in a 348 bp product from M13 template. However, the 2′-PO4-A residue at the 3′-end of the second primer effectively acts as a terminator. In order to serve as a primer, it must be activated by pyrophosphorolytic removal of the terminal residue.
Fluorescence data was analyzed to determine elbow values (C(t)) (emergence of fluorescence over baseline). C(t) values for G46E L329A E678G CS5 DNA polymerase are shown in
Several of the mutations isolated by the screen described in Example I were transferred to Thermus sp. Z05 DNA polymerase (see, e.g., U.S. Pat. No. 5,455,170, entitled “MUTATED THERMOSTABLE NUCLEIC ACID POLYMERASE ENZYME FROM THERMUS SPECIES Z05” issued Oct. 3, 1995 to Abramson et al. and U.S. Pat. No. 5,674,738, entitled “DNA ENCODING THERMOSTABLE NUCLEIC ACID POLYMERASE ENZYME FROM THERMUS SPECIES Z05” issued Oct. 7, 1997 to Abramson et al., which are both incorporated by reference). First, the amino acid position corresponding to the mutations were determined by using the alignment shown in
Expression plasmids created in this way were used to make purified protein of the various mutants, as described earlier in Example I. The nucleic acid extension rate of the various mutants was then determined. The reaction mixture contained 25 mM Tricine pH 8.3, 100 mM KOAc, 3 mM MgCl2, 2.5% v/v Storage Buffer (50% v/v glycerol, 100 mM KCl, 20 mM Tris pH 8.0, 0.1 mM EDTA, 1 mM DTT, 0.5% Tween 20), 1% DMSO, 1×SYBR® Green I, 0.5 nM primed M13, and 5 nM enzyme. To this, nucleotides were added to a final concentration of 0.1 mM dGTP, 0.1 mM dTTP, and 0.1 mM dCTP, and 0.1 mM dATP. Parallel reactions containing no nucleotides were also set up. All reactions were run in quadruplicate in 20 μl volume in 384 well thermocycler plates. The extension of primed M13 template was monitored by fluorescence in a kinetic thermocycler set at 64° C., taking readings every 10 seconds. Identical reactions were averaged and the parallel minus nucleotide reactions subtracted. Extension rate (see,
PAP-related HIV DNA template titrations were performed with and without the presence of genomic DNA.
More specifically, the reactions were performed using an ABI 5700 Sequence Detection System with the following temperature profile:
The following reaction conditions were common to all reactions:
Note, that “GLQDSE CS5 DNA polymerase” refers to a G46E L329A Q601R D640G S671F E678G CS5 DNA polymerase. Note further, that the “Tth Storage Buffer” included 0.2% Tween 20, 20 mM Tris pH8.0, 0.1 mM EDTA, 100 mM KCl, 1 mM DTT, and 50% v/v glycerol. In addition, each reaction volume was brought to 50 μl with diethylpyrocarbonate (DEPC) treated water.
The varied reaction components included unblocked primers (see, the reactions denoted “unblocked primers” in
Amplifications involving various copy numbers of mutant K-Ras plasmid template in a background of wild-type K-Ras plasmid template and comparing blocked and unblocked primers were performed.
The reactions were performed using an ABI 5700 Sequence Detection System with the following temperature profile:
The following reaction conditions were common to all reactions:
Note, that “GDSE CS5 DNA polymerase” refers to a G46E D640G S671F E678G CS5 DNA polymerase. In addition, each reaction volume was brought to 50 μl with DEPC treated water.
The varied reaction components included unblocked primers (see, the reactions denoted “unblocked” in
Amplifications involving K-Ras plasmid template with various enzymes at varied concentrations were performed.
The reactions were performed using an ABI 5700 Sequence Detection System with the following temperature profile:
The following reaction conditions were common to all reactions:
The reaction components included primers blocked with a 2′-Phosphate-U or a 2′-Phosphate-A (i.e., 2′-terminator nucleotides comprising phosphate groups at 2′ positions). The primer pairs created a 92 base pair amplicon on the linearized K-Ras plasmid template. In addition, each reaction volume was brought to 50 μl with diethylpyrocarbonate (DEPC) treated water.
The polymerase concentration and KOAc concentrations were optimized for each individual polymerase as follows:
Note, that “GLQDSE” refers to a G46E L329A Q601R D640G S671F E678G CS5 DNA polymerase, “GLDSE” refers to a G46E L329A D640G S671F E678G CS5 DNA polymerase, and “GLE” refers to a G46E E678G CS5 DNA polymerase.
The extension of an unblocked HCV RT primer was compared to the extension of a blocked primer on an HCV RNA template in reverse transcription reactions. These RT comparisons were performed using various polymerases. To illustrate,
The following reaction conditions were common to all RT reactions:
The varied reaction components included a 3′-OH unblocked primer (see, the reactions denoted “3′ OH Primer (Unblocked)” in
Z05 DNA polymerase (13 nM)
GLQDSE CS5 DNA polymerase (100 nM) combined with GLQDS CS5 DNA polymerase (25 nM)
GLQDSE CS5 DNA polymerase (50 nM) combined with GLQDS CS5 DNA polymerase (50 nM where “GLQDSE CS5 DNA polymerase” refers to a G46E L329A Q601R D640G S671F E678G CS5 DNA polymerase and “GLQDS CS5 DNA polymerase” refers to a G46E L329A Q601R D640G S671F CS5 DNA polymerase. In addition, each reaction was brought to 20 μl with diethylpyrocarbonate (DEPC) treated water.
The RT reactions were incubated at 60° C. for 60 minutes in an ABI 9600 Thermal Cycler. After the RT incubation, RT reactions were diluted 100-fold in DEPC treated water. The presence of cDNA was confirmed and quantitated by 5′ nuclease probe-based real-time HCV PCR reactions designed to specifically measure the HCV cDNA products of the RT reactions. These reactions were performed using an ABI Prism 7700 Sequence Detector with the following temperature profile:
The following reaction conditions were common to all RT reactions:
where “GLQDSE” refers to a G46E L329A Q601R D640G S671F E678G CS5 DNA polymerase.
The varied reaction components included the wild-type BRAF primers blocked with a 2′-Phosphate-A; a 2′-monophosphate-3′-hydroxyl adenosine nucleotide; a 2′-Phosphate-U; or a 2′-monophosphate-3′-hydroxyl uridine nucleotide (i.e., 2′ terminator nucleotides comprising a phosphate group at the 2′ position) (labeled “F5W/R5W” in
In addition, each reaction was brought to 50 μl with DEPC treated water. Wild-type reactions (labeled “WT” in
This prophetic example illustrates a real-time monitoring protocol that involves
PAP activation in which a blocked primer leads to the production of detectable signal as that primer is activated and extended.
The primer QX below is a DNA oligonucleotide that includes a quenching dye molecule, Black Hole Quencher® (BHQ) (Biosearch Technologies, Inc.) attached to the thirteenth nucleotide (A) from the 3′ terminus.
An oligonucleotide primer of the QX is mixed in solution with a complimentary oligonucleotide R1 (see, below) such that they form a hybrid duplex. This duplex is further mixed with the reagents in the Table 10 provided below which notably include a fluorescein-labeled deoxyriboadenine tetraphosphate (i.e., a fluorescein-labeled 2′-terminator nucleotide) and DNA polymerase capable of incorporating such labeled tetraphosphate. See, U.S. patent application Ser. Nos. 10/879,494, entitled “SYNTHESIS AND COMPOSITIONS OF 2′-TERMINATOR NUCLEOTIDES”, filed Jun. 28, 2004 and 10/879,493, entitled “2′-TERMINATOR NUCLEOTIDE-RELATED METHODS AND SYSTEMS,” filed Jun. 28, 2004, which are both incorporated by reference. Incubation of the mixture at a temperature of 60° C. for, e.g., one hour could causes the 3′ terminus of the sequence QX to be extended one nucleotide in a template directed manner, resulting in at least a portion of the QX oligonucleotides being extended at their 3′ ends with the fluorescein-labeled deoxyriboadenine 2′-phosphate nucleotides, represented below as Primer QXFAM.
The newly elongated Primer QXFAM are purified from the mixture above using any number of purification methods known to persons of skill in the art. An example of such a method capable of purifying Primer QXFAM from the mixture is High Peformance Liquid Chromatography (HPLC). HPLC purification parameters are selected such that the preparation of Primer QXFAM is substantially free of non-extended Primer QX and fluorescein-labeled adenine tetraphosphates. Dual HPLC (Reverse Phase and Anion Exchange HPLC) is known as a method for purifying such molecules.
Once purified, molecules such as Primer QXFAM which contain a BHQ quenching molecule and a fluorescein molecule on the same oligonucleotide generally exhibit a suppressed fluorescein signal due to energy absorbance by the BHQ2 “quencher” molecule.
Optionally, Primer QXFAM is synthesized chemically as described in, e.g., U.S. Patent Publication No. 2007/0219361.
The sequences referred to in this example are as follows:
Primer QX 5′-GCAAGCACCCTATCAQGGCAGTACCACA-3′ (SEQ ID NO:73) (Where Q represents the presence of a BHQ molecule)
R1 3′-PCGTTCGTGGGATAGTCCGTCATGGTGTT-5′ (SEQ ID NO:74) (Where P represents 3′ phosphate)
Primer QXFAM5′-GCAAGCACCCTATCAQGGCAGTACCACAF-3′ (SEQ ID NO:75) (Where Q represents the presence of a BHQ molecule, and F represents a fluorescein-labeled 2′ phosphate adenine)
Use of the Primer in PCR.
A Primer QXFAM is combined with the reagents in Table 11.
In addition each reaction is brought to 50 μl with DEPC treated water. Some reactions contain a target sequence which serves as a substrate for PCR amplification, while others contain no target. For example, the target can be a DNA sequence identical to the 5′UTR region of the HCV genome. Combinations of these primers in PCR are expected to produce an approximately 244 bp amplicon.
The reactions can be performed using an ABI Prism 7700 Sequence Detector with the following temperature profile:
For such a PCR to progress, PAP activation of the fluorescein-terminated Primer QXFAM is necessary, and would result in the removal of the fluorescein-labeled deoxyadenine tetraphosphate molecule. Such a release is expected to result in an increase in fluorescent signal at approximately 520 nm wavelength. With monitoring of signal at approximately 520 nm wavelength as the PCR progresses, one would expect to observe an increase in fluorescence in those reactions containing target nucleic acid while observing no increased fluorescence in reactions that do not contain target.
The effect of various substitutions at the D580 position on the nucleic acid extension rate of Z05 DNA polymerase was determined. First, the mutations were created in Z05 DNA polymerase, utilizing the technique of overlap PCR, and the mutant enzymes purified and quantified, as described previously. The extension rate on primed M13 (single-stranded DNA) template was determined, using both Mg+2 and Mn+2 as the metal co-factor, by monitoring the increase in SYBR® Green I florescence, as described in Example II above, and elsewhere. In this example, the reaction mixture contained 50 mM Tricine pH 8.3, 40 mM KOAc, 1 mM Mn(OAc)2 or 2.5 mM Mg(OAc)2, 1.25% v/v Storage Buffer (50% v/v glycerol, 100 mM KCl, 20 mM Tris pH 8.0, 0.1 mM EDTA, 1 mM DTT, 0.5% Tween 20), 1% DMSO, 0.6×SYBR® Green I, 1.0 nM primed M13, and or 5 nM enzyme. To this, nucleotides were added to a final concentration of 0.2 mM dGTP, 0.2 mM dTTP, and 0.2 mM dCTP, and 0.2 mM dATP. Parallel reactions containing no nucleotides were also set up. All reactions were run in quadruplicate in 20 μl volume in 384 well thermocycler plates. The extension of primed M13 template was monitored by fluorescence in a kinetic thermocycler set at 64° C., taking readings every 15 seconds. Replicate identical reactions were averaged and the parallel minus nucleotide reactions subtracted. Extension rate was estimated by linear regression analysis of the resulting data. Results are shown in Table 12 below:
The data indicate that all 5 amino acid substitutions at position 580 of Z05 DNA polymerase result in faster extension rate under the conditions tested.
Mn2+-based RT: The mutations D580G, D580K, and D580R were evaluated for their effect on RT-PCR efficiency in the presence of Mn+2. The reactions all contained the following components: 55 mM Tricine pH 8.3, 4% v/v glycerol, 5% v/v DMSO, 110 mM KOAc, 2.7 mM Mn(OAc)2, 3.6% v/v Storage Buffer (50% v/v glycerol, 100 mM KCl, 20 mM Tris pH 8.0, 0.1 mM EDTA, 1 mM DTT, 0.2% Tween 20), 0.04 units/μl UNG, 0.45 mM each dATP, dCTP, dUTP, dGTP; 750 nM of each primer, wherein each primer comprised a t-butyl benzyl dA at the 3′-end; and 150 nM of a TaqMan probe, labeled with a cyclohexyl-FAM, a black hole quencher (BHQ-2), a 3′-Phosphate. Together, the two primers generate a 241 bp amplicon on the HCV-1B transcript. 105 copies of RNA transcript HCV-1B was added to each 100 μl reaction.
Parallel reactions with no transcript were also set up. Each enzyme was added to a final concentration of 27 nM. Reactions were run in a Roche LC480 kinetic thermocycler. The thermocycling conditions were: 5 minutes at 50° C. (“UNG” step); 2, 5, or 30 minutes at 66° C. (“RT” step); 2 cycles of 95° C. for 15 seconds followed by 58° C. for 50 seconds; and 50 cycles of 91° C. for 15 seconds followed by 58° C. for 50 seconds.
Table 13 shows the Ct values obtained from the FAM signal increase due to cleavage of the TaqMan probe:
The results indicate these three mutations at position D580 allow for a much shorter RT time while maintaining equivalent RT efficiency.
Mg2+-based RT: The mutations D580G and D580K, were compared to ES112 (Z05 E683R) for their ability to perform RT-PCR in the presence of Mg+2. The parental enzyme, Z05 DNA polymerase, is known to perform Mg+2-based RT-PCR with greatly delayed Ct values relative to ES112, and was not re-tested in this study. The conditions used were identical to those described immediately above, except that the KOAc was changed to 50 mM, the Mn(OAc)2 was replaced with 2 mM Mg(OAC)2, and the enzyme concentration was reduced to 10 nM. Thermocycling conditions were identical, except that only the 30 minute RT time was tested.
Table 14 shows the Ct values obtained from the FAM signal increase due to cleavage of the TaqMan probe:
The results indicate the D580G mutant performs Mg+2-based RT PCR with roughly the same efficiency as does ES112, and that the D580K mutant results in significantly a earlier Ct value, indicative of a much higher RT efficiency under these conditions.
The effect of a mutant Z05 polymerase on catalytic efficiency in the presence of an intercalating dye was determined. The kinetics of primer extension with DNA and RNA substrates was determined for the wild type (Z05) and for the D580G mutant of Z05 (ZO5D) (Table 15). ZO5D shows 12.5-fold improvement in the catalytic efficiency (kcat/KM) of Mn2+-activated reverse transcription over wild-type Z05.
The inhibitory effect of Pico Green® dye on extension rate was also measured. Pico Green® dye inhibits activity on both DNA and RNA, mainly interfering with substrate binding. ZO5D largely overcomes this inhibition (Table 15), indicating that mutation at position D580 overcomes inhibitory effects by intercalating dyes. Therefore, these mutations are particularly useful for use with intercalating dyes.
The reactions all contained the following components: 50 mM Tricine pH 8.3, 60 mM KOAc, 0.03% TWEEN20, 3% v/v glycerol, 1 mM Mn(OAc)2, 125 mM each dNTP; about 2000 cpm/pmol [α-33P]dCTP. For DNA substrate, 1 nM M13 DNA template and 2 nM NJS40 primer were used. The extension product generated with this primer and template is 7 kb maximum.
For the RNA substrate, 5 nM HCV-NS5B transcript and 10 nM NJS201 were used. Complete extension of this primer annealed to this template generates ˜2.1 kb extension product. The concentration of Pico Green® used in the relevant reactions is 0.2%. Primer extension reactions were carried out at 60° C.
Rate of product formation was measured via TCA precipitation and filtration following [α-33P]dCTP incorporation on either primed M13 DNA or primed RNA transcript (HCV).
This example demonstrates that the DNA polymerases of the present invention have improved nucleic acid extension rates and reverse transcription efficiency in the presence of hemoglobin degradation products.
Hemoglobin, a critical component in blood, can be degraded to various heme breakdown products, such as hemin, hematin, hematoporphyrin, and eventually bilirubin. Since these molecules are both iron-chelators and purple pigments, they might utilize several mechanisms to inhibit real time PCR.
A model system using an HCV RNA transcript was used to determine the inhibitory effects of hemin in PCR using Z05, Z05 D580G, Z05 D580 K or Z05 D580R polymerases.
45 U DNA pol Z05, Z05 D580G, Z05 D580 K or Z05 D580R were tested in RT-PCR conditions (120 mM KOAc, 3.3 mM Mn2, 60 mM Tricine; 50 ul total) amplifying 1,000 copies of an HCV RNA transcript generated from plasmid pJP2-5 (Promega) with and without the addition of 2.5 μL hemin (40 fold molar excess to DNA pol). These reactions were run in a Roche LightCycler 480 Real Time PCR Instrument with a 12 minute RT step followed by 50 cycles of denaturation and extension. Real time fluorescence was detected in the JA270 and CY5.5 channels during the last 50 cycles. The Cp (crossing point) values from growth curves generated by fluorescent 5′ nuclease (TaqMan) activity for each reaction was determined using the instrument's “2nd derivative Max analysis” method. The Cps of all normal reactions were compared to those with hemin, as shown in Table 16. In the presence of 2.5 μM hemin, no amplification of HCV RNA was observed by Z05, whereas the variants Z05 D580G, D580K and D580R all detected HCV with Cp delays of 1.8-7.1 cycles.
Agarose gel electrophoresis confirmed that these effects were due to reduced amplification, not quenching by the porphyrin hemin molecule. Similar results were obtained with HCV DNA templates, suggesting that hemin acts as general PCR inhibitor.
This example demonstrates that the DNA polymerases of the present application have improved reverse transcription efficiency in the presence of heparin.
Heparin is a highly sulfated glycosaminoglycan and contains one of the highest negative charge densities of any known biological molecule. As such, it can mimic nucleic acid substrates and is often used as a non-specific competitor in protein-DNA/RNA binding assays. Whereas hemin acts a general PCR inhibitor, heparin preferentially inhibits reverse transcription by Z05-based DNA polymerases.
Using the HCV RNA RT-PCR amplification model system described above, the presence of 100 ng/μl or 200 ng/μl of heparin was tested to determine the inhibitory effects of heparin using Z05, Z05 D580G, Z05 D580 K or Z05 D580R polymerases. The Cps of all normal reactions were compared to those with heparin (Table 17). Whereas the wild-type Z05 enzyme was unable to amplify HCV RNA in the presence of 12.5 ng/ul heparin, the Z05 D580G/K/R mutants were able to tolerate up to 200 ng/μl heparin with minimal Cp delays, suggesting that these variants are tolerant of at least 10-15 fold more heparin.
A direct comparison between RNA and DNA substrates revealed that amplification of DNA by Z05 D580G is completely unaffected by the presence of high levels of heparin. Overall, these data support the notion that heparin is a nucleic acid mimic that specifically inhibits reverse transcription. The resistance of a DNA pol to heparin is directly correlated with the intrinsic RT activity for each particular enzyme.
This example demonstrates that the DNA polymerases of the present application have improved reverse transcription efficiency in the presence of melanin.
Previous groups have reported that melanin is a potent inhibitor of PCR with a more pronounced effect on the amplification of longer substrates (Giambernardi, T. A., et al., (1998) BioTechniques 25:564-566; Price, K., and Linge, C. (1999) Melanoma Res. 9:5-9.). More recently, it was demonstrated that melanin binds reversibly to a thermostable DNA polymerase and inhibits its activity (Eckhart, L., et al., (2000) Biochem Biophys Res Commun. 271(3):726-30). Together, these data suggest that melanin might negatively affect the DNA binding and processivity of DNA polymerases.
Using the HCV RNA RT-PCR amplification model system described above, the presence of 0.5, 1, 2, 5 or 10 ng/uL of melanin was tested to determine inhibitory effects using Z05 or Z05 D580G polymerases. Whereas the wild-type Z05 enzyme began to display inhibition at 1 ng/uL of melanin, Z05 D580G was unaffected at up to 5 ng/uL (Table 18).
The data indicate that Z05 D580G can tolerate 2-5 fold more melanin than Z05 without decreasing RT-PCR efficiency.
It is understood that the examples and embodiments described herein are for illustrative purposes only and that various modifications or changes in light thereof will be suggested to persons skilled in the art and are to be included within the spirit and purview of this application and scope of the appended claims. All publications, patents, and patent applications cited herein are hereby incorporated by reference in their entirety for all purposes.
The present application is a continuation-in-part of U.S. patent application. Ser. No. 12/425,303, filed Apr. 16, 2009, which is a continuation-in-part of U.S. patent application. Ser. No. 11/873,896, filed Oct. 17, 2007, which claims the benefit of U.S. Provisional Application No. 60/852,882, filed on Oct. 18, 2006. The entire disclosure of all of the above-referenced prior applications are hereby incorporated herein by reference.
| Number | Date | Country | |
|---|---|---|---|
| 60852882 | Oct 2006 | US |
| Number | Date | Country | |
|---|---|---|---|
| Parent | 12425303 | Apr 2009 | US |
| Child | 13088049 | US | |
| Parent | 11873896 | Oct 2007 | US |
| Child | 12425303 | US |
