Patent Application
20060199222
1. Field of the Invention
The present invention relates to Bacillus transformation, transformants, and mutant libraries.
2. Background
A widely used known method for altering the chromosome of Bacillus involves building plasmid constructs and transforming them into E. coli. Subsequently, the plasmids are isolated from E. coli and transformed into Bacillus. Widespread use of this method can be attributed, at least in part, to the notion that E. coli is easier to transform than Bacillus. In this regard, the in vitro ligation of plasmids results in nicked products that can transform E. coli but do not transform Bacillus.
The conventional approach to constructing libraries in Bacillus is based on replicating plasmids. Such an approach, unfortunately, is generally associated with a number of disadvantages, including:
Alternatively, integrating plasmids or vectors may be used. Integrating vectors do not contain an origin of replication and therefore require insertion into the host chromosome to be stably maintained. However, these are not without problems. Integration occurs via a Campbell-type recombination event that results in a duplication of the cloned region at either end of the inserted (now linear) vector. Depending on the position of the integration genes may be disrupted resulting in poor transformation efficiency.
With either method there is still a need to generate sufficient amounts of the desired sequence to effect an efficient transformation. This was usually accomplished by inserting the desired sequence into a shuttle vector that was inserted into E. coli, allowed to replicate to a high copy number, and recovering the amplified DNA. This process could run into problems due to sequence size; the larger the sequence the more difficult it could be to insert and replicate. Additionally, there were sequences that would not insert or replicate in E. coli resulting in a loss of diversity in the DNA library that was being built. Finally, the high copy number of some plasmids/vectors is deleterious to E. coli.
The prior art methods failed to reproducibly render cells hypercompetent nor did they generate large libraries easily in Bacillus and other host cells. In order to generate a small library the prior art utilized E. coli to amplify DNA of interest to obtain a sufficient quantity for transformation of host cells. The methods provided herein allows the generation of large libraries in a reproducible manner without the use of E. coli.
Thus, there is a need for a Bacillus transformation method that is relatively straightforward, efficient and reproducible. In particular, a method is needed that permits the efficient transformation of Bacillus, without requiring intervening steps involving the use of additional microorganisms, such as E. coli. Particularly advantageous would be a transformation method that is amenable to the construction of mutant libraries, and which avoids or overcomes one or more of the above-mentioned disadvantages.
The present invention provides methods for building polynucleotide constructs in vitro, directly transforming such constructs into competent Bacillus species and/or strains with good efficiency, and generating populations of mutants (e.g., a mutant library).
In one of its aspects, the present invention provides a method of producing a transformed microorganism. According to one embodiment, the method includes the steps of:
(i) selecting a competent microorganism of the genus Bacillus;
(ii) producing a polynucleotide construct in vitro; and
(iii) directly transforming the microorganism with the construct such that the construct becomes integrated into a chromosome of the microorganism.
In one embodiment, the construct includes mutagenized DNA.
In another embodiment, the construct includes a sequence of interest, flanked on each side by a homology box. Optionally, the construct can additionally include non-homologous outer flanks.
According to one embodiment, the construct is a non-plasmid DNA construct.
In one embodiment, the competent microorganism of the genus Bacillus is an ultra-competent strain, preferably Pxyl-comK.
In accordance with one embodiment, the above method additionally includes the steps of (i) selecting a target region in a chromosome of the competent Bacillus, and (ii) increasing (e.g., maximizing) the homology between the target region and the construct.
Another aspect of the present invention provides a library of mutants produced by the above method.
A further aspect of the present invention provides a method for the directed evolution of a sequence in the Bacillus chromosome. One embodiment of the method includes the steps of:
(i) in vitro mutagenesis of a selected sequence,
(ii) direct transformation of the mutagenized sequence into a competent Bacillus, e.g., a Bacillus carrying Pxyl-comK,
(iii) screening for, or selection of, mutants possessing or exhibiting a desired property; and
(iv) repeating steps (i)-(iii) for one or more rounds.
Advantageously, the methods disclosed herein allow one to evolve single-copy genes of a competent Bacillus strain.
In another of its aspects, the present invention provides a method for constructing a polynucleotide sequence in a target locus of a selected recipient strain, wherein the strain includes a selectable marker residing at the target locus. One embodiment of the method includes the steps of:
(i) assembling a construct comprising a sequence of interest, a selectable marker that differs from the residing marker of the recipient strain, and two flanks which are homologous to sequences of the target locus;
(ii) transforming the recipient strain with the construct under conditions permitting the incoming sequence and selectable marker to replace the residing marker, and selecting for transformants that include the incoming selectable marker;
(iii) repeating steps (i)-(iii), with the newly inserted selectable marker acting as the residing marker.
Optionally, after step (ii) the following additional step can be performed:
testing the transformants for loss of the residing marker, thereby verifying that the construct was incorporated into the correct locus of the chromosome.
These and other features, aspects and advantages of the present invention will become apparent from the following detailed description, in conjunction with the appended claims.
The present invention provides methods for building DNA constructs in vitro, transforming such constructs into competent Bacillus strains with good efficiency, and generating populations of mutants in Bacillus.
Definitions:
Unless defined otherwise herein, 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 belongs. Singleton, et al., D
Transforming DNA or DNA Construct
The transforming sequence or transforming DNA is generated in vitro by PCR or other suitable techniques. The typical structure of transforming DNA is shown in a schematic form in
Transforming DNA is DNA used to introduce sequences into a host cell or organism. The DNA may be generated in vitro by PCR or any other suitable techniques. In a preferred embodiment, mutant DNA sequences are generated with site saturation mutagenesis in at least one codon. In another preferred embodiment, site saturation mutagenesis is performed for two or more codons. In a further embodiment, mutant DNA sequences have more than 40%, more than 45%, more than 50%, more than 55%, more than 60%, more than 65%, more than 70%, more than 75%, more than 80%, more than 85%, more than 90%, more than 95%, or more than 98% homology with the wild-type sequence. Alternatively, mutant DNA may be generated in vivo using any known mutagenic procedure such as, for example, radiation, nitrosoguanidine and the like. The desired DNA sequence is then isolated and used in the methods provided herein.
The transforming sequences may be wild-type, mutant or modified. The sequences may be homologous or heterologous. Transforming sequence and DNA construct may be used interchangeably.
Incoming Sequence
This sequence can code for one or more proteins of interest. It can have other biological function. In many cases the incoming sequence will include a selectable marker, such as a gene that confers resistance to an antibiotic.
An incoming sequence as used herein means a DNA sequence that is newly introduced into the Bacillus chromosome or genome. The sequence may encode one or more proteins of interest. An incoming sequence comprises a sequence that may or may not already present in the genome of the cell to be transformed, i.e., either a homologous or heterologous sequence (defined herein).
In one embodiment, the incoming sequence encodes a heterologous protein, said protein(s) including, but not limited to hormones, enzymes, growth factors. In another embodiment, the enzyme includes, but is not limited to hydrolases, such as protease, esterase, lipase, phenol oxidase, permease, amylase, pullulanase, cellulase, glucose isomerase, laccase and protein disulfide isomerase.
In a second embodiment, the incoming sequence may encode a functional wild-type gene or operon, a functional mutant gene or operon, or a non-functional gene or operon. The non-functional sequence may be inserted into a target sequence to disrupt function thereby allowing a determination of function of the disrupted gene.
Flanking Sequence
A flanking sequence as used herein means any sequence that is either upstream or downstream of the sequence being discussed, e.g., for genes A B C, gene B is flanked by the A and C gene sequences. In a preferred embodiment, the incoming sequence is flanked by a homology box on each side. In a more preferred embodiment, the incoming sequence and the homology boxes comprise a unit that is flanked by stuffer sequence (as defined herein) on each side. A flanking sequence may be present on only a single side (either 3′ or 5′) but it is preferred that it be on each side of the sequence being flanked.
Stuffer Sequence
Stuffer sequence means any extra DNA that flanks the homology boxes, typically vector sequences, but could be any non-homologous DNA sequence. Not to be limited by any theory, a stuffer sequence provides a noncritical target for a cell to initiate DNA uptake.
Wild-Type Genes
The terms “wild-type sequence,” or “wild-type gene” are used interchangeably and refer to a sequence native or naturally occurring in a host cell. The wild-type sequence may encode either a homologous or heterologous protein. A homologous protein is one the host cell would produce without intervention. A heterologous protein is one that the host cell would not produce but for the intervention.
Mutant Genes
The terms “mutant sequence,” or “mutant gene” are used interchangeably and refer to a sequence that has an alteration in at least one codon occurring in a host cell's wild-type sequence. The expression product of the mutant sequence is a protein with an altered amino acid sequence relative to the wild-type. The expression product may have an altered functional capacity, e.g., enhanced enzymatic activity and the like.
Modified Genes
The terms “modified sequence” or “modified genes” are used interchangeably and means a deletion, insertion or interruption of naturally occurring nucleic acid sequence. The expression product of the modified sequence may be a truncated protein if the modification is a deletion or interruption of the sequence. The truncated protein may retain biological activity. The expression product of the modified sequence may be an elongated protein if the modification is an insertion into the nucleic acid sequence. An insertion may lead to a truncated protein as the expression product if the insertion results in the formation of a stop codon. Thus, an insertion may result in either a truncated protein or an elongated protein as an expression product.
Host Cell
“Host cell” means a cell that has the capacity to act as a host and expression vehicle for an incoming sequence according to the invention. In one embodiment, the host cell is a microorganism. In a preferred embodiment according to the present invention, “host cell” means the cells of Bacillus. As used herein, the genus Bacillus includes all members known to those of skill in the art, including but not limited to B. subtilis, B. licheniformis, B. lentus, B. brevis, B. stearothermophilus, B. alkalophilus, B. amyloliquefaciens, B. coagulans, B. ciculans, B. lautus and B. thuringiensis. Other cells useful in the present invention include Acinetobacter, Thermus, Deinococcus Radiodurans.
Homologous Sequence A homologous sequence is a sequence that is found in the same genetic source or species. For example, the host cell strain may be deficient in a specific gene. If that gene is found in other strains of the same species the gene would be considered a homologous sequence.
Heterologous Sequence
A heterologous sequence is a sequence derived from a separate genetic source or species. A heterologous sequence is a non-host sequence, a modified sequence, a sequence from a different host cell strain, or a homologous sequence from a different chromosomal location of the host cell.
Homology Box
Homology boxes may flank each side of the incoming sequence. The sequence of each homology box is homologous to a sequence in the Bacillus chromosome. These sequences direct where in the Bacillus chromosome the new construct gets integrated and what part of the bacillus chromosome will be replaced by the incoming sequence.
Chromosomal Integration
This is a process where the incoming sequence is introduced into the Bacillus chromosome. The homology boxes of the transforming DNA will align with homologous regions of the chromosome. Subsequently, the sequence between the homology boxes is replaced by the incoming sequence in a double crossover (i.e., homologous recombination).
Homologous Recombination
Homologous recombination means the exchange of DNA fragments between two DNA molecules or paired chromosomes (during crossing over) at the site of identical nucleotide sequences. In a preferred embodiment, chromosomal integration is by homologous recombination.
Target Sequence
A target sequence as used herein means the DNA sequence in the host cell that encodes the sequence where it is desired for the incoming sequence to be inserted into the host cell genome. The target sequence may encode a functional wild-type gene or operon, a functional mutant gene or operon, or a non-functional gene or operon.
Selectable Markers
Selectable markers are usually genes that confer antibiotic resistance or a metabolic advantage on the host cell to allow cells containing the exogenous DNA to be distinguished from cells that have not received any exogenous sequence during the transformation. A residing selectable marker is one that is located on the chromosome of the microorganism to be transformed. A residing selectable marker encodes a gene that is different from the selectable marker on the transforming DNA construct.
Sequence of Interest
As used herein, a sequence of interest may be an incoming sequence or a sequence to be generated in situ. The terms gene of interest and sequence of interest may be used interchangeably herein.
Library of Mutants
A population of cells which are identical in most of their genome but include different homologues of one or more genes. Such libraries can be used, for example, to identify genes or operons with improved traits.
Super Competent or Hypercompetent
As used herein, hypercompetent means that greater than 1% of a cell population is transformable with chromosomal Bacillus DNA. Alternatively, hypercompetent means that greater than 10% of a cell population is transformable with a self-replicating Bacillus plasmid. Preferably, the super competent cells will be transformed at a rate greater than observed for the wild-type or parental cell population. Super competent and hypercompetent are used interchangeably herein.
Although Bacillus is used throughout the specification it should be understood that any competent cell may be used in the inventive methods disclosed herein.
The incoming sequence may encode more than one protein. As shown in
The first and second sequences may encode different and distinct proteins, either full length or portions thereof. For example, the first sequence may encode a protease (or portion thereof) and the second sequence may encode a hormone (or portion thereof).
Alternatively, the first and second sequences may encode different portions of the same protein. For example, the first sequence may encode the amino terminal and the second sequence may encode the carboxy terminal of a single protein. This would allow either or both of the sequences to be selectively mutagenized with different mutagenizing protocols being used. Or the carboxy and amino terminal sequences of a protein may be joined while omitting an intervening sequence found in the native protein.
As another option, the first and second sequences may encode variants of a single protein. Thus, for example, sequence 1 may encode Type A hemoglobin while sequence 2 encodes Type S hemoglobin.
The various components of the DNA construct may be assembled by PCR and/or ligation. It should be noted that any technique may be used as long as the DNA construct has the final configuration desired.
Once the DNA construct is assembled in vitro it may be used to: 1) insert heterologous sequences into a desired target sequence of a host cell, or 2) mutagenize a region of the host cell chromosome, i.e., replace an endogenous sequence with a heterologous sequence, or 3) delete target genes.
As noted in
The DNA construct may further comprise flanking, non-homologous sequences, i.e., stuffer sequences, and is illustrated in
In one general embodiment, the present invention involves assembling a DNA construct in vitro, followed by direct cloning of such construct into a competent Bacillus, such that the construct becomes integrated into a chromosome of the Bacillus. For example, PCR fusion and/or ligation can be employed to assemble a DNA construct in vitro. In a preferred embodiment, the DNA construct is a non-plasmid DNA construct. In one embodiment, the DNA construct comprises a DNA into which a mutation has been introduced. Bacillus can then be transformed with the DNA construct. In this regard, highly competent mutants of Bacillus are preferably employed to facilitate the direct cloning of the constructs into the cells. For example, Bacillus carrying the comK gene under the control of a xylose-inducible promoter (Pxyl-comK) can be reliably transformed with very high efficiency, according to the teachings herein. Direct transformation means that an intermediate cell is not used to amplify, or otherwise process, the DNA construct prior to introduction into the host cell. Introduction of the DNA construct into the host cell includes those physical and chemical methods known in the art to introduce DNA into a host cell without insertion into a plasmid or vector. Such methods include but are not limited to calcium chloride precipitation, electroporation, naked DNA, liposomes and the like. The DNA constructs may be co-transformed with a plasmid without being inserted into the plasmid. A library of mutants can be generated.
As previously discussed, a widely used prior method for altering the chromosome of Bacillus involves building plasmid constructs and transforming them into E. coli. Subsequently, the plasmids are isolated from E. coli and transformed into Bacillus. The present invention, in contrast, provides in vitro construction and direct transformation into Bacillus, without the use of any such intervening microorganisms.
As also discussed above, the conventional approach to constructing libraries in Bacillus is based on replicating plasmids. Such an approach, unfortunately, is associated with a number of disadvantages, including:
Multimerize the Assembled Sequence
According to one embodiment of the present invention, the transforming DNA can be multimerized, for example, by ligation. This has a similar effect as adding non-homologous flanks, i.e., stuffer sequences. It increases the probability the DNA after uptake into the cell will still have both homology boxes flanking the incoming sequence, thereby increasing transformation efficiency.
Mutagenizing a Region of the Bacillus Chromosome
The present invention provides a process for mutagenizing a region of the Bacillus chromosome, an embodiment of which is illustrated in
In another embodiment, the assembly of long DNA sequences is accomplished in situ. Individual DNA constructs are utilized to introduce segments of the final heterologous DNA sequence into a target sequence or locus of the host cell.
Construction of Long Sequences by Iterative Marker Replacement
This method, as taught herein and illustrated in
According to one embodiment, the process comprises the steps of:
(i) by PCR fusion or other suitable technique one assembles a sequence comprising a sequence of interest, a selection marker and two flanks, which are homologous to the target locus;
(ii) the recipient strain is transformed with the constructed sequence and one selects for resistance to the incoming selection marker;
(iii) the transformants are then tested for loss of the residing marker which ensures that the construct was incorporated into the correct locus of the chromosome;
(iv) subsequently, the above cycle can be repeated by reversing the role of the incoming and residing markers.
In another embodiment, the microorganism doesn't possess an endogenous selection marker in the first round of transformation and cannot be tested for the loss of a residing marker. Thus, after being transformed the microorganism is screened for the incoming selection marker.
This method allows one to assemble large sequences (e.g., >>5 kb) in vivo from smaller pieces, which can be generated in vitro by PCR fusion or other suitable techniques. Only two antibiotic markers are required because each step displaces the marker gene used in the previous round.
The entire resulting construct can be moved between different strains using chromosomal transformation or transduction. Thus, by way of this method, one can accumulate various sequences during the course of a project and retain the ability to simultaneously move them into a new strain.
During the final cloning cycle, one can use a selected gene that is essential for growth under some conditions (e.g., synthesis of an amino acid, utilization of a certain sugar) instead of the incoming marker. The resulting strain would then be free of any antibiotic genes.
It should be appreciated that the iterative aspect of this method generates value as it permits the assembly of large sequences. This method allows one to introduce multiple sequences from various sources into a strain (e.g., bacteria, fungi, eukaryotic, etc.). This method permits one to generate tandem gene repeats as a method for increasing gene copy number. This method permits one to generate strains containing multiple mutations and inserted sequences but no antibiotic markers.
The methods disclosed herein directed to the assembly of transforming DNA constructs may be used to direct the evolution of a sequence or target locus within the host cell. Selection of the target sequence allows the design and/or in vitro mutagenesis of the target sequence. The mutagenesis of a locus of the host cell, i.e., recipient, chromosome is depicted in schematic form in
According to one preferred embodiment, the method comprises the following steps:
Identification of Transformants
Although the presence/absence of marker gene expression suggests that the gene of interest is also present, its presence and expression should be confirmed. For example, if the nucleic acid encoding a secretion factor is inserted within a marker gene sequence, recombinant cells containing the insert can be identified by the absence of marker gene function. Alternatively, a marker gene can be placed in tandem with nucleic acid encoding the secretion factor under the control of a single promoter. Expression of the marker gene in response to induction or selection usually indicates expression of the gene of interest as well.
Alternatively, host cells which contain the coding sequence for a sequence o interest and express the protein may be identified by a variety of procedures known to those of skill in the art. These procedures include, but are not limited to, DNA-DNA or DNA-RNA hybridization and protein bioassay or immunoassay techniques which include membrane-based, solution-based, or chip-based technologies for the detection and/or quantification of the nucleic acid or protein.
B. subtilis is a bacteria which is capable of entering sporulation during times of great stress in the environment, such as extreme lack of nutrients. Making this decision triggers a very elaborate and expensive conversion to the sporulation development state. Over 50 genes which need to be expressed for sporulation are under the control of eight sporulation control genes. These are Spo0A, 0B, 0E, 0F, 0H, 0J, 0K, and 0L, with spo0A being the most critical control factor. Mutation in the sporulation control genes allows the cells to ignore their environment so that they fail to enter sporulation and continue production of heterologous or homologous proteins. A mutation in the oppA gene of the oppA operon has been shown to enhance protein production. See WO 00/39323.
The degU gene of Bacillus subtilis encodes a protein involved in the control of expression of different cellular functions, including degradative enzyme synthesis, competence for DNA uptake and the presence of flagella. Two classes of mutations have been identified in both genes. One class of mutations leads to decreased expression (degU mutations) while the second one leads to enhanced expression [degU(Hy) mutations] of regulated genes, i.e., genes regulated by the degU system. This second class of mutations is associated with a pleiotropic phenotype which includes the ability to sporulate in the presence of glucose, loss of flagella and decreased genetic competence.
Many industrially important products, e.g., enzymes, hormones, growth factors, and other proteins, are produced from members of the genus Bacilli in large scale fermentation processes. Some of these include proteases, lipases, amylases, and beta-glucanases. The protein of interest to be expressed may be either homologous or heterologous to the host. In the first case overexpression should be read as expression above normal levels in said host. In the latter case basically any expression is of course overexpression. Thus, it is advantageous to have a cell that will fail to sporulate yet possesses enhanced expression of genes of interest.
An oppA (i.e., spo0K) mutation in combination with a degU(Hy) mutation would appear to be ideal for production of a gene of interest. However, it has been shown that mutation of the oppA gene results in a decreased competency. See Rudner et al., J. Bacteriology (1991) 173:1388-1398. As noted above the degU(Hy) mutant also results in decreased competency. Thus, introduction of a gene of interest or other genetic manipulation into such a host cell would be significantly more difficult than in the absence of such mutation.
It has been advantageously found that the inventive methods described herein overcome this difficulty. Use of a pyxl-comK Bacillus strain overcomes the decreased competency exhibited by degU(Hy) oppA− strains. It has been found that the introduction of pyxl-comK into Bacillus not only restores competency but the cells are hypercompetent relative to wild-type (or parental) cells. Thus, heterologous or homologous sequences may be introduced into previously low competency cells.
Transforming Bacillus with PCR-generated DNA and getting many transformants (>100). The methods provided by the present invention allows for the generation of large libraries.
The methods disclosed herein may be used with mutations that enhance competence. Employing other mutations to enhance competence, e.g., comS instead of comK, mutations to comS homologs and the like are contemplated by the present invention.
The methods described herein may be used in any microorganism that can be made competent. Direct transformation in other organisms which can be made competent (like Acinetobacter, Thermus, Deinococcus Radiodurans) is contemplated.
The methods herein should work for any recombination goal, such as insertions, deletions or replacements. Plasmids with temperature sensitive replication would facilitate the curing step. Ligating the PCR products to form concatamers are contemplated for improving the transformation frequency and allowing smaller homology boxes to be used.
Have Inactive Homologue Reside in the Host to Improve Transformation Efficiency
A mutagenesis experiment, in accordance with an embodiment of the present invention, is illustrated in
The following examples are illustrative and are not intended to limit the invention.
A fragment containing the xylR repressor gene and the xylA promoter was obtained by PCR using primers xylR.2.f (this primer will incorporate a HindIII site) and xylA.1.r and chromosomal DNA from BG168. A second fragment containing the comK gene including the first aa codon was obtained by PCR using primers comK.2.f and comK.2.r (this primer will incorporate a XbaI site) and same chromosomal DNA. After purification, the fragments were fused together by mixing them in a PCR reaction containing the external primers (xylR.2.f and comK.2.r). A PCR fragment of the expected size was purified, digested with HindIII/XbaI and ligated into the integration vector pJM103 (Kapp, Edwards et al., 1990) (containing carbenicillin and chloramphenicol resistance genes as markers) digested with the same restriction enzymes. Ligation products were transformed into MN296 E. coli cells, colonies were selected on 50 ug carbenicillin, plasmid DNA was isolated and screened for the 2.1 kbp xylR-PxylA-comK insert by DNA digest. The plasmid was integrated into B. subtilis. The resulting strain was grown overnight in L-broth medium, diluted to 1 OD600 in L-broth containing 1% xylose and grown 2 hours with shaking to induce comK expression. The resulting process produced a population of cells in which greater than 1% of cells are transformed by bacillus chromosomal DNA containing a marker, indicating that these cells were super competent. Cells were considered supercompetent if greater than 10% of the cells were transformable with a Bacillus self-replicating plasmid. These cells were utilized in the following examples.
The primer sequences used were as follows:
This Example describes an exemplary method to randomly mutagenize a large DNA fragment, containing a gene of interest (e.g. subtilisin gene) with an antibiotic marker and approximately 2 kb of homologous DNA on either side of the subtilisin gene. In this specific example, the mutagenized DNA fragment is 6.8 kb long comprising a left homology box (approx. 2.2 kb), the gene of interest and selectable marker (approx. 2.4 kb), and a right homology box (approx. 2.1 kb). See
Chromosomal DNA of Bacillus was extracted from an overnight culture of cells grown on semi solid nutrient agar plates (LA)+chloramphenicol plates. Usually three colonies from the overnight plate were resuspended into 0.1 ml of SMM medium (0.5M sucrose, 0.02M sodium maleate, 0.02M magnesium chloride-6H2O, pH 6.5) containing lysozyme (100,000 U). The cell suspension was incubated for 30 minutes at 37° C. with shaking. An additional 1 ml of SMM was added to the cells and the suspension microfuged for 1.5 minutes. The supernatant was removed and the step repeated. Finally the cell pellet was resuspended in 10 mM Tris (pH 8.0) and 0.5 mM EDTA, vigorously vortexed for 30 seconds and the sample was frozen at −20° C.
For PCR mutagenesis, a 100 ul PCR reaction was set up using the Z-Taq polymerase kit (TaKaRa Shuzo Co., Ltd.). A typical reaction mixture contained 0.25 uM of both primers, 125 uM of Z-Taq dNTP mixture, 5-10 ng of the chromosomal DNA, 2.5U of Z-Taq polymerase, 1×Z-Taq polymerase buffer. The PCR amplification parameters were: 98° C. for 10 sec (first cycle only) followed by 98° C. for 5 sec, 58° C. for 10 sec 72° C. for 2.5 minutes. The PCR reaction was run for a total of 30 cycles. The primer sequences to amplify the 6.8 Kb fragment were as follows:
After the amplification process, the PCR products were analyzed on an agarose gel. For a typical PCR reaction, the limited amount of dNTP used yielded approximately 15 ug of DNA. The mutagenized DNA was then transformed into Pxyl-comK Bacillus strains to generate a library.
This Example provides an exemplary method for randomly mutagenizing the signal sequence and propeptide of subtilisin.
Primers used in the random mutagenesis reactions were as follows:
To randomly mutagenize the signal sequence and propeptide of subtilisin gene, a PCR reaction using Primers 1 and 2 generated the 2.2 Kb left flanking region. Primers 3 and 4 were used to mutate a 646 bp region comprising of the signal sequence and propeptide region. Primers 5 and 6 were used to generate the 3.9 kb right flanking region. Primers 2 & 3 are complementary to one another, as are primers 4 & 5. See
The amplification parameters for the 2.2 Kb and 3.9 Kb fragments were: 95° C. for 3 min, 95° C. for 30 sec, 54° C. for 30 sec, and 68° C. for 2 min for a total of 30 cycles.
The PCR reaction products were analyzed on an agarose gel. If the correct size fragment was seen then the PCR product was purified using the QIAquick PCR Purification Kit.
The 646 bp fragment for mutagenizing the maturation site was amplified using Primers 3 and 4 (0.5 uM each), 33 ul 3×dNTP, 2 ul of liquid culture grown to OD600=0.5 (source of Bacillus chromosomal DNA), 0-0.3 mM MnCl2 (varies upon the rate of mutagenesis desired), 5.5 mM MgCl2, 5U Taq polymerase, 1×Taq polymerase buffer in a 100 ul reaction. The PCR amplification parameters were as follows: 95° C. for 30 sec, 54° C. for 30 sec, and 68° C. for 30 sec for a total of 30 cycles. The PCR reaction products were analyzed on an agarose gel. If the correct size fragment was seen, the PCR product was purified using the QIAquick PCR Purification Kit.
The assembly of the entire 6.8 kb fragment containing the mutagenized maturation site was done using 3-5 ul each of 646 bp, 2.2 kb, and 3.9 kb fragments, 0.5 uM each of Primers 1 and 6, 300 uM of dNTP, 4U of rTth XL polymerase, 1.25U Pfu of Turbo DNA polymerase, 1×rTth XL polymerase buffer, and 1.1 mM Mg (OAc)2 in a 100 ul reaction. The parameters for the assembly reaction were as follows: 95° C. for 30 sec, 48-50° C. for 30 sec, and 68° C. for 7 min for a total of 30 cycles. The PCR reaction products were analyzed on an agarose gel. If the correct size fragment was seen, the PCR product was transformed into Pxyl-comK Bacillus strains to generate a library. A total of 9,000 transformants were obtained.
This Example provides an exemplary method to increase the transformation efficiency of Bacillus for obtaining larger libraries. Although this example utilizes a plasmid that is amplified in E. coli, one skilled in the art will recognize any method that results in the addition of non-homologous flanks may be used with the present invention. The use of E. coli in the present example was a rapid and simple means for adding non-homologous flanks and should not be construed as limiting.
A 10.3 Kb fragment was generated as shown in
The non-homologous flanks were derived from the TOPO cloning vector and were of E. coli based plasmid origin; therefore, the sequences were not expected to have any significant homology to regions in the Bacillus chromosome.
Transformation efficiency of Pxyl-comK Bacillus competent cells for the two constructs: without (6.8 Kb fragment) and with (10.3 Kb fragment) the non-homologous flanking sequences was compared.
Transformation with 2.2×10−14 moles DNA (approximately 1 ug/ml) of the 6.8 kb DNA fragment (i.e. without the non-homologous flanks) yielded approximately 3.2×104 cfu/ml (0.01% transformation efficiency). Transformation with 2.2×10−14 moles DNA of the 10.3 Kb linearized fragment (i.e. with the non-homologous flanks) yielded approximately 7.2×105 cfu/ml (0.25% transformation efficiency).
As an alternative to using the TOPO cloning kit from Invitrogen, one could also ligate the 6.8 kb PCR product to itself. The multimerized DNA can then be transformed into Pxyl-comK Bacillus strains to generate a library.
This Example provides an exemplary method to evaluate transformation efficiency of Bacillus as a function of varying the size of the homology box and stuffer sequence.
Using primers of varying lengths that contained flanks corresponding to 100, 200, 400, 800, and 1600 bp homology boxes, a series of PCR fragments were generated containing genes coding for a protease, a selectable marker (CAT) and increasing amounts of flanking chromosome sequence. The DNA construct is shown in schematic form in
A typical PCR reaction (100 ul) contained 0.25 uM of Forward and Reverse Primers each, 300 uM of dNTP, 5-10 ng 6.8 Kb DNA fragment generated in Example 4, 2.5U Pfu Turbo DNA polymerase, and 1.5×Pfu Turbo DNA polymerase buffer. The cycling conditions for producing DNA fragments with different sized homology boxes were as follows: 95° C., 30 sec; 52° C., 30 sec, and 68° C. for 3 to 6 minutes for a total of 30 cycles (extension times depended on the expected product length, the rule being 1000 bp/min).
An aliquot of this reaction was saved for the direct transformation into Bacillus, while the rest was cloned into the Zero Blunt TOPO vector following manufacturer directions (Invitrogen). The cloned fragments were transformed into competent E. coli cells and plasmid DNA prepared.
The effect of homology box size on transformation efficiency was measured (
Experimental Discussion
Part of the improvement was due to having a larger length of DNA because the efficiency of transformation jumped over 10-fold when the PCR fragment was cloned into a vector. By cloning into a vector, the integrating DNA is flanked by stuffer sequence; presumably this stuffer sequence reduces the chance that the Bacillus DNA transporter will initiate in sequences between the homology boxes.
This example describes an exemplary method to perform site directed mutagenesis on the gene of interest and directly transform Bacillus strains with the mutagenized DNA.
Site-saturation libraries were created by PCR at 3 different sites in the gene of interest (in this case protease) by using QuikChange (Stratagene)
The primers used were as follows:
Three separate PCR reactions were set up using primer pairs A&B, C&D and E&F. A typical PCR reaction (100 ul) contained 1×Pfu Buffer, 1.5 ul 10 mM dNTPs, 1 ul of 25 uM primer, 1 ul Pfu Turbo DNA polymerase, 200 ng of plasmid DNA. The cycling conditions were: 95° C. for 35 seconds for one cycle; (95° C. for 35 seconds, 50° C. for 1 minute, 68° C. for 16.5 minutes) for 16 cycles, and 68° C. for 7 minutes The expected 7.8 Kb band was identified on the agarose gel (−100 ng/ul). The PCR products were digested with 1 uL DpnI at 37 C for an hour to eliminate the pMEO3 template. The digestion reaction was spiked with another 1 ul of DpnI and digested for another hour. A mock PCR reaction that did not undergo the PCR amplification was also digested to see how well DpnI works to get rid of the template DNA (template control).
A supercompetent Bacillus strain was directly transformed with the digested products. About 200 ng of the library was incubated with 100 ul of OD 600=0.5. The reaction was incubated at 37° C. for 1.5 hours with shaking. Two transformations were set up for each of the conditions, which included the three-mutagenesis reactions (with A&B, C&D, and E&F), a template control, a parent vector control and no DNA condition. Serial dilutions of the cell suspension were streaked on selection plates and following O/N incubation; the transformation efficiency was computed from the number of colonies obtained.
This example provides an exemplary method of mutagenizing the gene of interest with error prone PCR (forms separate PCR products which can be annealed together) and directly transforming the ligated product into Bacillus strain.
Generation of the Vector
The source of the vector DNA was the 800 bp homology box plasmid described in Example 5. Bbs I sites were incorporated into this vector and 20 ug of the plasmid was digested overnight at 37° C. in New England Biolabs Buffer 2 with Bbs I to generate the vector with flanking sites. See
Preparation of Insert
Insert DNA was generated from annealing two overlapping error prone PCR products. See
Error prone PCR products were formed by both sets of primers (P1&P2 [solid line product] and P3&P4 [hatched line product]) using conditions described in Example 3, with cycling conditions 94° C. for 1 min, 50° C. for 1 min, 68° C. for 2 min, for 30 cycles. Negative control was a reaction without MnCl2. PCR products (330 bp) were purified using Qiaquick PCR columns and pure DNA was pooled together.
For annealing of the two products, 1.3 ug of each was combined and heated at 95° C. for 5 min then allowed to cool to room temp in heat block. Only one out of four annealed products were expected to ligate properly with vector correctly.
Ligation
A 1:5 Molar ratio (vector:insert) was used for ligation (DNA ligation kit from PanVera TAK6021) with a total of 440 ng of DNA in the reaction mixture (10 ul of vector+insert DNA+10 ul of Takara Biomedicals Ligase solution). This 1:5 ratio was to the insert of interest (1 out of 4 of the reannealed products) so overall it was actually a 1:20 ratio (vector:annealed PCR product). Appropriate DNA controls were also made. The ligation reaction was incubated for 1 hr at 16° C. Incubating the reaction mixtures at 65° C. for 15 minutes inactivated the ligase. The incompletely digested template was destroyed by incubating the ligation mixture with 1000U of Bbs I in NEB2 buffer at 37° C. for 2 h. This mixture was then used for Bacillus transformation.
Transformation
5 ul (55 ng) of the 440 ng ligation was transformed into 200 ul of Bacillus competent cells. The cell suspension was shaken vigorously 1 hr at 37° C. One hundred ul of serial dilutions of the cell suspension was plated on selection plates. 129,000 CFU/ug ligation mixtures were obtained, useful for combinatorial library construction. Ligation conditions produced large tandem repeats, which facilitated Bacillus transformation. See
This example demonstrates the deletion of the metB gene of Bacillus. A PCR product was generated from sequences that flank the met B gene. This product and a replicating Bacillus plasmid were co-transformed into the competent Bacillus, and cells resistant to the antibiotic marker on the plasmid were selected. These cells were screened for the metB deletion by methionine auxotrophy and absence of metB sequence from a PCR product.
Preparation of Insert:
PCR with 100 f/r (Primers N1 and N2 in
The amplification used 0.5 uM of primer pairs N1/N2 or N3/N4, 300 uM of dNTP, 200 ng Bacillus chromosome DNA, 5U Herculase (Stratagene) and 1× Herculase buffer (Stratagene) in a 50 ul reaction volume.
The amplification parameters were: 94° C. for 3 min, 94° C. for 30 sec, 54° C. for 30 sec, and 68° C. for 7.1 min for a total of 30 cycles. PCR products were purified using the QIAquick PCR Purification Kit.
The assembly of the entire 7.4 kb fragment containing the mutagenized maturation site was done using 100 ng of each PCR fragment, 0.5 uM each of Primers N1 & N4, 300 uM of dNTP, 5U Herculase (Stratagene) and 1× Herculase buffer (Stratagene) in a 100 ul reaction volume. The parameters for the assembly reaction were as follows: 95° C. for 30 sec, 55° C. for 30 sec, and 68° C. for 7 min for a total of 30 cycles. The PCR reaction products were analyzed on an agarose gel.
Transformation
Transform 500 ng of the PCR fusion product along with 50 ng of Bacillus replicating plasmid DNA (provides chloramphenicol resistance) into 100 uL of hypercompetent Bacillus cells and plated on nutrient agar plates containing chloramphenicol (5γ) plates. The resulting colonies were screened for methionine auxotrophy and PCR for deletion of metB gene. This method produced >900 recombinant deletions per microgram of transformation mix (>6% of chloramphenicol resistant colonies).
Various other examples and modifications of the foregoing description and examples will be apparent to a person skilled in the art after reading the disclosure without departing from the spirit and scope of the invention, and it is intended that all such examples or modifications be included within the scope of the appended claims. All publications and patents referenced herein are hereby incorporated by reference in their entirety.
| Number | Date | Country | |
|---|---|---|---|
| 60224948 | Aug 2000 | US |
| Number | Date | Country | |
|---|---|---|---|
| Parent | 09927161 | Aug 2001 | US |
| Child | 11215675 | Aug 2005 | US |
