Rationally-designed mutations to the thrA gene for enhanced lysine production in E. coli

Abstract

The present disclosure relates to various different types of mutations in the thrA gene in E. coli leading to enhanced lysine production for, e.g., supplements and nutraceuticals.

Description

FIELD OF THE INVENTION

The present disclosure relates to mutations in the thrA gene in E. coli leading to enhanced lysine production.

BACKGROUND OF THE INVENTION

In the following discussion certain articles and methods will be described for background and introductory purposes. Nothing contained herein is to be construed as an “admission” of prior art. Applicant expressly reserves the right to demonstrate, where appropriate, that the articles and methods referenced herein do not constitute prior art under the applicable statutory provisions.

The amino acid lysine is an α-amino acid that is used in the biosynthesis of proteins and is a metabolite of E. coli, S. cerevisiae, plants, humans and other mammals, as well as algae. Lysine contains an α-amino group, an α-carboxylic acid group, and has a chemical formula of C6H14N202 One of nine essential amino acids in humans, lysine is required for growth and tissue repair and has a role as a micronutrient, a nutraceutical, an agricultural feed supplement, an anticonvulsant, as well as a precursor for the production of peptides. Because of these roles as, e.g., a supplement and nutraceutical, there has been a growing effort to produce lysine on a large scale.

Accordingly, there is a need in the art for organisms that produce enhanced amounts of lysine where such organisms can be harnessed for large scale lysine production. The disclosed nucleic acid sequences from E. coli satisfy this need.

SUMMARY OF THE INVENTION

This Summary is provided to introduce a selection of concepts in a simplified form that are further described below in the Detailed Description. This Summary is not intended to identify key or essential features of the claimed subject matter, nor is it intended to be used to limit the scope of the claimed subject matter. Other features, details, utilities, and advantages of the claimed subject matter will be apparent from the following written Detailed Description including those aspects illustrated in the accompanying drawings and defined in the appended claims.

The present disclosure provides E. coli variants having rationally-designed mutations in the thrA gene that produce enhanced amounts of lysine in culture. Thus, in some embodiments, the present disclosure provides a thrA variant comprising a variant amino acid sequence comprising any one of SEQ ID NOs. 1-40, or any combinations of variant amino acid sequences SEQ ID NOs: 1-40; i.e., variant sequences A660R, A720W, A840T, A808L, A080M, D648W, D763H, E649D, 5695G, E702R, F709W, G472N, G650F, G679P, G650P, G610H, G610K, G634I, G634K, I616P, I685H, I685K, K681F, K682G, K682I, K682Q, K682R, K772P, L631D, L631G, L768F, L811R, P467H, Q487D, Q487D, R661V, R668S, T8021, V468A and V468Y; or any combination of other variant sequences with any one or more of variant sequences A660R, A720W, A840T, A808L, A080M, D648W, D763H, E649D, 5695G, E702R, F709W, G472N, G650F, G679P, G650P, G610H, G610K, G634I, G634K, I616P, I685H, I685K, K681F, K682G, K682I, K682Q, K682R, K772P, L631D, L631G, L768F, L811R, P467H, Q487D, Q487D, R661V, R668S, T8021, V468A and V468Y.

These aspects and other features and advantages of the invention are described below in more detail.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other features and advantages of the present invention will be more fully understood from the following detailed description of illustrative embodiments taken in conjunction with the accompanying drawings in which:

FIG. 1A is an exemplary engine vector for creating edits in E. coli. FIG. 1B is an exemplary editing vector for creating edits in E. coli.

It should be understood that the drawings are not necessarily to scale, and that like reference numbers refer to like features.

DETAILED DESCRIPTION

All of the functionalities described in connection with one embodiment of the methods, devices or instruments described herein are intended to be applicable to the additional embodiments of the methods, devices and instruments described herein except where expressly stated or where the feature or function is incompatible with the additional embodiments. For example, where a given feature or function is expressly described in connection with one embodiment but not expressly mentioned in connection with an alternative embodiment, it should be understood that the feature or function may be deployed, utilized, or implemented in connection with the alternative embodiment unless the feature or function is incompatible with the alternative embodiment.

The practice of the techniques described herein may employ, unless otherwise indicated, conventional techniques and descriptions molecular biology (including recombinant techniques), cell biology, biochemistry, and genetic engineering technology, which are within the skill of those who practice in the art. Such conventional techniques and descriptions can be found in standard laboratory manuals such as Green and Sambrook, Molecular Cloning: A Laboratory Manual. 4th, ed., Cold Spring Harbor Laboratory Press, Cold Spring Harbor, N.Y., (2014); Current Protocols in Molecular Biology, Ausubel, et al. eds., (2017); Neumann, et al., Electroporation and Electrofusion in Cell Biology, Plenum Press, New York, 1989; and Chang, et al., Guide to Electroporation and Electrofusion, Academic Press, California (1992), all of which are herein incorporated in their entirety by reference for all purposes.

Note that as used herein and in the appended claims, the singular forms “a,” “an,” and “the” include plural referents unless the context clearly dictates otherwise. Thus, for example, reference to “a cell” refers to one or more cells, and reference to “the system” includes reference to equivalent steps, methods and devices known to those skilled in the art, and so forth.

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 belongs. All publications mentioned herein are incorporated by reference for the purpose of describing and disclosing devices, formulations and methodologies that may be used in connection with the presently described invention.

Where a range of values is provided, it is understood that each intervening value, between the upper and lower limit of that range and any other stated or intervening value in that stated range is encompassed within the invention. The upper and lower limits of these smaller ranges may independently be included in smaller ranges, and are also encompassed within the invention, subject to any specifically excluded limit in the stated range. Where the stated range includes one or both of the limits, ranges excluding either or both of those included limits are also included in the invention.

In the following description, numerous specific details are set forth to provide a more thorough understanding of the present invention. However, it will be apparent to one of skill in the art that the present invention may be practiced without one or more of these specific details. In other instances, features and procedures well known to those skilled in the art have not been described in order to avoid obscuring the invention. The terms used herein are intended to have the plain and ordinary meaning as understood by those of ordinary skill in the art.

The term DNA “control sequences” refers collectively to promoter sequences, polyadenylation signals, transcription termination sequences, upstream regulatory domains, origins of replication, internal ribosome entry sites, nuclear localization sequences, enhancers, and the like, which collectively provide for the replication, transcription and translation of a coding sequence in a recipient cell. Not all of these types of control sequences need to be present so long as a selected coding sequence is capable of being replicated, transcribed and—for some components—translated in an appropriate host cell.

The term “CREATE cassette” or “editing cassette” refers to a gRNA linked to a donor DNA or HA. Methods and compositions for designing and synthesizing CREATE editing cassettes are described in U.S. Pat. Nos. 10,240,167; 10,266,849; 9,982,278; 10,351,877; 10,364,442; 10,435,715; and 10,465,207; and U.S. Ser. No. 16/550,092, filed 23 Aug. 2019; U.S. Ser. No. 16/551,517, filed 26 Aug. 2019; U.S. Ser. No. 16/773,618, filed 27 Jan. 2020; and U.S. Ser. No. 16/773,712, filed 27 Jan. 2020, all of which are incorporated by reference herein in their entirety.

As used herein the term “donor DNA” or “donor nucleic acid” refers to nucleic acid that is designed to introduce a DNA sequence modification (insertion, deletion, substitution) into a locus (e.g., a target genomic DNA sequence or cellular target sequence) by homologous recombination using nucleic acid-guided nucleases. For homology-directed repair, the donor DNA must have sufficient homology to the regions flanking the “cut site” or site to be edited in the genomic target sequence. The length of the homology arm(s) will depend on, e.g., the type and size of the modification being made. In many instances and preferably, the donor DNA will have two regions of sequence homology (e.g., two homology arms) to the genomic target locus. Preferably, an “insert” region or “DNA sequence modification” region—the nucleic acid modification that one desires to be introduced into a genome target locus in a cell—will be located between two regions of homology. The DNA sequence modification may change one or more bases of the target genomic DNA sequence at one specific site or multiple specific sites. A change may include changing 1, 2, 3, 4, 5, 10, 15, 20, 25, 30, 35, 40, 50, 75, 100, 150, 200, 300, 400, or 500 or more base pairs of the genomic target sequence. A deletion or insertion may be a deletion or insertion of 1, 2, 3, 4, 5, 10, 15, 20, 25, 30, 40, 50, 75, 100, 150, 200, 300, 400, or 500 or more base pairs of the genomic target sequence.

The terms “guide nucleic acid” or “guide RNA” or “gRNA” refer to a polynucleotide comprising 1) a guide sequence capable of hybridizing to a genomic target locus, and 2) a scaffold sequence capable of interacting or complexing with a nucleic acid-guided nuclease.

“Homology” or “identity” or “similarity” refers to sequence similarity between two peptides or, more often in the context of the present disclosure, between two nucleic acid molecules. The term “homologous region” or “homology arm” refers to a region on the donor DNA with a certain degree of homology with the target genomic DNA sequence. Homology can be determined by comparing a position in each sequence which may be aligned for purposes of comparison. When a position in the compared sequence is occupied by the same base or amino acid, then the molecules are homologous at that position. A degree of homology between sequences is a function of the number of matching or homologous positions shared by the sequences.

“Operably linked” refers to an arrangement of elements where the components so described are configured so as to perform their usual function. Thus, control sequences operably linked to a coding sequence are capable of effecting the transcription, and in some cases, the translation, of a coding sequence. The control sequences need not be contiguous with the coding sequence so long as they function to direct the expression of the coding sequence. Thus, for example, intervening untranslated yet transcribed sequences can be present between a promoter sequence and the coding sequence and the promoter sequence can still be considered “operably linked” to the coding sequence. In fact, such sequences need not reside on the same contiguous DNA molecule (i.e. chromosome) and may still have interactions resulting in altered regulation.

As used herein, the terms “protein” and “polypeptide” are used interchangeably. Proteins may or may not be made up entirely of amino acids.

A “promoter” or “promoter sequence” is a DNA regulatory region capable of binding RNA polymerase and initiating transcription of a polynucleotide or polypeptide coding sequence such as messenger RNA, ribosomal RNA, small nuclear or nucleolar RNA, guide RNA, or any kind of RNA transcribed by any class of any RNA polymerase I, II or III. Promoters may be constitutive or inducible, and in some embodiments the transcription of at least one component of the nucleic acid-guided nuclease editing system is—and often at least three components of the nucleic acid-guided nuclease editing system are—under the control of an inducible promoter. A number of gene regulation control systems have been developed for the controlled expression of genes in plant, microbe, and animal cells, including mammalian cells, including the pL promoter (induced by heat inactivation of the CI857 repressor), the pPhIF promoter (induced by the addition of 2,4 diacetylphloroglucinol (DAPG)), the pBAD promoter (induced by the addition of arabinose to the cell growth medium), and the rhamnose inducible promoter (induced by the addition of rhamnose to the cell growth medium). Other systems include the tetracycline-controlled transcriptional activation system (Tet-On/Tet-Off, Clontech, Inc. (Palo Alto, Calif.); Bujard and Gossen, PNAS, 89(12):5547-5551 (1992)), the Lac Switch Inducible system (Wyborski et al., Environ Mol Mutagen, 28(4):447-58 (1996); DuCoeur et al., Strategies 5(3):70-72 (1992); U.S. Pat. No. 4,833,080), the ecdysone-inducible gene expression system (No et al., PNAS, 93(8):3346-3351 (1996)), the cumate gene-switch system (Mullick et al., BMC Biotechnology, 6:43 (2006)), and the tamoxifen-inducible gene expression (Zhang et al., Nucleic Acids Research, 24:543-548 (1996)) as well as others.

As used herein the term “selectable marker” refers to a gene introduced into a cell, which confers a trait suitable for artificial selection. General use selectable markers are well-known to those of ordinary skill in the art. Drug selectable markers such as ampicillin/carbenicillin, kanamycin, nourseothricin N-acetyl transferase, chloramphenicol, erythromycin, tetracycline, gentamicin, bleomycin, streptomycin, rifampicin, puromycin, hygromycin, blasticidin, and G418 may be employed. In other embodiments, selectable markers include, but are not limited to sugars such as rhamnose. “Selective medium” as used herein refers to cell growth medium to which has been added a chemical compound or biological moiety that selects for or against selectable markers.

The term “specifically binds” as used herein includes an interaction between two molecules, e.g., an engineered peptide antigen and a binding target, with a binding affinity represented by a dissociation constant of about 10⁻⁷ M, about 10⁻⁸ M, about 10⁻⁹ M, about 10⁻¹⁰ M, about 10⁻¹¹M, about 10⁻¹²M, about 10⁻¹³M, about 10⁻¹⁴M or about 10⁻¹⁵ M.

The terms “target genomic DNA sequence”, “cellular target sequence”, or “genomic target locus” refer to any locus in vitro or in vivo, or in a nucleic acid (e.g., genome) of a cell or population of cells, in which a change of at least one nucleotide is desired using a nucleic acid-guided nuclease editing system. The cellular target sequence can be a genomic locus or extrachromosomal locus.

The term “variant” may refer to a polypeptide or polynucleotide that differs from a reference polypeptide or polynucleotide but retains essential properties. A typical variant of a polypeptide differs in amino acid sequence from another reference polypeptide. Generally, differences are limited so that the sequences of the reference polypeptide and the variant are closely similar overall and, in many regions, identical. A variant and reference polypeptide may differ in amino acid sequence by one or more modifications (e.g., substitutions, additions, and/or deletions). A variant of a polypeptide may be a conservatively modified variant. A substituted or inserted amino acid residue may or may not be one encoded by the genetic code (e.g., a non-natural amino acid). A variant of a polypeptide may be naturally occurring, such as an allelic variant, or it may be a variant that is not known to occur naturally.

A “vector” is any of a variety of nucleic acids that comprise a desired sequence or sequences to be delivered to and/or expressed in a cell. Vectors are typically composed of DNA, although RNA vectors are also available. Vectors include, but are not limited to, plasmids, fosmids, phagemids, virus genomes, synthetic chromosomes, and the like. As used herein, the phrase “engine vector” comprises a coding sequence for a nuclease to be used in the nucleic acid-guided nuclease systems and methods of the present disclosure. The engine vector also comprises in E. coli, the λ Red recombineering system or an equivalent thereto which repairs the double-stranded breaks resulting from the cut by the nuclease. Engine vectors also typically comprise a selectable marker. As used herein the phrase “editing vector” comprises a donor nucleic acid, optionally including an alteration to the cellular target sequence that prevents nuclease binding at a PAM or spacer in the cellular target sequence after editing has taken place, and a coding sequence for a gRNA. The editing vector may also and preferably does comprise a selectable marker and/or a barcode. In some embodiments, the engine vector and editing vector may be combined; that is, all editing and selection components may be found on a single vector. Further, the engine and editing vectors comprise control sequences operably linked to, e.g., the nuclease coding sequence, recombineering system coding sequences (if present), donor nucleic acid, guide nucleic acid(s), and selectable marker(s).

Library Design Strategy and Nuclease-Directed Genome Editing

Lysine is naturally synthesized in E. coli along the diaminopimelate (DAP) biosynthetic pathway. The thrA gene in E. coli is not directly involved in the biosynthetic pathway; instead, the thrA has two enzymatic activities required for the biosynthesis of threonine, aspartokinase I and homoserine dehydrogenase I. The thrA gene was cloned in a bacterial plasmid and its complete nucleotide sequence was established. It contains 2460 base pairs that encode for a polypeptide chain of 820 amino acids. The gene contains an internal sequence that resembles the structure of bacterial ribosome-binding sites, with an AUG preceded by four triplets, each of which can be converted to a nonsense codon by a single mutation, suggesting that the single polypeptide chain was formed by the fusion of two genes and that initiation of translation may occur inside the gene to give a protein fragment having only the homoserine dehydrogenase activity.

Strain engineering strategies for increasing lysine production in E. coli and other industrially-relevant production hosts such as Corynebacterium glutamicum have historically focused on the genes in the DAP pathway as obvious targets for mutagenesis and over-expression. Beyond this short list of genes encoding the lysine biosynthetic enzymes, it is likely that additional loci throughout the E. coli genome may also contribute appreciably (if less directly) to improved lysine yields in an industrial production setting. For this reason, targeted mutagenesis strategies which enable a broader query of the entire genome are also of significant value to the lysine metabolic engineer.

The variants presented in this disclosure are the result of nucleic acid-guided nuclease editing of 370+ loci in the thrA gene in the MG1655 strain of E. coli harboring an engine plasmid such as that shown in FIG. 1A (such transformed MG1655 strain is referred to herein as E. coli strain EC83). The mutations included amino acid swaps, premature terminations via stop codons and mutations to the thrA promoter. The engine plasmid comprises a coding sequence for the MAD7 nuclease under the control of the inducible pL promoter, the λ Red operon recombineering system under the control of the inducible pBAD promoter (inducible by the addition of arabinose in the cell growth medium), the c1857 gene under the control of a constitutive promoter, as well as a selection marker and an origin of replication. As described above, the λ Red recombineering system repairs the double-stranded breaks resulting from the cut by the MAD7 nuclease. The c1857 gene at 30° C. actively represses the pL promoter (which drives the expression of the MAD7 nuclease and the editing or CREATE cassette on the editing cassette such as the exemplary editing vector shown in FIG. 1B); however, at 42° C., the c1857 repressor gene unfolds or degrades, and in this state the c1857 repressor protein can no longer repress the pL promoter leading to active transcription of the coding sequence for the MAD7 nuclease and the editing (e.g., CREATE) cassette.

FIG. 1B depicts an exemplary editing plasmid comprising the editing (e.g., CREATE) cassette (crRNA, spacer and HA) driven by a pL promoter, a selection marker, and an origin of replication.

The 370+ nucleic acid mutations or edits made were generated using MAD7, along with a gRNA and donor DNA. A nucleic acid-guided nuclease such as MAD7 is complexed with an appropriate synthetic guide nucleic acid in a cell and can cut the genome of the cell at a desired location. The guide nucleic acid helps the nucleic acid-guided nuclease recognize and cut the DNA at a specific target sequence. By manipulating the nucleotide sequence of the guide nucleic acid, the nucleic acid-guided nuclease may be programmed to target any DNA sequence for cleavage as long as an appropriate protospacer adjacent motif (PAM) is nearby. In certain aspects, the nucleic acid-guided nuclease editing system may use two separate guide nucleic acid molecules that combine to function as a guide nucleic acid, e.g., a CRISPR RNA (crRNA) and trans-activating CRISPR RNA (tracrRNA). In other aspects, the guide nucleic acid may be a single guide nucleic acid that includes both the crRNA and tracrRNA sequences.

A guide nucleic acid comprises a guide sequence, where the guide sequence is a polynucleotide sequence having sufficient complementarity with a target sequence to hybridize with the target sequence and direct sequence-specific binding of a complexed nucleic acid-guided nuclease to the target sequence. The degree of complementarity between a guide sequence and the corresponding target sequence, when optimally aligned using a suitable alignment algorithm, is about or more than about 50%, 60%, 75%, 80%, 85%, 90%, 95%, 97.5%, 99%, or more. Optimal alignment may be determined with the use of any suitable algorithm for aligning sequences. In some embodiments, a guide sequence is about or more than about 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 35, 40, 45, 50, 75, or more nucleotides in length. In some embodiments, a guide sequence is less than about 75, 50, 45, 40, 35, 30, 25, 20 nucleotides in length. Preferably the guide sequence is 10-30 or 15-20 nucleotides long, or 15, 16, 17, 18, 19, or 20 nucleotides in length.

In the methods to generate the 370+ thrA mutations, the guide nucleic acids were provided as a sequence to be expressed from a plasmid or vector comprising both the guide sequence and the scaffold sequence as a single transcript under the control of an inducible promoter. The guide nucleic acids are engineered to target a desired target sequence by altering the guide sequence so that the guide sequence is complementary to a desired target sequence, thereby allowing hybridization between the guide sequence and the target sequence. In general, to generate an edit in the target sequence, the gRNA/nuclease complex binds to a target sequence as determined by the guide RNA, and the nuclease recognizes a protospacer adjacent motif (PAM) sequence adjacent to the target sequence. The target sequences for the genome-wide mutagenesis here encompassed 370+ loci throughout the thrA gene in the E. coli genome.

The guide nucleic acid may be and in the processes generating the variants reported herein were part of an editing cassette that also encoded the donor nucleic acid. The target sequences are associated with a proto-spacer mutation (PAM), which is a short nucleotide sequence recognized by the gRNA/nuclease complex. The precise preferred PAM sequence and length requirements for different nucleic acid-guided nucleases vary; however, PAMs typically are 2-7 base-pair sequences adjacent or in proximity to the target sequence and, depending on the nuclease, can be 5′ or 3′ to the target sequence.

In certain embodiments, the genome editing of a cellular target sequence both introduces the desired DNA change to the cellular target sequence and removes, mutates, or renders inactive a proto-spacer mutation (PAM) region in the cellular target sequence. Rendering the PAM at the cellular target sequence inactive precludes additional editing of the cell genome at that cellular target sequence, e.g., upon subsequent exposure to a nucleic acid-guided nuclease complexed with a synthetic guide nucleic acid in later rounds of editing. Thus, cells having the desired cellular target sequence edit and an altered PAM can be selected for by using a nucleic acid-guided nuclease complexed with a synthetic guide nucleic acid complementary to the cellular target sequence. Cells that did not undergo the first editing event will be cut rendering a double-stranded DNA break, and thus will not continue to be viable. The cells containing the desired cellular target sequence edit and PAM alteration will not be cut, as these edited cells no longer contain the necessary PAM site and will continue to grow and propagate.

As for the nuclease component of the nucleic acid-guided nuclease editing system, a polynucleotide sequence encoding the nucleic acid-guided nuclease can be codon optimized for expression in particular cell types, such as archaeal, prokaryotic or eukaryotic cells. The choice of nucleic acid-guided nuclease to be employed depends on many factors, such as what type of edit is to be made in the target sequence and whether an appropriate PAM is located close to the desired target sequence. Nucleases of use in the methods described herein include but are not limited to Cas 9, Cas 12/CpfI, MAD2, or MAD7 or other MADzymes. As with the guide nucleic acid, the nuclease is encoded by a DNA sequence on a vector (e.g., the engine vector—see FIG. 1A) and be under the control of an inducible promoter. In some embodiments—such as in the methods described herein—the inducible promoter may be separate from but the same as the inducible promoter controlling transcription of the guide nucleic acid; that is, a separate inducible promoter drives the transcription of the nuclease and guide nucleic acid sequences but the two inducible promoters may be the same type of inducible promoter (e.g., both are pL promoters). Alternatively, the inducible promoter controlling expression of the nuclease may be different from the inducible promoter controlling transcription of the guide nucleic acid; that is, e.g., the nuclease may be under the control of the pBAD inducible promoter, and the guide nucleic acid may be under the control of the pL inducible promoter.

Another component of the nucleic acid-guided nuclease system is the donor nucleic acid comprising homology to the cellular target sequence. In some embodiments, the donor nucleic acid is on the same polynucleotide (e.g., editing vector or editing cassette) as the guide nucleic acid. The donor nucleic acid is designed to serve as a template for homologous recombination with a cellular target sequence nicked or cleaved by the nucleic acid-guided nuclease as a part of the gRNA/nuclease complex. A donor nucleic acid polynucleotide may be of any suitable length, such as about or more than about 20, 25, 50, 75, 100, 150, 200, 500, or 1000 nucleotides in length. In certain preferred aspects, the donor nucleic acid can be provided as an oligonucleotide of between 20-300 nucleotides, more preferably between 50-250 nucleotides. The donor nucleic acid comprises a region that is complementary to a portion of the cellular target sequence (e.g., a homology arm). When optimally aligned, the donor nucleic acid overlaps with (is complementary to) the cellular target sequence by, e.g., about 20, 25, 30, 35, 40, 50, 60, 70, 80, 90 or more nucleotides. In many embodiments, the donor nucleic acid comprises two homology arms (regions complementary to the cellular target sequence) flanking the mutation or difference between the donor nucleic acid and the cellular target sequence. The donor nucleic acid comprises at least one mutation or alteration compared to the cellular target sequence, such as an insertion, deletion, modification, or any combination thereof compared to the cellular target sequence. Various types of edits were introduced herein, including site-directed mutagenesis, saturation mutagenesis, promoter swaps and ladders, knock-in and knock-out edits, SNP or short tandem repeat swaps, and start/stop codon exchanges.

In addition to the donor nucleic acid, an editing cassette may comprise one or more primer sites. The primer sites can be used to amplify the editing cassette by using oligonucleotide primers; for example, if the primer sites flank one or more of the other components of the editing cassette. In addition, the editing cassette may comprise a barcode. A barcode is a unique DNA sequence that corresponds to the donor DNA sequence such that the barcode can identify the edit made to the corresponding cellular target sequence. The barcode typically comprises four or more nucleotides. In some embodiments, the editing cassettes comprise a collection or library gRNAs and of donor nucleic acids representing, e.g., gene-wide or genome-wide libraries of gRNAs and donor nucleic acids. The library of editing cassettes is cloned into vector backbones where, e.g., each different donor nucleic acid is associated with a different barcode.

Variants of interest from the 370+ variants created and assayed with at least 900-fold improvement over wildtype (MG1655) in minimal media are those listed in Table 1 below. The thrA gene has an NCBI Gene ID of 945803 and the edit is in relation to the amino acid residue of the amino acid sequence.

TABLE 1
Variants
		Phenotype, FOWT	Phenotype, FIOPC
		(MG1655,	(dapA E84T,
SEQ ID No.	Edit	min medium)	min medium
SEQ ID No. 1	A660R	932.018	1.864
SEQ ID No. 2	A720W	2039.723	4.079
SEQ ID No. 3	A804T	1148.346	2.297
SEQ ID No. 4	A808L	3034.660	6.069
SEQ ID No. 5	A080M	1490.789	2.982
SEQ ID No. 6	D648W	2464.749	4.929
SEQ ID No. 7	D763H	2862.677	5.725
SEQ ID No. 8	E649D	1853.637	3.707
SEQ ID No. 9	E695G	1583.835	3.168
SEQ ID No. 10	E702R	2186.557	4.373
SEQ ID No. 11	F709W	1658.502	3.317
SEQ ID No. 12	G472N	2233.949	4.468
SEQ ID No. 13	G650F	1161.607	2.323
SEQ ID No. 14	G679P	1599.642	3.199
SEQ ID No. 15	G650P	2197.597	4.395
SEQ ID No. 16	G610H	2958.276	5.917
SEQ ID No. 17	G610K	4823.148	9.646
SEQ ID No. 18	G634I	1818.493	3.637
SEQ ID No. 19	G634K	2313.266	4.627
SEQ ID No. 20	I616P	1768.396	3.537
SEQ ID No. 21	I685H	1385.364	2.771
SEQ ID No. 22	I685K	2522.543	5.045
SEQ ID No. 23	K681F	1368.820	2.738
SEQ ID No. 24	K682G	2188.667	4.377
SEQ ID No. 25	K682I	2258.957	4.518
SEQ ID No. 26	K682Q	2497.239	4.994
SEQ ID No. 27	K682R	3976.188	7.952
SEQ ID No. 28	K772P	1455.380	2.911
SEQ ID No. 29	L631D	2457.977	4.916
SEQ ID No. 30	L631G	1053.649	2.107
SEQ ID No. 31	L768F	2534.033	5.068
SEQ ID No. 32	L811R	1059.535	2.119
SEQ ID No. 33	P467H	1491.993	2.984
SEQ ID No. 34	Q487D	2078.026	4.156
SEQ ID No. 35	Q487F	2085.004	4.170
SEQ ID No. 36	R661V	1399.696	2.799
SEQ ID No. 37	R668S	1118.189	2.236
SEQ ID No. 38	T802I	2069.932	4.140
SEQ ID No. 39	V468A	1192.620	2.385
SEQ ID No. 40	V468Y	1481.573	2.963
In Table 1, “FOWT” is fold over wild type in minimal medium (FOWT MC1655 (minimal medium), “FIOPC” is fold improved over positive control (MG1655, dapA E84T).

EXAMPLES

Mutagenesis libraries specifically targeting the thrA gene. All thrA variants were screened at shallow sampling for lysine production via mass spec as described below.

Example 1: Editing Cassette and Backbone Amplification and Assembly

Editing Cassette Preparation: 5 nM oligonucleotides synthesized on a chip were amplified using Q5 polymerase in 50 μL volumes. The PCR conditions were 95° C. for 1 minute; 8 rounds of 95° C. for 30 seconds/60° C. for 30 seconds/72° C. for 2.5 minutes; with a final hold at 72° C. for 5 minutes. Following amplification, the PCR products were subjected to SPRI cleanup, where 30 μL SPRI mix was added to the 50 μL PCR reactions and incubated for 2 minutes. The tubes were subjected to a magnetic field for 2 minutes, the liquid was removed, and the beads were washed 2× with 80% ethanol, allowing 1 minute between washes. After the final wash, the beads were allowed to dry for 2 minutes, 50 μL 0.5× TE pH 8.0 was added to the tubes, and the beads were vortexed to mix. The slurry was incubated at room temperature for 2 minutes, then subjected to the magnetic field for 2 minutes. The eluate was removed and the DNA quantified.

Following quantification, a second amplification procedure was carried out using a dilution of the eluate from the SPRI cleanup. PCR was performed under the following conditions: 95° C. for 1 minute; 18 rounds of 95° C. for 30 seconds/72° C. for 2.5 minutes; with a final hold at 72° C. for 5 minutes. Amplicons were checked on a 2% agarose gel and pools with the cleanest output(s) were identified. Amplification products appearing to have heterodimers or chimeras were not used.

Backbone Preparation: A 10-fold serial dilution series of purified backbone was performed, and each of the diluted backbone series was amplified under the following conditions: 95° C. for 1 minute; then 30 rounds of 95° C. for 30 seconds/60° C. for 1.5 minutes/72° C. for 2.5 minutes; with a final hold at 72° C. for 5 minutes. After amplification, the amplified backbone was subjected to SPRI cleanup as described above in relation to the cassettes. The backbone was eluted into 100 μL ddH₂O and quantified before nucleic acid assembly.

Isothermal Nucleic Acid Assembly: 150 ng backbone DNA was combined with 100 ng cassette DNA. An equal volume of 2× Gibson Master Mix was added, and the reaction was incubated for 45 minutes at 50° C. After assembly, the assembled backbone and cassettes were subjected to SPRI cleanup, as described above.

Example 2: Transformation of Editing Vector Library into E Cloni®

Transformation: 20 μL of the prepared editing vector Gibson Assembly reaction was added to 30 μL chilled water along with 10 μL E Cloni® (Lucigen, Middleton, Wis.) supreme competent cells. An aliquot of the transformed cells were spot plated to check the transformation efficiency, where >100× coverage was required to continue. The transformed E Cloni® cells were outgrown in 25 mL SOB+100 μg/mL carbenicillin (carb). Glycerol stocks were generated from the saturated culture by adding 500 μL 50% glycerol to 1000 μL saturated overnight culture. The stocks were frozen at −80° C. This step is optional, providing a ready stock of the cloned editing library. Alternatively, Gibson or another assembly of the editing cassettes and the vector backbone can be performed before each editing experiment.

Example 3: Creation of New Cell Line Transformed With Engine Vector

Transformation: 1 μL of the engine vector DNA (comprising a coding sequence for MAD7 nuclease under the control of the pL inducible promoter, a chloramphenicol resistance gene, and the λ Red recombineering system) was added to 50 μL EC83 strain E. coli cells. The transformed cells were plated on LB plates with 25 μg/mL chloramphenicol (chlor) and incubated overnight to accumulate clonal isolates. The next day, a colony was picked, grown overnight in LB+25 μg/mL chlor, and glycerol stocks were prepared from the saturated overnight culture by adding 500 μL 50% glycerol to 1000 μL culture. The stocks of EC1 comprising the engine vector were frozen at −80° C.

Example 4: Preparation of Competent Cells

A 1 mL aliquot of a freshly-grown overnight culture of EC83 cells transformed with the engine vector was added to a 250 mL flask containing 100 mL LB/SOB+25 μg/mL chlor medium. The cells were grown to 0.4-0.7 OD, and cell growth was halted by transferring the culture to ice for 10 minutes. The cells were pelleted at 8000×g in a JA-18 rotor for 5 minutes, washed 3× with 50 mL ice cold ddH₂O or 10% glycerol, and pelleted at 8000× g in JA-18 rotor for 5 minutes. The washed cells were resuspended in 5 mL ice cold 10% glycerol and aliquoted into 200 μL portions. Optionally at this point the glycerol stocks could be stored at −80° C. for later use.

Example 5: Screening of Edited Libraries for Lysine Production

Library stocks were diluted and plated onto 245×245 mm LB agar plates (Teknova) containing 100 μg/mL carbenicillin (Teknova) and 25 μg/mL chloramphenicol (Teknova) using sterile glass beads. Libraries were diluted an appropriate amount to yield ˜2000-3000 colonies on the plates. Plates were incubated ˜16 h at 30° C. and then stored at 4° C. until use. Colonies were picked using a QPix™ 420 (Molecular Devices) and deposited into sterile 1.2 mL square 96-well plates (Thomas Scientific) containing 300 μL of overnight growth medium (EZ Rich Defined Medium, w/o lysine (Teknova), 100 μg/mL carbenicillin and 25 μg/mL chloramphenicol). Plates were sealed (AirPore sheets (Qiagen)) and incubated for ˜19 h in a shaker incubator (Climo-Shaker ISF1-X (Kuhner), 30° C., 85% humidity, 250 rpm). Plate cultures were then diluted 20-fold (15 μL culture into 285 μL medium) into new 96-well plates containing lysine production medium (20 g/L ammonium sulfate (Teknova), 200 mM MOPS buffer (Teknova), 3 mg/L Iron(II) sulfate heptahydrate (Sigma), 3 mg/L Manganese (II) sulfate monohydrate (Sigma), 0.5 mg/L Biotin (Sigma), 1 mg/L Thiamine hydrochloride (Sigma), 0.7 g/L Potassium chloride (Teknova), 20 g/L glucose (Teknova), 5 g/L Potassium phosphate monobasic (Sigma), 1 mL/L Trace metal mixture (Teknova), 1 mM Magnesium sulfate (Teknova), 100 μg/mL carbenicillin and 25 μg/mL chloramphenicol). Production plates were incubated for 24 h in a shaker incubator (Climo-Shaker ISF1-X (Kuhner), 30° C., 85% humidity, 250 rpm).

Production plates were centrifuged (Centrifuge 5920R, Eppendorf) at 3,000 g for 10 min to pellet cells. The supernatants from production plates were diluted 100-fold into water (5 μL of supernatant with 495 μL) of water in 1.2 mL square 96-well plates. Samples were thoroughly mixed and then diluted a subsequent 10-fold further into a 50:50 mixture of acetonitrile and water (20 μL sample with 180 μL of the acetonitrile/water mixture) into a 96-well Plate (polypropylene, 335 μL/well, Conical Bottom (Thomas Scientific). Plates were heat sealed and thoroughly mixed.

Lysine concentrations were determined using a RapidFire high-throughput mass spectrometry system (Agilent) coupled to a 6470 Triple Quad mass spectrometer (Agilent). The RapidFire conditions were as follows: Pump 1: 80% acetonitrile (LC/MS grade, Fisher), 20% water (LC/MS grade, Fisher), 1.5 mL/min, Pump 2: 100% water, 1.25 mL/min, Pump 3: 5% acetonitrile, 95% water, 1.25 mL/min. RapidFire method: Aspirate: 600 ms, Load/wash: 2000 ms, Extra wash: 0 ms, Elute: 3000 ms, Re-equilibrate: 500 ms. 10 μL injection loop.

Example 6: Mass Spectrometry Conditions for Lysine Detection

Precursor ion: 147.1 m/z, Product ion (quantifying): 84 m/z, Dwell: 20, Fragmentor: 80, Collision energy: 20, Cell accelerator voltage: 4, Polarity: positive Precursor ion: 147.1 m/z, Product ion (qualifying): 130 m/z, Dwell: 20, Fragmentor: 80, Collision energy: 8, Cell accelerator voltage: 4, Polarity: positive Source conditions: Gas Temp: 300° C., Gas Flow: 10 L/min, Nebulizer: 45 psi, Sheath gas temp: 350° C., Sheath gas flow: 11 L/min, Capillary voltage: 3000V (positive), Nozzle voltage: 1500V (positive)

Data was analyzed using MassHunter Quantitative Analysis software (Agilent) with a standard curve of lysine used for quantitation of lysine in the samples. Each 96-well plate of samples contained 4 replicates of the wildtype strain and 4 replicates of the dapA E84T positive control strain to calculate the relative lysine yield of samples compared to the controls. Hits from the primary screen were re-tested in quadruplicate using a similar protocol as described above.

While this invention is satisfied by embodiments in many different forms, as described in detail in connection with preferred embodiments of the invention, it is understood that the present disclosure is to be considered as exemplary of the principles of the invention and is not intended to limit the invention to the specific embodiments illustrated and described herein. Numerous variations may be made by persons skilled in the art without departure from the spirit of the invention. The scope of the invention will be measured by the appended claims and their equivalents. The abstract and the title are not to be construed as limiting the scope of the present invention, as their purpose is to enable the appropriate authorities, as well as the general public, to quickly determine the general nature of the invention. In the claims that follow, unless the term “means” is used, none of the features or elements recited therein should be construed as means-plus-function limitations pursuant to 35 U.S.C. § 112, ¶6.

Claims

1. A modified E. coli cell comprising a mutated thrA gene encoding a polypeptide comprising an amino acid sequence selected from the group consisting of SEQ ID NO:1 to SEQ ID NO: 40, wherein the modified E. coli cell produces an enhanced amount of lysine in comparison to a wildtype cell.

2. The modified E. coli cell of claim 1, wherein the mutated thrA gene encodes a polypeptide comprising the amino acid sequence as set forth in SEQ ID NO: 17.

3. The modified E. coli cell of claim 1, wherein the E. coli cell is of E. coli strain MG1655.

4. The modified E. coli cell of claim 1, wherein the mutated thrA gene is generated using a nucleic acid-guided nuclease.

5. The modified E. coli cell of claim 1, wherein the modified cell further comprises a selectable marker.

6. The modified E. coli cell of claim 1, wherein the modified cell further comprises a barcode DNA sequence.