Patent Publication Number: US-2022213515-A1

Title: Method for producing 2-methyl-butyric acid by bacterial fermentation

Description:
This application is a Continuation of, and claims priority under 35 U.S.C. § 120 to, International Application No. PCT/JP2020/035935, filed Sep. 24, 2020, and claims priority therethrough under 35 U.S.C. § 119 to Russian Patent Application No. 2019130090, filed Sep. 25, 2019, the entireties of which are incorporated by reference herein. Also, the Sequence Listing filed electronically herewith is hereby incorporated by reference (File name: 2022-03-24T_US-635_seq_as_filed.txt; File size: 126 KB; Date recorded: Mar. 24, 2022). 
     BACKGROUND 
     General Field 
     The present invention relates to the microbiological industry, and specifically to a method for producing 2-methyl-butyric acid by fermentation of a bacterium belonging to the order Enterobacterales that has been modified to attenuate expression of a gene encoding a protein having tyrosine aminotransferase activity, so that production of a byproduct substance of 2-methyl-butyric acid is reduced, as compared with a non-modified bacterium. 
     Brief Description of the Related Art 
     2-Methyl-butyric acid is notably different in character from other compounds closely related by having a similar chemical structure regarding unbranched and branched aliphatic fatty acids, such as pentanoic acid (also referred to as “valeric acid”) and 3-methyl-butyric acid (also referred to as “isovaleric acid”). 2-Methyl-butyric acid has a mild, soft, dried fruit character, whereas the related fatty acids are stronger, but much more cheesy in character. 2-Methyl butyric acid is widely distributed in nature and has a wide spectrum of use in the flavor industry (Wright J. 2-Methyl butyric acid. Use in berry, fruit, brown, fermented and savory flavors,  Perfumer and flavorist,  2011, 36(7):18-19). In addition, lower alcohol esters of 2-methyl-butyric acid are used for production of fragrances, and polyol esters of 2-methyl-butyric acid are used for production of synthetic lubricants. 
     For particular applications, such as in the production of fragrances and some lubricants, demand is high for highly pure 2-methyl-butyric acid with only a residual amount of byproduct substances such as, for example, 3-methyl-butyric acid, that is, below 0.2% by weight. Therefore, demand is high for preparation methods of 2-methyl-butyric acid having a reduced amount of byproduct substances. 
     2-Methyl-butyric acid can be prepared using chemical synthesis or by fermentation of microorganisms in appropriate nutrient media. Commercially, 2-methyl-butyric acid is typically manufactured chemically by oxidizing 2-methyl-butyraldehyde which is generally produced by dehydrogenation of alcohols in the presence of a catalyst (Ullmann&#39;s Encyclopedia of industrial chemistry, 7 th  edition, 2011, Wiley-VCH). The alcohols are often inexpensive and available in good purity. Synthesis of 2-methyl-butyraldehyde via the oxo process (also called as “hydroformylation”), a process in which butenes are reacted with a mixture of carbon monoxide and hydrogen (so-called “synthesis gas”) in the presence of transition metal compounds, is less suitable as the resultant products are often not sufficiently pure. To increase the purity of 2-methyl-butyric acid, a method for producing 2-methyl-butyric acid having a reduced amount of 3-methyl-butyric acid was developed (US20160264503 A1). The method includes controlling the composition of the butene feed mixture and optimization of the hydroformylation conditions, which are followed by catalytic hydrogenation reduction at elevated temperature and pressure and treatment by oxidizing agent. The resultant 2-methyl-butyric acid can be obtained with the amount of 3-methyl-butyric acid of less than 0.2% by weight. 
     Methods for producing 2-methyl-butyric acid by fermentation of bacteria include, for example, fermentative production of (S)-2-methyl-butyric acid using bacteria belonging to the genus  Bacillus  in an L-isoleucine-containing medium (JP5271606 B2). In another example, a 2-methyl-butyric acid-producing bacterium was constructed from  E. coli  L-threonine-producing strain ATCC 98082 by inactivation of a threonine exporter and an alcohol dehydrogenase encoded by the rhtA and yqhD genes, respectively, and introduction of native thrABC and artificial ilvAGMCD operons under the control of P L lacO1 promoter on low-copy plasmids. The ability to produce 2-methyl-butyric acid was imparted to the bacterium by enhancing expression of genes encoding ketoacid decarboxylase and aldehyde dehydrogenase, which are responsible for the last two steps in the 2-methyl-butyric acid biosynthesis pathway (US20150132813 A1). 
     The tyrB gene encodes tyrosine aminotransferase (TyrB) which catalyzes the final step in tyrosine, leucine, and phenylalanine biosynthesis. The enzyme is feedback-inhibited by leucine (Powell J. T. and Morrison J. F., Role of the  Escherichia coli  aromatic amino acid aminotransferase in leucine biosynthesis,  J. Bacteriol.,  1978, 136(1):1-4), tyrosine (Collier R. H. and Kohlhaw G., Nonidentity of the aspartate and the aromatic aminotransferase components of transaminase A in  Escherichia coli, J. Bacteriol.,  1972, 112(1):365-371) and 2-keto-isovalerate (CN109402034 A; Vartak N. B. et al., A functional leuABCD operon is required for leucine synthesis by the tyrosine-repressible transaminase in  Escherichia coli  K-12,  J. Bacteriol.,  1991, 173(12):3864-3871). 
     Tyrosine aminotransferase TyrB having enhanced activity was used to construct L-tyrosine-producing strains (U.S. Pat. No. 9,540,652 B2, KR101869869 B1, CN109266592 A, US20080102499 A1, U.S. Pat. No. 7,700,328 B2; Lutke-Eversloh T. and Stephanopoulos G., Combinatorial pathway analysis for improved L-tyrosine production in  Escherichia coli : identification of enzymatic bottlenecks by systematic gene overexpression,  Metabolic engineering,  2008, 10(2):69-77), L-phenylalanine (CN104531597 B, CN100352928 C; Zhang C. et. al., Rational engineering of multiple module pathways for the production of L-phenylalanine in  Corynebacterium glutamicum, J. Ind. Microbiol. Biotechnol.,  2015, 42(5):787-797; Liu S. P. et al., A systems level engineered  E. coli  capable of efficiently producing L-phenylalanine,  Proc. Biochem.,  2014, 49(5):751-757; Wo Y. Q. et al., Co-expression of five genes in  E. coli  for L-phenylalanine in  Brevibacterium flavum, World J. Gastroenterol.,  2003, 9(2):342-346), L-tryptophan (EP2147972 A1), L-homophenylalanine (U.S. Pat. No. 6,146,859 A), and L-2-aminobutyric acid (CN106148259 A; Fotheringham I. G. et al., Engineering of a novel biochemical pathway for the biosynthesis of L-2-aminobutyric acid in  Escherichia coli  K12,  Bioorg. Med. Chem.,  1999, 7(10):2209-2213). 
     Methods for reducing production of byproduct substances of target compounds by deleting or attenuating the tyrB gene have been reported (see, for example, Li F. F. et al., Engineering  Escherichia coli  for production of 4-hydroxymandelic acid using glucose-xylose mixture,  Microb. Cell Fact.,  2016, 15:90; Zhu Y. et al., Metabolic engineering of indole pyruvic acid biosynthesis in  Escherichia coli  with tdiD,  Microb. Cell Fact.,  2017, 16(1):2; Liu S. P. et al., Heterologous pathway for the production of L-phenylglycine from glucose by  E. coli, J. Biotechnol.,  2014, 186:91-97). 
     In a series of reports, an  Escherichia coli  strain having increased expression of genes from the leucine biosynthesis pathway that can be used to generate 2-keto-isocaproate, and in which the ilvE and tyrB genes were deleted, was utilized in a method for production of alcohols, in particular, 3-methyl-1-butanol by fermentation of the bacterium (US2009081746 A1, US20100209986 A1, WO2010045629 A2; Connor M. R. and Liao J. C., Engineering of an  Escherichia coli  strain for the production of 3-methyl-1-butanol,  App. Env. Microbiol.,  2008, 74(18):5769-5775). It was shown that elimination of the leucine synthesis genes ilvE and tyrB led to increased production of 2-keto-isocaproate, which can then be converted to 3-methyl-1-butanol via decarboxylation and reduction steps. 
     However, no data has been previously reported that describes the effect of attenuation of expression of tyrB gene on production of 2-methyl-butyric acid and byproduct substances of 2-methyl-butyric acid by fermentation of a 2-methyl-butyric acid-producing bacterium belonging to the order Enterobacterales. From the viewpoint of industrial production, reducing the amount of a byproduct substance of 2-methyl-butyric acid in a method for producing 2-methyl-butyric acid by fermentation of a bacterium belonging to the order Enterobacterales is of considerable importance as the improved method would allow production of highly pure 2-methyl-butyric acid at a low price. 
     SUMMARY 
     An improved method for producing 2-methyl-butyric acid by fermentation of a bacterium belonging to the order Enterobacterales is described herein. According to the presently disclosed subject matter, production of 2-methyl-butyric acid by fermentation of a bacterium belonging to the order Enterobacterales can be improved by attenuating expression of tyrB gene in the bacterium, so that production of a byproduct substance of 2-methyl-butyric acid such as, for example, 3-methyl-butyric acid, isobutyric acid, L-allo-isoleucine, and/or D-allo-isoleucine, by the modified bacterium can be reduced, as compared with a non-modified bacterium. When a bacterium belonging to the order Enterobacterales and modified to attenuate expression of the tyrB gene in the bacterium is cultured in a medium to produce 2-methyl-butyric acid, the amount of the byproduct substance of 2-methyl-butyric acid can be reduced further by attenuating expression of one or more of the leuABCD operon genes. As a result, 2-methyl-butyric acid can be produced at higher grade of purity and lower price as compared with a method in which the modified bacterium as described herein is not used. 
     It is one aspect of the present invention to provide a method for producing 2-methyl-butyric acid comprising: (i) cultivating in a culture medium a 2-methyl-butyric acid-producing bacterium belonging to the order Enterobacterales to produce and accumulate 2-methyl-butyric acid in the culture medium or cells of the bacterium, or both, and (ii) collecting 2-methyl-butyric acid from the culture medium or the cells of the bacterium, or both, wherein said bacterium has been modified to attenuate expression of a gene encoding a protein having tyrosine aminotransferase activity. 
     It is another aspect of the invention to provide the method as described above, wherein said gene encoding a protein having tyrosine aminotransferase activity is a tyrB gene. 
     It is another aspect of the invention to provide the method as described above, wherein said protein having tyrosine aminotransferase activity is selected from the group consisting of: (A) a protein comprising the amino acid sequence shown in SEQ ID NO: 2, (B) a protein comprising the amino acid sequence shown in SEQ ID NO: 2, but which includes substitution, deletion, insertion and/or addition of 1 to 150 amino acid residues, and wherein said protein has tyrosine aminotransferase activity, and (C) a protein comprising an amino acid sequence having an identity of not less than 60% with respect to the entire amino acid sequence shown in SEQ ID NO: 2, and wherein said protein has tyrosine aminotransferase activity. 
     It is another aspect of the invention to provide the method as described above, wherein said gene is selected from the group consisting of: (a) a gene comprising the nucleotide sequence shown in SEQ ID NO: 1, (b) a gene comprising a nucleotide sequence that is able to hybridize under stringent conditions with a nucleotide sequence complementary to the sequence shown in SEQ ID NO: 1, and wherein the gene encodes a protein having tyrosine aminotransferase activity, (c) a gene encoding a protein comprising the amino acid sequence shown in SEQ ID NO: 2, but which includes substitution, deletion, insertion and/or addition of 1 to 150 amino acid residues, and wherein said protein has tyrosine aminotransferase activity, and (d) a gene comprising a variant nucleotide sequence of SEQ ID NO: 1, wherein the variant nucleotide sequence is due to the degeneracy of the genetic code. 
     It is another aspect of the invention to provide the method as described above, wherein said expression of the gene encoding a protein having tyrosine aminotransferase activity is attenuated due to inactivation of the gene. 
     It is another aspect of the invention to provide the method as described above, wherein said gene encoding a protein having tyrosine aminotransferase activity is deleted. 
     It is another aspect of the invention to provide the method as described above, wherein said bacterium belongs to the family Enterobacteriaceae or Erwiniaceae. 
     It is another aspect of the invention to provide the method as described above, wherein said bacterium belongs to the genus  Escherichia  or  Pantoea.    
     It is another aspect of the invention to provide the method as described above, wherein said bacterium is  Escherichia coli  or  Pantoea ananatis.    
     It is another aspect of the invention to provide the method as described above, wherein said bacterium has been modified further to attenuate expression of a gene selected from the group consisting of leuA, leuB, leuC, leuD, and combinations thereof. 
     It is another aspect of the invention to provide the method as described above, wherein the amount of a byproduct substance of 2-methyl-butyric acid is reduced as compared with a non-modified bacterium. 
     It is another aspect of the invention to provide the method as described above, wherein the byproduct substance is selected from the group consisting of 3-methyl-butyric acid, isobutyric acid, L-allo-isoleucine, D-allo-isoleucine, and combinations thereof. 
     Still other objects, features, and attendant advantages of the present invention will become apparent to those skilled in the art from a reading of the following detailed description of embodiments constructed in accordance therewith. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  shows the scheme of biosynthesis of 2-methyl-butyric acid and byproduct substances of 2-methyl-butyric acid. KdcA (also indicated as 1 in a circle): 2-ketoacid decarboxylase (EC 4.1.1.72) encoded by kdcA gene native to  Lactococcus lactis  those codons were optimized for the expression in  E. coli , AldH (also indicated as 2 in a circle): aldehyde dehydrogenase (EC 1.2.1.3), IlvE: branched chain amino acid aminotransferase, aminotransferase B (EC 2.6.1.42), TyrB: aromatic amino acid aminotransferase (EC 2.6.1.57), LeuA: 2-isopropylmalate sythase (EC 2.3.3.13), LeuCD: 3-isopropylmalate dehydratase (EC 4.2.1.33), LeuB: 3-isopropylmalate dehydrogenase (EC 1.1.1.85), E-4-P: D-erythrose-4-phosphate, PEP: phosphoenolpyruvate, PhePyr: 3-phenyl-pyruvate, hPhePyr: 4-hydroxy-phenyl-pyruvate, Pyr: pyruvate, 2-KB: 2-keto-butanoate, Prop: propanoic acid, AHB: 2-aceto-2-hydroxy-butanoate, (S)-KMV: (S)-2-keto-3 -methyl-valerate, (R)-KMV: (R)-2-keto-3 -methyl-valerate, allolle: allo-isoleucine, (S)-2-MB: (S)-2-methyl-butyric acid, (R)-2-MB: (R)-2-methyl-butyric acid, AL: 2-aceto-lactate, KIV: 2-keto-isovalerate, isoBut: isobutyric acid, KIC: 2-keto-isocaproate, 3-MB: 3-methyl-butyric acid. 
         FIG. 2  shows the DNA sequence of P tac  promoter (SEQ ID NO: 69). The −35 and −10 sequences of the P tac  promoter are underlined. The lac repressor-binding site is indicated with capital letters. The sequence of P tac  fragment which contains a half of lac operator is indicated in bold. 
         FIG. 3  shows the scheme for construction of an  E. coli  2-methyl-butyric acid-producing strain L1201-1. SD1: modified Shine-Dalgarno sequence, *: mutant allele of a gene. 
     
    
    
     DETAILED DESCRIPTION OF THE EXEMPLARY EMBODIMENTS 
     The invention of the present application will now be described in more detail with reference to the exemplary embodiments, given only by way of example, and with reference to the accompanying figures. 
     1. Bacterium 
     The bacterium as described herein is a 2-methyl-butyric acid-producing bacterium belonging to the order Enterobacterales that has been modified to attenuate expression of a gene encoding a protein having tyrosine aminotransferase activity. The bacterium as described herein can be used in the method as described herein. Hence, the explanations given hereinafter to the bacterium can be similarly applied to any bacterium that can be used interchangeably or equivalently for the method as described herein. 
     Any 2-methyl-butyric acid-producing bacterium belonging to the order Enterobacterales and modified to attenuate expression of a gene encoding a protein having tyrosine aminotransferase activity can be used. 
     The phrase “a 2-methyl-butyric acid-producing bacterium” may be used interchangeably or equivalently to the phrase “a bacterium that is able to produce 2-methyl-butyric acid”, the phrase “a bacterium having an ability to produce 2-methyl-butyric acid”, or the phrase “a bacterium having a 2-methyl-butyric acid-producing ability”. 
     The phrase “2-methyl-butyric acid-producing bacterium” can mean a bacterium belonging to the order Enterobacterales that has an ability to produce, excrete or secrete, and/or cause accumulation of 2-methyl-butyric acid in a culture medium and/or cells of the bacterium (i.e. the bacterial cells) when the bacterium is cultured in the medium. 
     The phrase “2-methyl-butyric acid-producing bacterium” can also mean a bacterium that is able to produce, excrete or secrete, and/or cause accumulation of 2-methyl-butyric acid in a culture medium in an amount larger than a non-modified bacterium. The term “a non-modified bacterium” may be used interchangeably or equivalently to the term “a non-modified strain”. The term “a non-modified strain” can mean a control strain that has not been modified to attenuate expression of a gene encoding a protein having tyrosine aminotransferase activity. Examples of the non-modified strain can include a wild-type or parental strain, such as  E. coli  K-12 strains such as W3110 (ATCC 27325) and MG1655 (ATCC 47076). The term “2-methyl-butyric acid-producing bacterium” can also mean a bacterium that is able to cause accumulation in a culture medium of an amount, for example, not less than 0.1 g/L, not less than 0.5 g/L, or not less than 1.0 g/L of 2-methyl-butyric acid. 
     Furthermore, the bacterium belonging to the order Enterobacterales and modified to attenuate expression of a gene encoding a protein having tyrosine aminotransferase activity such as tyrB gene, which has 2-methyl-butyric acid-producing ability, can also be used. The bacterium may inherently have 2-methyl-butyric acid-producing ability or may be modified to have 2-methyl-butyric acid-producing ability. Such modification can be attained by using, for example, a mutation method or DNA recombination techniques. The bacterium can be obtained by attenuating expression of a gene encoding a protein having tyrosine aminotransferase activity such as tyrB gene in a bacterium that inherently has 2-methyl-butyric acid-producing ability. Alternatively, the bacterium can be obtained by imparting 2-methyl-butyric acid-producing ability to a bacterium already modified to attenuate expression of a gene encoding a protein having tyrosine aminotransferase activity such as tyrB gene. Alternatively, the bacterium may have been imparted with 2-methyl-butyric acid-producing ability by being modified to attenuate expression of a gene encoding a protein having tyrosine aminotransferase activity such as tyrB gene. The bacterium as described herein can be obtained, specifically, for example, by modifying a bacterial strain described hereinafter. 
     The phrase “2-methyl-butyric acid-producing ability” can mean the ability of a bacterium belonging to the order Enterobacterales to produce, excrete or secrete, and/or cause accumulation of 2-methyl-butyric acid in a culture medium and/or the bacterial cells. The phrase “2-methyl-butyric acid-producing ability” can specifically mean the ability of the bacterium to produce, excrete or secrete, and/or cause accumulation of 2-methyl-butyric acid in a culture medium and/or the bacterial cells to such a level that 2-methyl-butyric acid can be collected from the culture medium and/or the bacterial cells, when the bacterium is cultured in the medium. 
     The term “cultured” with reference to a bacterium which is grown in a medium and used according to the method as described herein may be used interchangeably or equivalently to the terms “cultivated”, “grown”, or the like, that are well-known to persons skilled in the art. 
     The bacterium can produce 2-methyl-butyric acid in S-form or R-form enantiomers of the 2-methyl-butyric acid, or as a mixture of S-form and R-form of the enantiomers in various proportions. Among these, an S-form of enantiomer is a particular example. 
     The bacterium can produce 2-methyl-butyric acid either alone or as a mixture of 2-methyl-butyric acid and one or more kinds of substances that are different from 2-methyl-butyric acid. For example, the bacterium can produce 2-methyl-butyric acid either alone, or a mixture of 2-methyl-butyric acid and one or more kinds of amino acids, for example, amino acids in L-form (also referred to as “L-amino acids”). Furthermore, the bacterium can produce 2-methyl-butyric acid either alone or as a mixture of 2-methyl-butyric acid and one or more kinds of other organic acids such as, for example, carboxylic acids that are different from 2-methyl-butyric acid. 
     Furthermore, any 2-methyl-butyric acid-producing bacterium belonging to the order Enterobacterales and modified to attenuate expression of a gene encoding a protein having tyrosine aminotransferase activity such as tyrB gene, such that production of a byproduct substance of 2-methyl-butyric acid is reduced as compared with a non-modified strain, for example, a wild-type or parental strain as described hereinafter, can be used. The byproduct substance can consist of one, two, or more kinds of substances. That is, specifically, the bacterium can be used, which is modified to attenuate expression of a gene encoding a protein having tyrosine aminotransferase activity such as tyrB gene and which is able to produce one, two, or more kinds of byproduct substances of 2-methyl-butyric acid at a lower amount as compared with a non-modified bacterium, or by which by-production of one, two, or more kinds of byproduct substances of 2-methyl-butyric acid is reduced as compared with a non-modified bacterium. The amount of a byproduct substance of 2-methyl-butyric acid may be expressed as an absolute value such as, for example, in grams per liter (g/L), and the like, or a relative value such as, for example, in %. That is, the production amount of a byproduct substance of 2-methyl-butyric acid in terms of an absolute value and/or a relative value may be reduced. The phrase “an amount of a byproduct substance of 2-methyl-butyric acid is expressed as a relative value” can mean that an amount of a byproduct substance of 2-methyl-butyric acid is expressed as a ratio of the amount of the byproduct substance of 2-methyl-butyric acid relative to a reference substance and is, preferably, multiplied by 100%. A particular example of “a reference substance” can be 2-methyl-butyric acid. It is, therefore, acceptable that when a method for producing 2-methyl-butyric acid as described herein is used, an absolute amount of a byproduct substance of 2-methyl-butyric acid which is produced by a modified bacterium can remain unchanged or even be increased as compared with a non-modified bacterium, whereas the relative amount of the byproduct substance is decreased as compared with a non-modified bacterium, so that 2-methyl-butyric acid can be produced at a higher grade of purity as compared with a method in which the modified bacterium as described herein is not used. 
     The phrase “able to produce a byproduct substance of 2-methyl-butyric acid” as used herein with regard to a bacterium can mean the ability of a bacterium belonging to the order Enterobacterales to produce, excrete or secrete, and/or cause accumulation of one, two or more kinds of byproduct substances of 2-methyl-butyric acid in a culture medium or the bacterial cells. The phrase “able to produce a byproduct substance of 2-methyl-butyric acid” as used herein with regard to a bacterium can specifically mean the ability of a bacterium belonging to the order Enterobacterales to produce, excrete or secrete, and/or cause accumulation of one, two, or more kinds of byproduct substances of 2-methyl-butyric acid in a culture medium or the bacterial cells, or both, to such a level that the one, two, or more byproduct substances of 2-methyl-butyric acid can be collected from the culture medium and/or the bacterial cells when the bacterium is cultured in the medium. The phrase “a byproduct substance of 2-methyl-butyric acid” is explained hereinafter. 
     Examples of L-amino acids include, but are not limited to, L-alanine, L-arginine, L-asparagine, L-aspartic acid, L-citrulline, L-cysteine, L-glutamic acid, L-glutamine, glycine, L-histidine, L-isoleucine, L-leucine, L-lysine, L-methionine, L-ornithine, L-phenylalanine, L-proline, L-serine, L-threonine, L-tryptophan, L-tyrosine, and L-valine, and derivatives thereof 
     Examples of carboxylic acids include, but are not limited to, formic acid, acetic acid, propionic acid, butyric acid, lactic acid, citric acid, and pentanoic acid, and derivatives thereof. 
     The terms “L-amino acid” and “carboxylic acid” can refer not only to an amino acid and a carboxylic acid in a free form, but can also refer to a derivative form thereof, such as a salt, a hydrate, an adduct, or a combination of these. An adduct can be a compound formed by the amino acid or the carboxylic acid and another organic or inorganic compound. Hence, the terms “L-amino acid” and “carboxylic acid” can mean, for example, an L-amino acid and a carboxylic acid in a free form, a derivative form, or a mixture of them. The terms “L-amino acid” and “carboxylic acid” can particularly mean, for example, an L-amino acid and a carboxylic acid in a free form, a salt thereof, or a mixture of these. The terms “L-amino acid” and “carboxylic acid” can mean, for example, any of sodium, potassium, ammonium, mono-, di- and trihydrate, mono- and dichlorhydrate, and salts thereof. Unless otherwise stated, the terms “L-amino acid” and “carboxylic acid” without referring to hydration, such as the phrases “an L-amino acid or a carboxylic acid in a free form” and “a salt of an L-amino acid or a carboxylic acid”, can refer to an L-amino acid and a carboxylic acid not in a hydrate form, a hydrate of an L-amino acid and a carboxylic acid, or or a mixture of these. 
     The phrase “a byproduct substance of 2-methyl-butyric acid” can refer to one, two or more byproduct substances of 2-methyl-butyric acid and can mean a substance, such as, for example, an organic compound, which is different from 2-methyl-butyric acid and which is produced as a byproduct, co-product, or side-product in a method for production of 2-methyl-butyric acid by fermentation of a bacterium as described herein. The phrase “a byproduct substance of 2-methyl-butyric acid” can also refer to a substance that can be produced and excreted or secreted by a 2-methyl-butyric acid-producing bacterium belonging to the order Enterobacterales when the bacterium is cultured to produce 2-methyl-butyric acid, such that the byproduct substance accumulates in a culture medium or the bacterial cells, or both, to such a level that the byproduct substance can be collected from the culture medium and/or the bacterial cells when the bacterium is cultured in the medium. An amount of a byproduct substance of 2-methyl-butyric acid in the culture medium and/or the bacterial cells can be lower, equal, or higher than the amount of 2-methyl-butyric acid produced by fermentation of a bacterium belonging to the order Enterobacterales that has an ability to produce 2-methyl-butyric acid. 
     As 2-methyl-butyric acid biosynthesis pathway branches off from the biosynthesis pathway of L-isoleucine, specific examples of a byproduct substance of 2-methyl-butyric acid include, but are not limited to, intermediates in a biosynthesis pathway of 2-methyl-butyric acid, and intermediates of other biosynthesis pathways, from which the biosynthesis pathway of 2-methyl-butyric acid branches off, and so forth, or their combination. The intermediate is not limited to an intermediate in the biosynthesis pathway of 2-methyl-butyric acid, and it also may be a precursor, an intermediate, or a substrate in a metabolic pathway of other one, two, or more substances, for example, a precursor, an intermediate, or a substrate in a biosynthesis pathway of a branched-chain L-amino acid. 
     The phrase “a branched-chain L-amino acid” can refer to an L-amino acid such as L-valine, L-leucine, and L-isoleucine. As pyruvate, which may be also referred to as “alpha-ketopropionic acid”, is a precursor in the biosynthesis pathways of L-valine, L-leucine, and L-isoleucine, a byproduct of 2-methyl-butyric acid can be the byproduct of another biosynthesis pathway that branches off from pyruvate in the biosynthesis pathway of 2-methyl-butyric acid. A byproduct substance of 2-methyl-butyric acid is a particular example of the byproduct of another biosynthesis pathway that branches off from pyruvate in the biosynthesis pathway of 2-methyl-butyric acid, and can include 3-methyl-butyric acid and isobutyric acid that have the common precursor 2-keto-isovalerate, which may be also referred to as “alpha-oxoisovaleric acid”, L-allo-isoleucine and D-allo-isoleucine that have the common precursor 2-keto-3-methyl-valerate, which may be also referred to as “alpha-oxomethylvaleric acid”, which 2-keto-isovalerate and 2-keto-3-methyl-valerate have the common precursor pyruvate ( FIG. 1 ). 
     Therefore, a byproduct substrate of 2-methyl-butyric acid may be, but is not limited to, 3-methyl-butyric acid, isobutyric acid, L-allo-isoleucine, D-allo-isoleucine or a combination thereof, in a method for producing 2-methyl-butyric acid by fermentation of a bacterium belonging to the order Enterobacterales and modified to attenuate expression of a gene encoding a protein having tyrosine aminotransferase activity as described hereinafter. 
     The bacteria belonging to the family Enterobacteriaceae were recently reclassified based on a comprehensive comparative genomic analyses, which includes phylogenetic reconstructions based on 1548 core proteins, 53 ribosomal proteins, and four multilocus sequence analysis proteins (Adelou M. et al., Genome-based phylogeny and taxonomy of the Enterobacteriales&#39; : proposal for Enterobacterales ord. nov. divided into the families Enterobacteriaceae, Erwiniaceae fam. nov., Pectobacteriaceae fam. nov., Yersiniaceae fam. nov., Hafniaceae fam. nov., Morganellaceae fam. nov., and Budviciaceae fam. nov.,  Int. J. Syst. Evol. Microbiol.,  2016, 66:5575-5599). 
     According to the reclassification, the genera previously belonging to the family Enterobacteriaceae now are a part of different families within the order Enterobacterales. Based on the above analyses, a bacterium that can be used in the method as described herein and belonging to the order Enterobacterales can be from the genera  Enterobacter, Escherichia, Klebsiella, Salmonella, Erwinia, Pantoea, Morganella, Photorhabdus, Providencia, Yersinia , and so forth, and can have the ability to produce 2-methyl-butyric acid. Specifically, those classified into the order Enterobacterales according to the taxonomy used in the NCBI (National Center for Biotechnology Information) database (ncbi.nlm.nih.gov/Taxonomy/Browser/wwwtax.cgi?id=91347) can be used. Examples of strains belonging to the order Enterobacterales that can be modified include a bacterium of the family Enterobacteriaceae or Erwiniaceae, and, specifically, the genus  Escherichia, Enterobacter , or  Pantoea.    
     The  Escherichia  bacterial species are not particularly limited, and examples include species classified into the genus  Escherichia  according to the taxonomy known to those skilled in the field of microbiology. Examples of the  Escherichia  bacterium include, for example, those described in the work of Neidhardt et al. (Bachmann B. J., Derivations and genotypes of some mutant derivatives of  E. coli  K-12, p. 2460-2488. In F. C. Neidhardt et al. (ed.),  E. coli  and  Salmonella : cellular and molecular biology, 2 nd  ed. ASM Press, Washington, D.C., 1996). The species  Escherichia coli  ( E. coli ) is a particular example of  Escherichia  bacteria. Specific examples of  E. coli  include  E. coli  K-12 strain, which is a prototype wild-type strain, such as  E. coli  W3110 (ATCC 27325),  E. coli  MG1655 (ATCC 47076), and so forth. 
     The  Enterobacter  bacteria are not particularly limited, and examples include species classified into the genus  Enterobacter  according to classification known to a person skilled in the art of microbiology. Examples of the  Enterobacter  bacterium include, for example,  Enterobacter agglomerans, Enterobacter aerogenes , and so forth. Specific examples of  Enterobacter agglomerans  strains include, for example, the  Enterobacter agglomerans  ATCC 12287. Specific examples of  Enterobacter aerogenes  strains include, for example, the  Enterobacter aerogenes  ATCC 13048, NBRC 12010 (Sakai S. and Yaqishita T., Microbial production of hydrogen and ethanol from glycerol-containing wastes discharged from a biodiesel fuel production plant in a bioelectrochemical reactor with thionine,  Biotechnol. Bioeng.,  2007, 98(2):340-348), and AJ110637 (FERM BP-10955). Examples of the  Enterobacter  bacterial strains also include, for example, the strains described in European Patent Application Laid-open (EP-A) No. 0952221. In addition,  Enterobacter agglomerans  also include some strains classified as  Pantoea agglomerans.    
     The  Pantoea  bacteria are not particularly limited, and examples include species classified into the genus  Pantoea  according to classification known to a person skilled in the art of microbiology. Examples of the  Pantoea  bacterial species include, for example,  Pantoea ananatis  ( P. ananatis ),  Pantoea stewartii, Pantoea agglomerans , and  Pantoea citrea . Specific examples of  P. ananatis  strains include, for example, the  P. ananatis  LMG20103, AJ13355 (FERM BP-6614), AJ13356 (FERM BP-6615), AJ13601 (FERM BP-7207), SC17 (FERM BP-11091), and SC17(0) (VKPM B-9246). Some strains of  Enterobacter agglomerans  were recently reclassified into  Pantoea agglomerans, Pantoea ananatis, Pantoea stewartii , or the like on the basis of nucleotide sequence analysis of 16S rRNA etc. (Mergaert J. et al., Transfer of  Envinia ananas  (synonym,  Erwinia uredovora ) and  Envinia stewartii  to the Genus  Pantoea  emend. as  Pantoea ananas  (Serrano 1928) comb. nov. and  Pantoea stewartii  (Smith 1898) comb. nov., respectively, and description of  Pantoea stewartii  subsp. indologenes subsp. nov.,  Int. J. Syst. Evol. Microbiol.,  1993, 43:162-173). The  Pantoea  bacteria include those reclassified into the genus  Pantoea  as described above. 
     Examples of the  Erwinia  bacteria include  Envinia amylovora  and  Envinia carotovora . Examples of the  Klebsiella  bacteria include  Klebsiella planticola.    
     These strains are available from, for example, the American Type Culture Collection (Address: P.O. Box 1549, Manassas, Va. 20108, United States of America). That is, registration numbers are given to the respective strains, and the strains can be ordered by using these registration numbers (refer to atcc.org). The registration numbers of the strains are listed in the catalogue of the American Type Culture Collection. 
     The bacterium may be a bacterium inherently having a 2-methyl-butyric acid-producing ability or may be a bacterium modified so that it has 2-methyl-butyric acid-producing ability. The bacterium having a 2-methyl-butyric acid-producing ability can be obtained by imparting a 2-methyl-butyric acid-producing ability to such a bacterium as mentioned above, or by enhancing a 2-methyl-butyric acid-producing ability of such a bacterium as mentioned above. 
     To impart or enhance a 2-methyl-butyric acid-producing ability, methods conventionally employed in the breeding of amino acid-producing strains of  Escherichia  bacteria, and so forth (see “Amino Acid Fermentation”, Gakkai Shuppan Center (Ltd.), 1st Edition, published May 30, 1986, pp.77-100) can be used. Examples of such methods include, for example, acquiring an auxotrophic mutant strain, acquiring a 2-methyl-butyric acid analogue-resistant strain, acquiring a metabolic regulation mutant strain, and constructing a recombinant strain in which the activity of a 2-methyl-butyric acid biosynthetic enzyme is enhanced. In the breeding of 2-methyl-butyric acid-producing bacteria, one of the above-described properties such as auxotrophy, analogue resistance, and metabolic regulation mutation may be imparted alone, or two, or three, or more of such properties may be imparted in combination. Also, in the breeding of 2-methyl-butyric acid-producing bacteria, the activity of one of 2-methyl-butyric acid biosynthetic enzymes may be enhanced alone, or the activities of two or three or more of such enzymes may be enhanced in combination. Furthermore, imparting property(s) such as auxotrophy, analogue resistance, and metabolic regulation mutation can be combined with enhancing the activity(s) of biosynthetic enzyme(s). 
     An auxotrophic mutant strain, analogue-resistant strain, or metabolic regulation mutant strain having a 2-methyl-butyric acid-producing ability can be obtained by subjecting a parental strain or wild-type strain to a typical mutagenesis treatment, and then selecting a strain exhibiting auxotrophy, analogue resistance, or a metabolic regulation mutation, and having a 2-methyl-butyric acid-producing ability from the obtained mutant strains. Examples of typical mutagenesis treatments include irradiation of X-ray or ultraviolet and a treatment with a mutation agent such as N-methyl-N′-nitro-N-nitrosoguanidine (MNNG), ethyl methanesulfonate (EMS), and/or methyl methanesulfonate (MMS). 
     A 2-methyl-butyric acid-producing ability can also be imparted or enhanced by enhancing the activity of an enzyme involved in biosynthesis of 2-methyl-butyric acid. An enzyme activity can be enhanced by, for example, modifying a bacterium so that the expression of a gene encoding the enzyme is enhanced. Methods for enhancing gene expression are described in WO0018935, EP1010755 A, and so forth. 
     Furthermore, a 2-methyl-butyric acid-producing ability can also be imparted or enhanced by reducing the activity of an enzyme that catalyzes a reaction branching away from the biosynthesis pathway of 2-methyl-butyric acid to generate a compound other than the 2-methyl-butyric acid. The “enzyme that catalyzes a reaction branching away from the biosynthesis pathway of 2-methyl-butyric acid to generate a compound other than the 2-methyl-butyric acid” includes an enzyme involved in decomposition of the 2-methyl-butyric acid. An enzyme activity can be reduced by, for example, modifying a bacterium so that the gene encoding the enzyme is inactivated. The method for reducing enzyme activity will be described later. 
     Hereafter, 2-methyl-butyric acid-producing bacteria and methods for imparting or enhancing a 2-methyl-butyric acid-producing ability will be specifically exemplified. All of the properties of the 2-methyl-butyric acid-producing bacteria and modifications for imparting or enhancing a 2-methyl-butyric acid-producing ability may be used independently or in any appropriate combination. 
     2-Methyl-Butyric Acid-Producing Bacteria 
     Examples of 2-methyl-butyric acid-producing bacteria and parental strains which can be used to derive 2-methyl-butyric acid-producing bacteria include, but are not limited to, strains in which the expression of one or more genes encoding proteins responsible for the last two steps in the 2-methyl-butyric acid biosynthesis pathway is enhanced (FIG.1). Examples of genes encoding proteins responsible for the penultimate step in the 2-methyl-butyric acid biosynthesis pathway include the kivD gene encoding branched-chain-2-ketoacid decarboxylase (KivD), which can be native to, for example,  Lactococcus lactis  (US20150132813 A1; de la Plaza et al., Biochemical and molecular characterization of alpha-keto-isovalerate decarboxylase, an enzyme involved in the formation of aldehydes from amino acids by  Lactococcus lactis, FEMS Microbiol Lett.,  2004, 238(2):367-374), the kdcA gene encoding branched-chain 2-ketoacid decarboxylase, which can be native to, for example,  Lactococcus lactis  B1157 that shows 89.8% identity with KivD (Smit B.A. et al., Identification, cloning, and characterization of a  Lactococcus lactis  branched-chain a-ketoacid decarboxylase involved in flavor formation,  Appl. Env. Microbiol.,  2005, 71(1):303-311; Savrasova E. A. et al., Use of the valine biosynthesis pathway to convert glucose into isobutanol,  J. Ind. Microbiol. Biotechnol.,  2011, 38(9):1287-1294), and ipdC gene encoding indolepyruvate decarboxylase, which can be native to, for example,  Salmonella typhimurium  (US20150132813 A1). Examples of genes encoding proteins responsible for the ultimate step in the 2-methyl-butyric acid biosynthesis pathway include the feaB gene (synonym: padA gene) encoding phenylacetaldehyde dehydrogenase, which can be native to, for example,  E. coli  (US20150132813 A1; Rodriguez-Zavala J. S. et al., Characterization of  E. coli  tetrameric aldehyde dehydrogenases with atypical properties compared to other aldehyde dehydrogenases,  Protein Sci.,  2006, 15(6):1387-1396), the aldB gene encoding aldehyde dehydrogenase, which can be native to, for example,  E. coli  (US20150132813 A1; Ho K. K. and Weiner H., Isolation and characterization of an aldehyde dehydrogenase encoded by the aldB gene of  Escherichia coli, J. Bacteriol.,  2005, 187(3):1067-1073), and the puuC gene (synonym: aldH gene) encoding γ-glutamyl-γ-aminobutyraldehyde dehydrogenase, which can be native to, for example,  E. coli  (US20150132813 A1; Kurihara S. et al., A novel putrescine utilization pathway involves γ-glutamylated intermediates of  Escherichia coli  K-12,  J. Biol. Chem.,  2005, 280(6):4602-4608; Jo J.-E. et al., Cloning, expression, and characterization of an aldehyde dehydrogenase from  Escherichia coli  K-12 that utilizes 3-hydroxypropionaldehyde as a substrate,  Appl. Microbiol. Biotechnol.,  2008, 81(1):51-60). 
     As 2-methyl-butyric acid biosynthesis pathway branches off from 2-keto-3-methyl-valerate in the biosynthesis pathway of L-isoleucine, examples of 2-methyl-butyric acid-producing bacteria and parent strains which can be used to derive 2-methyl-butyric acid-producing bacteria, include but are not limited to, recombinant strains transformed with genes encoding proteins involved in L-isoleucine biosynthesis, such as threonine deaminase (JP 2-458 A) and acetohydroxy acid synthase (EP0356739 A1). Other examples of L-isoleucine-producing bacteria and parental strains which can be used to derive 2-methyl-butyric acid-producing bacteria include, but are not limited to, strains belonging to the genus  Escherichia  such as  E. coli  AJ12919 (Japanese Patent Laid-open Publication No. 8-47397);  E. coli  strains VL1892 and KX141 (VKPM B-4781) (US5658766), and the like. 
     As the L-isoleucine biosynthesis pathway starts from L-threonine, examples of 2-methyl-butyric acid-producing bacteria and parent strains which can be used to derive 2-methyl-butyric acid-producing bacteria, can also include, but are not limited to, strains in which expression of one or more genes encoding an L-threonine biosynthetic enzyme(s) is enhanced. For example, to enhance expression of the threonine operon thrABC, the attenuator region which affects the transcription is desirably removed from the operon (WO2005049808 A1, WO2003097839 A1). Other examples of L-threonine-producing bacteria and parental strains which can be used to derive 2-methyl-butyric acid-producing bacteria include, but are not limited to, strains belonging to the genus  Escherichia  such as  E. coli  TDH-6/pVIC40 (VKPM B-3996) (U.S. Pat. No. 5,175,107 and U.S. Pat. No. 5,705,371),  E. coli  472T23/pYN7 (ATCC 98081) (U.S. Pat. No. 5,631,157),  E. coli  NRRL-21593 (U.S. Pat. No. 5,939,307),  E. coli  MG442 (U.S. Pat. No. 4,278,765; Gusyatiner M. M. et al., Study of relA gene function in the expression of amino acid operons. II. Effect of the relA gene allelic state on threonine over-production by  Escherichia coli  K-12 mutants resistant to β-hydroxynorvaline acid,  Genetika  (Russian), 1978, 14(6):957-968),  E. coli  VL643 and VL2055 (EP1149911 A2),  E. coli  VKPM B-5318 (EP0593792 A1), and the like. 
     Based on the above L-threonine-producing strain VKPM B-3996, L-isoleucine-producing strain AJ12919 was obtained by introducing the ilvGMEDA operon on the plasmid pMWD5 containing the ilvA gene encoding threonine deaminase, of which inhibition by L-isoleucine is substantially desensitized and from which a region required for attenuation is removed (EP0685555 B1). 
     A 2-methyl-butyric acid-producing bacterium can be obtained from any L-isoleucine-producing bacterium by inactivation of branched-chain amino acid aminotransferase encoded by ilvE gene. Methods for inactivation of genes are described herein. 
     The genes and proteins used for breeding 2-methyl-butyric acid-producing bacteria may have, for example, known nucleotide sequences and amino acid sequences of the genes and proteins exemplified above, respectively. Also, the genes and proteins used for breeding 2-methyl-butyric acid-producing bacteria may be variants of the genes and proteins exemplified above, such as variants of genes and proteins having known nucleotide sequences and amino acid sequences, respectively, so long as the original function thereof, such as respective enzymatic activities in cases of proteins, is maintained. As for variants of genes and proteins, the descriptions concerning variants of a gene encoding a protein having tyrosine aminotransferase activity and tyrosine aminotransferase encoded thereby described herein can be similarly applied. 
     The bacterium as described herein belonging to the order Enterobacterales has been modified to, at least, attenuate expression of a gene encoding a protein having tyrosine aminotransferase activity. 
     The phrase “a gene encoding a protein having tyrosine aminotransferase activity” can mean a gene encoding the protein having enzymatic activity of catalyzing the following reactions: an aromatic amino acid+2-ketoglutarate &lt;--&gt; an aromatic 2-ketoacid+L-glutamate (Enzyme Commission number, EC: 2.6.1.57) and L-leucine+2-ketoglutarate &lt;--&gt; 4-methyl-2-keto-pentanoate+L-glutamate (EC: 2.6.1.6). A specific example of the gene which encodes the enzyme having tyrosine aminotransferase activity includes the tyrB gene which encodes tyrosine aminotransferase. The gene encoding an enzyme having tyrosine aminotransferase activity can be the tyrB gene and its homolog(s) or variant nucleotide sequence(s). The more specific description of tyrB and its homologs and variant nucleotide sequences is given hereinafter. 
     The tyrB gene encodes tyrosine aminotransferase TyrB (synonym: aromatic-amino-acid aminotransferase, leucine aminotransferase) (KEGG, Kyoto Encyclopedia of Genes and Genomes, entry No. b4054; Protein Knowledgebase, UniProtKB/Swiss-Prot, accession No. P04693). 
     The tyrB gene (GenBank, accession No. NC_000913.3; nucleotide positions: 4267114 to 4268307, complement; Gene ID: 948563) is located between the alr gene on the same strand and the yjbS gene on the opposite strand on the chromosome of  E. coli  strain K-12. The nucleotide sequence of the tyrB gene (SEQ ID NO: 1) of  E. coli  strain K-12 and the amino acid sequence of the TyrB protein (SEQ ID NO: 2) encoded by this gene native to  E. coli  strain K-12 are known. 
     That is, the gene encoding a protein having tyrosine aminotransferase activity such as the tyrB gene may be a gene, such as DNA, having the nucleotide sequence of SEQ ID NO: 1, and the protein having tyrosine aminotransferase activity such as the TyrB protein may be a protein having the amino acid sequence of SEQ ID NO: 2. The phrase “a gene or protein has a nucleotide or amino acid sequence” can mean that a gene or protein includes the nucleotide or amino acid sequence as a part of a larger sequence unless otherwise stated, and can also mean that a gene or protein has only the nucleotide or amino acid sequence. 
     The production of a byproduct substance of 2-methyl-butyric acid by the bacterium that can be used in the method as described herein may be further reduced, as compared with a non-modified bacterium, by the attenuation of expression of one or more genes involved in L-leucine biosynthesis pathway which branches off from pyruvate in the biosynthesis pathway of 2-methyl-butyric acid. The first step in L-leucine biosynthesis pathway is the formation of 2-isopropylmalate from 2-keto-isovalerate, acetyl-CoA and H 2 O, and it is catalyzed by 2-isopropylmalate synthase which is encoded by leuA gene. The second step is the conversion of 2-isopropylmalate to 3-isopropylmalate which is catalyzed by 3-isopropylmalate dehydratase which is encoded by the leuCD genes. The next reaction is the formation of 2-isopropyl-3-keto-succinate from 3-isopropylmalate, and it is catalyzed by 3-isopropylmalate dehydrogenase which is encoded by leuB gene. The synthesized 2-isopropyl-3-keto-succinate spontaneously turns to 2-keto-isocaproate which is a precursor of 3-methyl-butyric acid, a byproduct substance that is produced during the production of 2-methyl-butyric acid by fermentation of a 2-methyl-butyric acid-producing bacterium belonging to the order Enterobacterales. 
     By attenuating expression of one or more genes in the L-leucine biosynthesis pathway such as leuA, leuB, leuC, and/or leuD, one can impart to a bacterium that can be used in the method as described herein the property to produce one or more byproduct substance(s) in a lower amount as compared with the bacterium in which the leuA, leuB, leuC, and/or leuD gene(s) is/are not attenuated. As a result of attenuating the expression of said gene(s), the production of one or more byproduct substance(s) such as, for example, 3-methyl-butyric acid by the modified bacterium can be decreased further and 2-methyl-butyric acid can be produced at a higher purity. 
     The term “leuA gene” can mean a gene which encodes an enzyme having a 2-isopropylmalate synthase activity. A specific example of the gene which encodes the enzyme having the 2-isopropylmalate synthase activity includes the leuA gene which encodes 2-isopropylmalate synthase. The gene encoding the enzyme having the 2-isopropylmalate synthase activity can be the leuA gene and its homolog(s) or variant nucleotide sequence(s). The more specific description of leuA and its variant nucleotide sequences is given hereinafter. 
     The leuA gene encodes the 2-isopropylmalate synthase protein LeuA (KEGG, entry No. b0074; Protein Knowledgebase, UniProtKB/Swiss-Prot, accession No. P09151). The leuA gene (GenBank accession No. NC_000913.3; nucleotide positions: 81958 to 83529, complement; Gene ID: 947465) is located between the leuL and leuB genes on the same strand on the chromosome of  E. coli  strain K-12. The leuA gene is a part of the leuABCD operon. The nucleotide sequence of the leuA gene and the amino acid sequence of the LeuA protein encoded by the leuA gene are shown in SEQ ID NO: 3 and SEQ ID NO: 4, respectively. 
     The phrase “a 2-isopropylmalate synthase activity” can mean enzymatic activity of catalyzing the following reaction: 3-methyl-2-keto-butanoate+acetyl-CoA+H 2 O-&gt;(2S)-2-isopropylmalate+coenzyme A+H +  (EC: 2.3.3.13). 
     The term “leuB gene” can mean a gene which encodes an enzyme having a 3-isopropylmalate dehydrogenase activity. A specific example of the gene which encodes the enzyme having the 3-isopropylmalate dehydrogenase activity includes the leuB gene which encodes 3-isopropylmalate dehydrogenase. The gene encoding the enzyme having the 3-isopropylmalate dehydrogenase activity can be the leuB gene and its homolog(s) or variant nucleotide sequence(s). The more specific description of leuB and its variant nucleotide sequences is given hereinafter. 
     The leuB gene encodes the 3-isopropylmalate dehydrogenase protein LeuB (synonyms: IMDH, 3-carboxy-2-hydroxy-4-methylpentanoate:NAD +  oxidoreductase; KEGG, entry No. b0073; Protein Knowledgebase, UniProtKB/Swiss-Prot, accession No. P30125). The leuB gene (GenBank accession No. NC_000913.3; nucleotide positions: 80867 to 81958, complement; Gene ID: 944798) is located between the leuA and leuC genes on the same strand on the chromosome of  E. coli  strain K-12. The leuB gene is a part of the leuABCD operon. The nucleotide sequence of the leuB gene and the amino acid sequence of the LeuB protein encoded by the leuB gene are shown in SEQ ID NO: 5 and SEQ ID NO: 6, respectively. 
     The phrase “a 3-isopropylmalate dehydrogenase activity” can mean enzymatic activity of catalyzing the following reaction: (2R,3S)-3-isopropylmalate+NAD + -&gt;4-methyl-2-keto-pentanoate+CO 2  NADH (EC: 1.1.1.85). 
     The term “leuCD genes” can mean genes which encode an enzyme having a 3-isopropylmalate dehydratase activity. A specific example of the genes which encode the enzyme having the 3-isopropylmalate dehydratase activity includes the leuC and leuD genes which encode large and small subunits of 3-isopropylmalate dehydratase respectively. The genes encoding the enzyme having the 3-isopropylmalate dehydratase activity can be the leuC and leuD genes and their homologs or variant nucleotide sequences. The more specific description of leuC, leuD, and their variant nucleotide sequences is given hereinafter. 
     The leuC gene encodes the large subunit of 3-isopropylmalate dehydratase LeuC (KEGG, entry No. b0072; Protein Knowledgebase, UniProtKB/Swiss-Prot, accession No. POA6A6). The leuC gene (GenBank accession No. NC_000913.3; nucleotide positions: 79464 to 80864, complement; Gene ID: 945076) is located between the leuB and leuD genes on the same strand on the chromosome of  E. coli  strain K-12. The leuC gene is a part of the leuABCD operon. The nucleotide sequence of the leuC gene and the amino acid sequence of the LeuC protein encoded by the leuC gene are shown in SEQ ID NO: 7 and SEQ ID NO: 8, respectively. 
     The leuD gene encodes the small subunit of 3-isopropylmalate dehydratase LeuD (KEGG, entry No. b0071; Protein Knowledgebase, UniProtKB/Swiss-Prot, accession No. P30126). The leuD gene (GenBank accession No. NC_000913.3; nucleotide positions: 78848 to 79453, complement; Gene ID: 945642) is located between the leuC gene on the same strand and the setA gene on the opposite strand on the chromosome of  E. coli  strain K-12. The leuD gene is a part of the leuABCD operon. The nucleotide sequence of the leuD gene and the amino acid sequence of the LeuD protein encoded by the leuD gene are shown in SEQ ID NO: 9 and SEQ ID NO: 10, respectively. 
     LeuC and LeuD constitute 3-isopropylmalate dehydratase LeuCD (synonym: 3-isopropylmalate isomerase). 
     The phrase “a 3-isopropylmalate dehydratase activity” can mean enzymatic activity of catalyzing the following reaction: (2R,3S)-3-isopropylmalate &lt;--&gt; (2S)-2-isopropylmalate (EC: 4.2.1.33). 
     That is, the leuA, leuB, leuC, and leuD genes may be genes, such as DNA, having the nucleotide sequences of SEQ ID NOS: 3, 5, 7, and 9, respectively, and the LeuA, LeuB, LeuC, and LeuD proteins may be proteins having the amino acid sequence of SEQ ID NOS: 4, 6, 8, and 10, respectively. 
     The protein concentration can be determined by the Bradford protein assay or the method of Lowry using bovine serum albumin (BSA) as a standard and a Coomassie dye (Bradford M.M., A rapid and sensitive method for the quantitation of microgram quantities of protein utilizing the principle of protein-dye binding,  Anal. Biochem.,  1976, 72:248-254; Lowry O. H. et al., Protein measurement with the Folin phenol reagent,  J. Biol. Chem.,  1951, 193 :265-275), or a Western blot analysis (Hirano S., Western blot analysis,  Methods Mol. Biol.,  2012, 926:87-97). 
     The explanations given hereinafter with reference to, for example, gene expression attenuation, a variant protein and a variant nucleotide sequence can be applied with appropriate changes to any protein and gene described in this specification including, but is not limited to, TyrB, LeuA, LeuB, LeuC, LeuD, KdcA, and AldH proteins, and the genes encoding them such as tyrB, leuA, leuB, leuC, leuD, kdcA, and aldH genes, and homologs thereof. 
     The phrase “a bacterium has been modified to attenuate expression of a gene” can mean that the bacterium has been modified in such a way that in the modified bacterium expression of the gene is attenuated. For example, the expression of the gene can be attenuated due to inactivation of the gene. 
     The phrase “a gene is inactivated” can mean that the modified gene encodes a completely inactive or non-functional protein as compared with a wild-type or non-modified gene. It is also acceptable that the modified DNA region is unable to naturally express the gene due to deletion of a part of the gene or deletion of the entire gene, replacement of one base or more to cause an amino acid substitution in the protein encoded by the gene (missense mutation), introduction of a stop codon (nonsense mutation), deletion of one or two bases to cause a reading frame shift of the gene, insertion of a drug-resistance gene and/or transcription termination signal, or modification of an adjacent region of the gene, including sequences controlling gene expression such as promoter, enhancer, attenuator, ribosome-binding site, etc. Inactivation of the gene can also be performed, for example, by conventional methods such as a mutagenesis treatment using UV irradiation or nitrosoguanidine (N-methyl-N′-nitro-N-nitrosoguanidine), site-directed mutagenesis, gene disruption using homologous recombination, and/or insertion-deletion mutagenesis (Yu D. et al., An efficient recombination system for chromosome engineering in  Escherichia coli, Proc. Natl. Acad. Sci. USA,  2000, 97(11):5978-5983; Datsenko K. A. and Wanner B. L., One-step inactivation of chromosomal genes in  Escherichia coli  K-12 using PCR products,  Proc. Natl. Acad. Sci. USA,  2000, 97(12):6640-6645; Zhang Y. et al., A new logic for DNA engineering using recombination in  Escherichia coli, Nature Genet.,  1998, 20:123-128) based on “Red/ET-driven integration” or “λRed/ET-mediated integration”. 
     The phrase “a bacterium has been modified to attenuate expression of a gene” can mean that the modified bacterium contains a regulatory region operably linked to the gene, including sequences controlling gene expression such as promoters, enhancers, attenuators, and transcription termination signals, ribosome-binding sites, and other expression control elements, which is modified resulting in the decrease of the expression level of the gene; and other examples (see, for example, WO9534672 A1; Carrier T. A. and Keasling J. D., Library of synthetic 5′ secondary structures to manipulate mRNA stability in  Escherichia coli, Biotechnol Prog.,  1999, 15:58-64). 
     The phrase “operably linked to the gene” in reference to a regulatory region can mean that the regulatory region(s) is/are linked to the nucleotide sequence of the nucleic acid molecule or gene in such a manner which allows for expression, for example, enhanced, increased, constitutive, basal, antiterminated, attenuated, deregulated, decreased, or repressed expression, of the nucleotide sequence, specifically, the expression of a gene product encoded by the nucleotide sequence. 
     The phrase “a bacterium has been modified to attenuate expression of a gene” can also mean that the bacterium has been modified in such a way that in the modified bacterium, the expression level (that is, expression amount) of a gene is decreased as compared with a non-modified strain, for example, a wild-type or parental strain. A decrease in the expression level of a gene can be measured as, for example, a decrease in the expression level of the gene per cell, which may be an average expression level of the gene per cell. The phrase “the expression level of a gene” or “the expression amount of a gene” can mean, for example, that the amount of the expression product of the gene, such as the amount of mRNA of the gene or the amount of the protein encoded by the gene. The bacterium may be modified so that the expression level of the gene per cell is reduced to, for example, 50% or less, 20% or less, 10% or less, 5% or less, or 0% of that in a non-modified bacterium. 
     The phrase “a bacterium has been modified to attenuate expression of a gene” can also mean that the bacterium has been modified in such a way that in the modified bacterium the total amount and/or the total activity of the corresponding gene product (i.e. the encoded protein) is decreased as compared with a non-modified bacterium. A decrease in the total amount and/or the total activity of a protein can be measured as, for example, a decrease in the amount or activity of the protein per cell, which may be an average amount or activity of the protein per cell. The bacterium can be modified so that the activity of the protein per cell is decreased to, for example, 50% or less, 20% or less, 10% or less, 5% or less, or 0% of that in a non-modified bacterium. 
     Examples of a non-modified bacterium serving as a reference for the above comparisons can include wild-type strains of a bacterium belonging to the genus  Escherichia , such as the  E. coli  MG1655 strain (ATCC 47076),  E. coli  W3110 strain (ATCC 27325), or a bacterium belonging to the genus  Pantoea , such as the  P. ananatis  AJ13355 strain (FERM BP-6614), and so forth. Examples of a non-modified bacterium serving as a reference for the above comparisons can also include a parental strain which has not been modified to attenuate expression of the gene or a bacterium in which expression of the gene is not attenuated. 
     Expression of a gene can be attenuated by replacing an expression control sequence of the gene, such as a promoter on the chromosomal DNA, with a weaker one. The strength of a promoter is defined by the frequency of initiation acts of RNA synthesis. Examples of methods for evaluating the strength of promoters are described in Goldstein M. A. et al. (Goldstein M. A. and Doi R. H., Prokaryotic promoters in biotechnology,  Biotechnol. Annu. Rev.,  1995, 1:105-128), and so forth. Furthermore, it is also possible to introduce one or more nucleotide substitutions in a promoter region of the gene and thereby modify the promoter to be weakened as disclosed in WO0018935 A1. Furthermore, it is known that substitution of several nucleotides in the Shine-Dalgarno (SD) sequence, and/or in the spacer between the SD sequence and the start codon, and/or a sequence immediately upstream and/or downstream from the start codon in the ribosome-binding site (RBS) greatly affects the translation efficiency of mRNA. This modification of the RBS may be combined with decreasing transcription of the gene. 
     Expression of a gene can also be attenuated by inserting a transposon or an insertion sequence (IS) into the coding region of the gene (U.S. Pat. No. 5,175,107) or in the region controlling gene expression, or by conventional methods such as mutagenesis with ultraviolet (UV) irradiation or nitrosoguanidine (N-methyl-N′-nitro-N-nitrosoguanidine, NTG). Furthermore, the incorporation of a site-specific mutation can be conducted by known chromosomal editing methods based, for example, on λRed/ET-mediated recombination (Datsenko K. A. and Wanner B. L.,  Proc. Natl. Acad. Sci. USA,  2000, 97(12):6640-6645). [000102] The copy number and/or the presence or absence of the gene can be measured, for example, by restricting the chromosomal DNA followed by Southern blotting using a probe based on the gene sequence, fluorescence in situ hybridization (FISH), and the like. The level of gene expression can be determined by measuring the amount of mRNA transcribed from the gene using various well-known methods, including Northern blotting, quantitative RT-PCR, and the like. The amount of the protein encoded by the gene can be measured by known methods including SDS-PAGE followed by immunoblotting assay (Western blotting analysis), or mass spectrometry analysis of the protein samples, and the like. 
     Methods for manipulation with recombinant molecules of DNA and molecular cloning such as preparation of plasmid DNA, digestion, ligation, and transformation of DNA, selection of an oligonucleotide as a primer, incorporation of mutations, and the like may be ordinary methods well-known to the persons of ordinary skill in the art. These methods are described, for example, in Sambrook J., Fritsch E. F. and Maniatis T., “Molecular Cloning: A Laboratory Manual”, 2 nd  ed., Cold Spring Harbor Laboratory Press (1989) or Green M. R. and Sambrook J. R., “Molecular Cloning: A Laboratory Manual”, 4 th  ed., Cold Spring Harbor Laboratory Press (2012); Bernard R. Glick, Jack J. Pasternak and Cheryl L. Patten, “Molecular Biotechnology: principles and applications of recombinant DNA”, 4 th  ed., Washington, D.C., ASM Press (2009). 
     Any methods for manipulation with recombinant DNA can be used including conventional methods such as, for example, transformation, transfection, infection, conjugation, and mobilization. Transformation, transfection, infection, conjugation, or mobilization of a bacterium with the DNA encoding a protein can impart to the bacterium the ability to synthesize the protein encoded by the DNA. Methods of transformation, transfection, infection, conjugation, and mobilization include any known methods. For example, a method of treating recipient cells with calcium chloride to increase permeability of the cells of  E. coli  K-12 to DNA has been reported for efficient DNA transformation and transfection (Mandel M. and Higa A., Calcium-dependent bacteriophage DNA infection,  J. Mol. Biol.,  1970, 53:159-162). Methods of specialized and/or generalized transduction were described (Morse M. L. et al., Transduction in  Escherichia coli  K-12,  Genetics,  1956, 41(1):142-156; Miller J. H.,  Experiments in Molecular Genetics . Cold Spring Harbor, N.Y.: Cold Spring Harbor La. Press, 1972). Other methods for random and/or targeted integration of DNA into the host microorganism can be applied, for example, “Mu-driven integration/amplification” (Akhverdyan et al.,  Appl. Microbiol. Biotechnol.,  2011, 91:857-871), “Red/ET-driven integration” or “λRed/ET-mediated integration” (Datsenko K. A. and Wanner B. L.,  Proc. Natl. Acad. Sci. USA  2000, 97(12):6640-45; Zhang Y., et al.,  Nature Genet.,  1998, 20:123-128). Moreover, for multiple insertions of desired genes in addition to Mu-driven replicative transposition (Akhverdyan et al.,  Appl. Microbiol. Biotechnol.,  2011, 91:857-871) and chemically inducible chromosomal evolution based on recA-dependent homologous recombination resulted in an amplification of desired genes (Tyo K. E. J. et al.,  Nature Biotechnol.,  2009, 27:760-765), and other methods can be used, which utilize different combinations of transposition, site-specific, and/or homologous Red/ET-mediated recombinations, and/or P1-mediated generalized transduction (see, for example, Minaeva N. et al.,  BMC Biotechnology,  2008, 8:63; Koma D. et al.,  Appl. Microbiol. Biotechnol.,  2012, 93(2):815-829). 
     There may be some differences in DNA sequences between the families, genera, species, or strains belonging to the order Enterobacterales. Therefore, the tyrB, leuA, leuB, leuC, and leuD genes are not limited to the genes having the nucleotide sequences shown in SEQ ID NOs: 1, 3, 5, 7, and 9, accordingly, but may include genes having variant nucleotide sequences of or homologous to SEQ ID NOs: 1, 3, 5, 7, and 9, accordingly, and which encode variants of the TyrB, LeuA, LeuB, LeuC, and LeuD proteins. Similarly, the TyrB, LeuA, LeuB, LeuC, and LeuD proteins are not limited to the proteins having the amino acid sequences shown in SEQ ID NOS: 4, 6, 8, and 10, but may include proteins having variant amino acid sequences of or homologous to SEQ ID NO: 4, 6, 8, and 10. 
     The phrase “a variant protein” can mean a protein having a variant amino acid sequenc. [000107] The phrase “a variant protein” can mean a protein which has one or more mutations in the amino acid sequence as compared with the wild-type amino acid sequence of the protein whether they are substitutions, deletions, insertions, and/or additions of one or several amino acid residues, but still maintains an activity or function similar to that of the wild-type protein, or the three-dimensional structure of the variant proteins is not significantly changed relative to the wild-type or non-modified protein. The number of changes in a variant protein depends on the position of amino acid residues in the three-dimensional structure of the protein or the type of amino acid residues. It can be, but is not strictly limited to, 1 to 150, in another example 1 to 100, in another example 1 to 50, in another example 1 to 30, in another example 1 to 15, in another example 1 to 10, and in another example 1 to 5, in the wild-type amino acid sequence of the protein. This is because some amino acids have high homology to one another so that the activity or function is not affected by such a change, or the three-dimensional structure of the protein is not significantly changed relative to the wild-type or non-modified protein. Therefore, the variant protein may be a protein having an amino acid sequence having a homology, defined as the parameter “identity” when using the computer program blastp, of not less than 60%, of not less than 65%, not less than 70%, not less than 75%, not less than 80%, not less than 85%, not less than 90%, not less than 95%, not less than 98%, or not less than 99% with respect to the entire wild-type amino acid sequence of the protein, as long as the activity or function of the protein is maintained, or the three-dimensional structure of the protein is not significantly changed relative to the wild-type or non-modified protein. In this specification, “homology” may mean “identity”, that is the identity of amino acid residues. The sequence identity between two sequences is calculated as the ratio of residues matching in the two sequences when aligning the two sequences to achieve a maximum alignment with each other. 
     The exemplary substitution, deletion, insertion, and/or addition of one or several amino acid residues can be a conservative mutation(s), so that the activity and features of the variant protein are maintained, or the three-dimensional structure of the protein is not significantly changed relative to the non-modified protein such as, for example, the wild-type protein. The representative conservative mutation is a conservative substitution. The conservative substitution can be, but is not limited to, a substitution, wherein substitution takes place mutually among Phe, Trp, and Tyr, if the substitution site is an aromatic amino acid; among Ala, Leu, Ile, and Val, if the substitution site is a hydrophobic amino acid; between Glu, Asp, Gln, Asn, Ser, His, and Thr, if the substitution site is a hydrophilic amino acid; between Gln and Asn, if the substitution site is a polar amino acid; among Lys, Arg, and His, if the substitution site is a basic amino acid; between Asp and Glu, if the substitution site is an acidic amino acid; and between Ser and Thr, if the substitution site is an amino acid having hydroxyl group. Examples of conservative substitutions include substitution of Ser or Thr for Ala, substitution of Gln, His, or Lys for Arg, substitution of Glu, Gln, Lys, His, or Asp for Asn, substitution of Asn, Glu, or Gln for Asp, substitution of Ser or Ala for Cys, substitution of Asn, Glu, Lys, His, Asp or Arg for Gln, substitution of Asn, Gln, Lys, or Asp for Glu, substitution of Pro for Gly, substitution of Asn, Lys, Gln, Arg, or Tyr for His, substitution of Leu, Met, Val, or Phe for Ile, substitution of Ile, Met, Val, or Phe for Leu, substitution of Asn, Glu, Gln, His, or Arg for Lys, substitution of Ile, Leu, Val, or Phe for Met, substitution of Trp, Tyr, Met, Ile or Leu for Phe, substitution of Thr or Ala for Ser, substitution of Ser or Ala for Thr, substitution of Phe or Tyr for Trp, substitution of His, Phe, or Trp for Tyr, and substitution of Met, Ile, or Leu for Val. 
     The exemplary substitution, deletion, insertion, and/or addition of one or several amino acid residues can also be a non-conservative mutation(s) provided that the mutation(s) is/are compensated by one or more secondary mutations in the different position(s) of amino acids sequence so that the activity or function of the variant protein is maintained, or the three-dimensional structure of the protein is not significantly changed relative to the non-modified protein such as, for example, the wild-type protein. 
     The calculation of a percent identity of an amino acid sequence can be carried out using the algorithm blastp. More specifically, the calculation of a percent identity of an amino acid sequence can be carried out using the algorithm blastp in the default settings of Scoring Parameters (Matrix: BLOSUM62; Gap Costs: Existence=11 Extension=1; Compositional Adjustments: Conditional compositional score matrix adjustment) provided by National Center for Biotechnology Information (NCBI). The calculation of a percent identity of a nucleotide sequence can be carried out using the algorithm blastn. More specifically, the calculation of a percent identity of a nucleotide sequence can be carried out using the algorithm blastn in the default settings of Scoring Parameters (Match/Mismatch Scores=1,−2; Gap Costs=Linear) provided by NCBI. 
     The protein homologues of TyrB native to different bacteria belonging to the order Enterobacterales are known that have the tyrosine aminotransferase activity as described above. Examples of such homologous proteins that are native to the bacteria belonging to the order Enterobacterales are described in Table 1 with accession numbers of amino acid sequences in the NCBI database (National Center for Biotechnology Information, ncbi.nlm.nih.gov/protein/), taxonomy data, and indication of a homology value (as “identity”, that is the identity of amino acids). 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 Protein homologues of TyrB. 
               
            
           
           
               
               
               
            
               
                 Accession No.* 
                 Organism 
                 Identity** 
               
               
                   
               
               
                 WP_000486985.1 
                   Escherichia coli  K-12 MG1655 
                 100%  
               
               
                 WP_128875460.1 
                 
                   Shigella dysenteriae 
                 
                 99% 
               
               
                 WP_044709161.1 
                 
                   Escherichia albertii 
                 
                 94% 
               
               
                 WP_112001357.1 
                 
                   Citrobacter koseri 
                 
                 93% 
               
               
                 EAA9367477.1 
                 
                   Salmonella enterica 
                 
                 92% 
               
               
                 WP_063941226.1 
                 
                   Enterobacter cloacae 
                 
                 91% 
               
               
                 WP_047045861.1 
                 
                   Klebsiella aerogenes 
                 
                 88% 
               
               
                 WP_090136777.1 
                 
                   Kosakonia oryziphila 
                 
                 88% 
               
               
                 WP_015742653.1 
                 
                   Cronobacter turicensis 
                 
                 87% 
               
               
                 WP_103947500.1 
                 
                   Lelliottia aquatilis 
                 
                 87% 
               
               
                 WP_034456957.1 
                 
                   Buttiauxella noackiae 
                 
                 87% 
               
               
                 WP_045289609.1 
                 
                   Pluralibacter gergoviae 
                 
                 85% 
               
               
                 WP_016517436.1 
                 
                   Cedecea davisae 
                 
                 85% 
               
               
                 WP_067434221.1 
                 
                   Erwinia gerundensis 
                 
                 83% 
               
               
                 WP_085069663.1 
                 
                   Pantoea alhagi 
                 
                 82% 
               
               
                 ADD75434.1 
                   Pantoea ananatis  LMG 20103 
                 81% 
               
               
                 WP_085982588.1 
                 
                   Pantoea agglomerans 
                 
                 80% 
               
               
                 WP_061321992.1 
                 
                   Serratia rubidaea 
                 
                 79% 
               
               
                 WP_050082921.1 
                 
                   Yersinia frederiksenii 
                 
                 78% 
               
               
                 WP_022634786.1 
                 
                   Dickeya solani 
                 
                 77% 
               
               
                 WP_116155443.1 
                 
                   Pectobactcrium aquaticum 
                 
                 76% 
               
               
                 WP_115459870.1 
                 
                   Enterobacillus tribolii 
                 
                 75% 
               
               
                 WP_094100856.1 
                 
                   Lonsdalea iberica 
                 
                 74% 
               
               
                 WP_038023955.1 
                 
                   Tatumella morbirosei 
                 
                 73% 
               
               
                 WP_074023805.1 
                 
                   Xenorhabdus eapokensis 
                 
                 72% 
               
               
                 SER25667.1 
                 
                   Rosenbergiella nectarea 
                 
                 71% 
               
               
                 WP_046974208.1 
                 
                   Photorhabdus thracensis 
                 
                 70% 
               
               
                 WP_015462403.1 
                 
                   Edwardsiella piscicida 
                 
                 69% 
               
               
                 WP_093316479.1 
                 
                   Thorsellia anophelis 
                 
                 68% 
               
               
                 WP_067400647.1 
                 
                   Morganella psychrotolerans 
                 
                 67% 
               
               
                 WP_060556387.1 
                 
                   Proteus mirabilis 
                 
                 65% 
               
               
                 WP_008913813.1 
                 
                   Providencia burhodogranariea 
                 
                 64% 
               
               
                 WP_039855722.1 
                 
                   Providencia rustigianii 
                 
                 62% 
               
               
                   
               
               
                 *in the NCBI database (National Center for Biotechnology Information, www.ncbi.nlm.nih.gov/) 
               
               
                 **Identity was calculated with respect to the TyrB protein native to  E. coli  K-12 substr. MG1655 (GenBank: NP_418478.1, WP_000486985.1) using blastp and default settings provided by the NCBI database. 
               
            
           
         
       
     
     The phrase “a variant nucleotide sequence” can mean the nucleotide sequence which encodes a protein having the wild-type amino acid sequence using any synonymous amino acid codons according to the standard genetic code table (see, for example, Lewin B., “Genes VIII”, 2004, Pearson Education, Inc., Upper Saddle River, N.J. 07458). Therefore, a gene encoding a protein having the wild-type amino acid sequence can be a gene having a variant nucleotide sequence due to the degeneracy of the genetic code. 
     The phrase “a variant nucleotide sequence” can also mean, but is not limited to, a nucleotide sequence that is able to hybridize under stringent conditions with the nucleotide sequence complementary to the wild-type nucleotide sequence or a probe that can be prepared from the nucleotide sequence provided that it encodes a protein having tyrosine aminotransferase activity. “Stringent conditions” can include those under which a specific hybrid, for example, a hybrid having homology, defined as the parameter “identity” when using the computer program blastn, of not less than 60%, not less than 65%, not less than 70%, not less than 75%, not less than 80%, not less than 85%, not less than 90%, not less than 95%, not less than 96%, not less than 97%, not less than 98%, or not less than 99% is formed, and a non-specific hybrid, for example, a hybrid having homology lower than the above is not formed. For example, stringent conditions can be exemplified by washing one time or more, or in another example, two or three times, at a salt concentration of 1×SSC (standard sodium citrate or standard sodium chloride), 0.1% SDS (sodium dodecyl sulphate), or in another example, 0.1×SSC, 0.1% SDS at 60° C. or 65° C. Duration of washing depends on the type of membrane used for blotting and, as a rule, can be what is recommended by the manufacturer. For example, the recommended duration of washing for the Amersham Hybond™-N+ positively charged nylon membrane (GE Healthcare) under stringent conditions is 15 minutes. The washing step can be performed 2 to 3 times. As the probe, a part of the sequence complementary to the wild-type nucleotide sequence may also be used. Such a probe can be produced by PCR using oligonucleotides as primers prepared based on the wild-type nucleotide sequence and a DNA fragment containing the nucleotide sequence as a template. The length of the probe is recommended to be &gt;50 bp; it can be suitably selected depending on the hybridization conditions, and is usually 100 bp to 1 kbp. For example, when a DNA fragment having a length of about 300 bp is used as the probe, the washing conditions after hybridization can be exemplified by 2×SSC, 0.1% SDS at 50° C., 60° C. or 65° C. 
     The phrase “a variant nucleotide sequence” can also mean a nucleotide sequence that encodes a variant protein. 
     As the gene encoding the wild-type protein native to the species  E. coli  has already been elucidated (see above), a variant nucleotide sequence encoding a variant protein of the wild-type protein can be obtained by PCR (polymerase chain reaction; refer to White T. J. et al., The polymerase chain reaction,  Trends Genet.,  1989, 5(6):185-189) utilizing primers prepared based on the nucleotide sequence of the wild-type gene; or the site-directed mutagenesis method by treating a DNA containing a wild-type gene in vitro, for example, with hydroxylamine, or a method for treating a microorganism, for example, a bacterium belonging to the species  E. coli  harboring a wild-type gene with ultraviolet (UV) irradiation or a mutating agent such as N-methyl-N′-nitro-N-nitrosoguanidine (NTG) and nitrous acid usually used for the such treatment; or chemically synthesized as full-length gene structure. Genes encoding the proteins or its variant proteins from other bacteria belonging to the order Enterobacterales can be obtained in a similar manner. 
     The phrase “wild-type”, which can be equivalent to the phrases “native” and “natural”, as used herein as to a protein (for example, “a wild-type protein”) and a gene (for example, “a wild-type gene”) can mean, respectively, a native protein and a native gene that exist, and/or is expressed naturally in, and/or produced by a wild-type bacterium, for example, a wild-type strain of a bacterium belonging to the order Enterobacterales such as, for example, the family Enterobacteriaceae or Erwiniaceae such as, for example, the  E. coli  MG1655 strain (ATCC 47076), the  E. coli  W3110 strain (ATCC 27325), the  P. ananatis  AJ13355 strain (FERM BP-6614), and so forth. As a protein is encoded by a gene, “a wild-type protein” can be encoded by “a wild-type gene” naturally occurring in genome of a wild-type bacterium. 
     The phrase “a wild-type protein” can mean a native protein naturally produced by a wild-type or parent bacterial strain belonging to the order Enterobacterales, for example, by the wild-type  E. coli  MG1655 strain or  P. ananatis  AJ13355 strain. A wild-type protein can be encoded by the “wild-type gene”, or the “non-modified gene” naturally occurring in genome of a wild-type bacterium. A wild-type protein and wild-type gene can have “a wild-type amino acid sequence” and “a wild-type nucleotide sequence”, accordingly, as a primary structure of proteins and genes. 
     The phrase “native to” in reference to a protein or a nucleic acid native to a particular organism such as, for example, a bacterial species can refer to a protein or a nucleic acid that is native to that organism. That is, a protein or a nucleic acid native to a particular organism can mean the protein or the nucleic acid, respectively, that exists naturally in the organism and can be isolated from that organism and sequenced using means known to the one of ordinary skill in the art. Moreover, as the amino acid sequence or the nucleotide sequence of a protein or nucleic acid, respectively, isolated from an organism in which the protein or nucleic acid exists, can easy be determined, the phrase “native to” in reference to a protein or a nucleic acid can also refer to a protein or a nucleic acid that can be obtained using, for example, a genetic engineering technique, including recombinant DNA technology, or a chemical synthesis method, or the like, so long as the amino acid sequence of the protein or the nucleotide sequence of the nucleic acid thus obtained is identical, accordingly, to the amino acid sequence of the protein or the nucleotide sequence of the nucleic acid that exists naturally in the organism. Examples of amino acid sequences native to particular species include, but are not limited to, peptides, oligopeptides, polypeptides, including proteins, specifically enzymes, and so forth. Examples of nucleotide sequences native to particular species include deoxyribonucleic acid (DNA) and ribonucleic acid (RNA), and these are not limited to expression regulatory sequences, including promoters, attenuators, terminators, and the like, genes, intergenic sequences, and nucleotide sequences encoding signal peptides, pro-moieties of proteins, artificial amino acid sequences, and so forth. Specific examples of amino acid sequences and nucleotide sequences, and homologues thereof native to various species are described herein, and these examples include the proteins having the amino acid sequences shown in SEQ ID NOs: 2, 4, 6, 8, and 10, which are native to the bacterium of the species  E. coli  and can be encoded by the genes having the nucleotide sequences shown in SEQ ID NOs: 1, 3, 5, 7, and 9, accordingly. 
     The bacterium can have, in addition to the properties already mentioned, other specific properties such as various nutrient requirements, drug resistance, drug sensitivity, and drug dependence. 
     2. Method 
     The method as described herein includes a method of producing 2-methyl-butyric acid using a bacterium as described herein. The method of producing 2-methyl-butyric acid includes the steps of cultivating (also called “culturing”) the bacterium in a culture medium to allow 2-methyl-butyric acid to be produced, excreted, and/or accumulated in the culture medium or in the bacterial cells, or both, and collecting 2-methyl-butyric acid from the culture medium and/or the bacterial cells. The method may further include, optionally, the step of purifying 2-methyl-butyric acid from the culture medium and/or the bacterial cells. 2-Methyl-butyric acid can be produced in a free form or as a salt thereof, or as a mixture of them. Hence, the phrase “2-Methyl-butyric acid” can mean, for example, 2-Methyl-butyric acid in a free form, a salt thereof, or a mixture of them. For example, sodium, potassium, ammonium, and the like salts of 2-methyl-butyric acid can be produced by the method. This is possible as carboxylic acids, to which 2-methyl-butyric acid belongs, can react under fermentation conditions with a neutralizing agent such as an inorganic or organic substance in a typical acid-base neutralization reaction to form a salt that is the chemical feature of carboxylic acids which is apparent to the person skilled in the art. 
     The cultivation of the bacterium, and collection and, optionally, purification of 2-methyl-butyric acid from the medium and the like may be performed in a manner similar to the conventional fermentation methods wherein an L-amino acid is produced using a microorganism. That is, the cultivation of the bacterium, and collection and purification of 2-methyl-butyric acid from the medium and the like may be performed by applying the conditions that are suitable for the cultivation of the bacterium, and appropriate for the collection and purification of an L-amino acid, which conditions are well-known to the persons of ordinary skill in the art. 
     The culture medium to be used is not particularly limited, so long as the medium contains, at least, a carbon source, and the bacterium as described herein can proliferate in it and produce 2-methyl-butyric acid. The culture medium can be either a synthetic or natural medium such as a typical medium that contains a carbon source, a nitrogen source, a sulphur source, a phosphorus source, inorganic ions, and other organic and inorganic components as required. As the carbon source, saccharides such as glucose, sucrose, lactose, galactose, fructose, arabinose, maltose, xylose, trehalose, ribose, and hydrolyzates of starches; alcohols such as ethanol, glycerol, mannitol, and sorbitol; organic acids such as gluconic acid, fumaric acid, citric acid, malic acid, and succinic acid; fatty acids, and the like can be used. As the nitrogen source, inorganic ammonium salts such as ammonium sulfate, ammonium chloride, and ammonium phosphate; organic nitrogen such as of soy bean hydrolysate; ammonia gas; aqueous ammonia; and the like can be used. Furthermore, peptone, yeast extract, meat extract, malt extract, corn steep liquor, and so forth can also be utilized. The medium may contain one or more types of these nitrogen sources. The sulphur source can include ammonium sulphate, magnesium sulphate, ferrous sulphate, manganese sulphate, and the like. The medium can contain a phosphorus source in addition to the carbon source, the nitrogen source and the sulphur source. As the phosphorus source, potassium dihydrogen phosphate, dipotassium hydrogen phosphate, phosphate polymers such as pyrophosphoric acid and so forth can be utilized. Vitamins such as vitamin B1, vitamin B2, vitamin B6, nicotinic acid, nicotinamide, vitamin B12, required substances, for example, organic nutrients such as nucleic acids such as adenine and RNA, amino acids, peptone, casamino acid, yeast extract, and the like may be present in appropriate, even if trace, amounts. Other than these, small amounts of calcium phosphate, iron ions, manganese ions, and so forth may be added, if necessary. 
     Cultivation can be performed under the conditions suitable for cultivating a bacterium chosen for the use in the method for producing 2-methyl-butyric acid. For example, the cultivation can be performed under aerobic conditions for from 16 to 72 hours or for from 16 to 65 hours, the culture temperature during cultivation can be controlled within from 30 to 45° C. or within from 30 to 37° C., and the pH can be adjusted between 5 and 8 or between 6.0 and 7.5. The pH can be adjusted using an inorganic or organic acidic or alkaline substance such as urea, calcium carbonate or ammonia gas. 
     After cultivation, 2-methyl-butyric acid can be collected from the culture medium. Specifically, 2-methyl-butyric acid present outside of cells can be collected from the culture medium. Also, after cultivation, 2-methyl-butyric acid can be collected from cells of the bacterium. Specifically, the cells can be disrupted, a supernatant can be obtained by removing solids such as the cells and the cell-disrupted suspension (so-called cell debris), and then 2-methyl-butyric acid can be collected from the supernatant. Disruption of the cells can be performed using, for example, methods that are well-known in the art, for example, ultrasonic lysis using high frequency sound waves, or the like. Removal of solids can be performed by, for example, centrifugation or membrane filtration. Collection of 2-methyl-butyric acid from the culture medium or the supernatant etc. can be performed using, for example, conventional techniques such as, for example, concentration, crystallization, membrane treatment, ion-exchange chromatography, flash chromatography, thin-layer chromatography, medium or high pressure liquid chromatography. These methods may be independently used, or may be used in an appropriate combination. 
     EXAMPLES 
     The present invention will be more precisely explained below with reference to the following non-limiting examples. 
     Example 1 
     Construction of  E. coli  Strain L1190-1 
     A 2-methyl-butyric acid-producing strain was constructed from an L-isoleucine-producing  E. coli  strain NS1547. The NS1547 strain had the MG1655 Δtdh rhtA*mini-Mu::P lac -lacI-ilvA*P L -SD1-ilvG*MEDA genotype. 
     The strain NS1547 has a mutation in the rhtA gene (rhtA23) (US20010049126 A1) that confers the resistance to high concentration of threonine (&gt;40 mg/mL) or homoserine (&gt;5 mg/mL) and improves threonine production, and the deletion of the tdh gene (SEQ ID NO: 11) (Shakalis I. O. and Gusyatiner M. M., Participation of threonine dehydrogenase, threonine desaminase and serine transhydroxymethylase in threonine degradation in  Escherichia coli  cells K-12,  Biotekhnologiya,  1990, (2):16-17) that prevents threonine degradation. The strain NS1547 also contains the expression cassette mini-Mu: P lac -lacI-ilvA*-kan (SEQ ID NO: 12) (Sycheva E. V. et al., Aerobic catabolism of threonine in  Escherichia coli  strain with feedback resistant biosynthetic threonine deaminase,  Biotekhnologiya,  2003, (4):22-34), which includes the mutant ilvA gene encoding feedback-resistant threonine deaminase and the lacI repressor gene under the control of P lac  promoter. This cassette allows the IPTG-inducible expression of threonine deaminase, which carries out the first step in isoleucine biosynthesis pathway such as the conversion of threonine into 2-ketobutyrate. To ensure a sufficient 2-keto-3-methyl-valerate supply for 2-methyl-butyric acid production, the strain NS1547 possesses the expression cassette P L -SD1-ilvG*MEDA (SEQ ID NO: 13) which provides the expression of ilvG*MEDA operon under control of the lambda phage P L  promoter operably linked to the modified Shine-Dalgarno sequence named as SD1 (SD sequence from pET22b(+) plasmid (Novagen). This artificial operon (ilvG*MEDA) also contains the mutant ilvG gene (ilvG*) having the insertion of two base pairs (aa) between the nucleotides in position numbers 981 and 982 from the start-codon of the gene, upstream of the sequence TGACTGGCA (EP1627884 B1) restoring the frame-shift in the wild-type ilvG gene that provides the expression of feedback-resistant acetolactate synthase II. 
     The cassette dacA::mini-Mu::cat-P thr -attB-thrA*BC (SEQ ID NO: 14) that provides the expression of the threonine operon from which a region required for attenuation is removed, was introduced into the strain NS1547 by P1-transduction (Sambrook J. et al., “Molecular Cloning A Laboratory Manual, 2nd Edition”, Cold Spring Harbor Laboratory Press (1989)). The threonine operon contains a mutation in thrA gene (thrA442) (Akhverdyan V. Z. et al., Development of the mini-Mu system providing effective integration and amplification of the genetic material into the  Escherichia coli  chromosome,  Biotekhnologiya,  2007, (3):3-20), which confers aspartokinase I-homoserine dehydrogenase I insensitivity to feedback inhibition by threonine. This cassette is localized in the  E. coli  dacA gene and it is flanked by the phage mini-Mu L/R-ends for mini-Mu-mediated transposition (Akhverdyan V. Z. et al., Application of the bacteriophage Mu-driven system for the integration/amplification of target genes in the chromosomes of engineered Gram-negative bacteria—mini review,  Appl. Microbiol. Biotechnol.,  2011, 91(4):857-871), with the chloramphenicol resistance (Cm R ) marker inserted into the Mu-attL end. 
     Cm R  transductants were selected on the plates containing LB-medium (also referred to as Luria-Bertani medium as described in Sambrook, J. and Russell, D. W. “Molecular Cloning: A Laboratory Manual”, 3 rd  ed., Cold Spring Harbor Laboratory Press (2001)), agar (1.5%) and Cm (20 mg/L) and verified by PCR using primers P1 (SEQ ID NO: 15) and P2 (SEQ ID NO: 16). Conditions for PCR verification were as follows: denaturation step for 5 min at 94° C.; profile for the 25 cycles: 30 sec at 94° C., 30 sec at 57° C., 1 min at 72° C.; final elongation for 7 min at 72° C. DNA fragment 1 (SEQ ID NO: 17), obtained in the reaction with DNA of the cells of the parental strain NS1547 as a template, was 1256 bp in length. DNA fragment 2 (SEQ ID NO: 18), obtained in the reaction with DNA of the cells of the strain NS1547 dacA::mini-Mu::cat-P thr -attB-thrA*BC as a template, was 2452 bp in length. After that, λ-Int/Xis-mediated excision of the Cm R -marker from the strain NS1547 dacA::mini-Mu::cat-P thr attB-thrA*BC was performed. As a result, the strain L1178-1 was obtained. Excision was confirmed by PCR as described above. DNA fragment 2 (SEQ ID NO: 18), obtained in the reaction with DNA of the cells of the parental strain NS1547 dacA::mini-Mu::cat-P thr attB-thrA*BC as a template, was 2452 bp in length. DNA fragment 3 (SEQ ID NO: 19), obtained in the reaction with DNA of the cells of the strain L1178-1 as a template, was 842 bp in length. 
     The expression cassette ΔcynX::cat-P L -ilvA* (SEQ ID NO: 20) was introduced into the strain L1178-1 by P1-transduction. This cassette is localized in the  E. coli  cynX gene and includes the mutant ilvA* gene (iivA 1237 ) having the replacement of g with a at position number 1237 from the start-codon of the gene that results in replacement of Glu 412  with Lys (Hashiguchi K. et al., Construction of an L-isoleucine overproducing strain of  Escherichia coli  K-12,  Biosci. Biotechnol. Biochem.,  1999, 63(4):672-679), encoding feedback resistant threonine deaminase, under control of the lambda phage P L  promoter. 
     Cm R  transductants were selected and verified by PCR using primers P3 (SEQ ID NO: 21) and P4 (SEQ ID NO: 22). Conditions for PCR verification were as follows: denaturation for 5 min at 94° C.; profile for 25 cycles: 30 sec at 94° C., 30 sec at 59° C., 1 min at 72° C.; final elongation for 7 min at 72° C. There was no DNA fragment in the reaction with DNA of the cells of the parent strain L1178-1 used as a template. DNA fragment 4 (SEQ ID NO: 23), obtained in the reaction with DNA of the cells of the strain L1178-1 ΔcynX:cat-P L -ilvA* as a template, was 2131 bp in length. After that, λ-Int/Xis-mediated excision of Cm R -marker from the strain L1178-1 ΔcynX::cat-P L -ilvA* was performed. As a result, the strain L1190-1 was obtained. Excision was confirmed by PCR as described above. DNA fragment 4 (SEQ ID NO: 23), obtained in the reaction with DNA of the cells of the parent strain L1178-1 ΔcynX:cat-P L -ilvA* used as a template, was 2131 bp in length. DNA fragment 5 (SEQ ID NO: 24), obtained in the reaction with DNA of the cells of the strain L1190-1 as a template, was 534 bp in length. 
     Example 2 
     Constuction of  E. coli  Strain L1194-2 
     To provide 2-methyl-butyric acid biosynthesis, 2-methyl-butyric acid biosynthesis genes were inserted into the chromosome of the strain L1190-1 (Example 1). The plasmid pAH162-tetA-tetR-kdcA-aldH was used which contains the heterologous kdcA gene native to Lactococcus lactis having codons optimized for the expression in  E. coli  (Savrasova E. A. et al., Use of the valine biosynthetic pathway to convert glucose into isobutanol,  J. Ind. Microbiol. Biotechnol.,  2011, 38(9):1287-1294) and the aldH gene native to  E. coli . The promoter-less 3195 bp DNA fragment of kdcA-aldH (SEQ ID NO: 25) was inserted into the integrative vector pAH162-λaaL-Tc R -λattR (Minaeva N. I. et al., Dual-In/Out strategy for genes integration into bacterial chromosome: a novel approach to step-by-step construction of plasmid-less marker-less recombinant  E. coli  strains with predesigned genome structure,  BMC Biotechnol.,  2008, 8:63) digested with MlsI/Sa/I The transformants of the  E. coli  CC118 λpir +  strain (Herrero M. et al., Transposon vectors containing non-antibiotic resistance selection markers for cloning and stable chromosomal insertion of foreign genes in gram-negative bacteria,  J. Bacteriol.,  1990, 172:6557-6567) carrying the recombinant plasmid pAH162-tetA-tetR-kdcA-aldH, were selected on the medium supplemented with tetracycline (Tc). The correct insertion was confirmed by restriction analysis and PCR analysis using primers P5 (SEQ ID NO: 26) and P6 (SEQ ID NO: 27), P7 (SEQ ID NO: 28) and P8 (SEQ ID NO: 29). Conditions for PCR verification were as follows: denaturation for 5 min at 94° C.; profile for 25 cycles: 30 sec at 94° C., 30 sec at 61° C., 1 min at 72° C.; final elongation for 7 min at 72° C. There was no DNA fragment in the reaction with DNA of the cells of the parent CC118 λpir +  strain used as a template. DNA fragment 6 (SEQ ID NO: 30), obtained in the reaction with DNA of the cells of CC118 λpir +  pAH162-tetA-tetR-kdcA-aldH strain as a template and primers P5 (SEQ ID NO: 26) and P6 (SEQ ID NO: 27), was 1045 bp in length. DNA fragment 7 (SEQ ID NO: 31), obtained in the reaction with DNA of the cells of CC118 λpir +  pAH162-tetA-tetR-kdcA-aldH strain as a template and primers P7 (SEQ ID NO: 28) and P8 (SEQ ID NO: 29), was 865 bp in length. 
     Then, the plasmid pAH162-tetA-tetR-kdcA-aldH was integrated into the artificial locus trs5-10::φ80-attB of the  E. coli  strain MG1655 Δ(φ80-attB) trs5-10::φ80-attB (Minaeva N. I. et al., Dual-In/Out strategy for genes integration into bacterial chromosome: a novel approach to step-by-step construction of plasmid-less marker-less recombinant  E. coli  strains with predesigned genome structure,  BMC Biotechnol.,  2008, 8:63) containing the plasmid pAH123 by φ80-driven integration. The cells of the strain MG1655 Δ(φ80-attB), that harbor trs5-10::pAH162-tetA-tetR-kdcA-aldH cassette, were selected on the plates containing LB-medium, agar (1.5%) and Tc (40 mg/L). The correct integration was confirmed by PCR using primers P9 (SEQ ID NO: 32) and P10 (SEQ ID NO: 33). Conditions for PCR verification were as follows: denaturation for 5 min at 94° C.; profile for 25 cycles: 30 sec at 94° C., 30 sec at 57° C., 1 min at 72° C.; final elongation for 7 min at 72° C. There was no DNA fragment in the reaction with DNA of the cells of the parent strain MG1655 Δ(φ80-attB) trs5-10::φ80-attB used as a template. DNA fragment 8 (SEQ ID NO: 34), obtained in the reaction with DNA of the cells of the strain MG1655 Δ(φ80-attB) trs5-10::pAH162-tetA-tetR-kdcA-aldH as a template, was 1629 bp in length. After λ-Int/Xis-mediated excision of the vector part of integrated plasmid, the strain MG1655 Δ(φ80-attB) trs5-10::kdcA-aldH was obtained. Excision of Tc R -marker was confirmed by PCR using primers P9 (SEQ ID NO: 32) and P11 (SEQ ID NO: 35). Conditions for PCR verification were as follows: denaturation for 5 min at 94° C.; profile for 25 cycles: 30 sec at 94° C., 30 sec at 57° C., 1 min at 72° C.; final elongation for 7 min at 72° C. DNA fragment 9 (SEQ ID NO: 36), obtained in the reaction with DNA of the cells of the strain MG1655 Δ(φ80-attB) trs5-10::pAH162-tetA-tetR-kdcA-aldH as a template, was 4603 bp in length. DNA fragment 10 (SEQ ID NO: 37), obtained in the reaction with DNA of the cells of the strain MG1655 Δ(φ80-attB) trs5-10::kdcA-aldH as a template, was 1312 bp in length. 
     Then, to ensure a high level of constitutive expression, the regulatory region of the P tac  promoter (De Boer H. A. et al., The tac promoter: a functional hybrid derived from the trp and lac promoters,  Proc. Natl. Acad. Sci. USA,  1983, 80(1): 21-25) containing a half part of lac repressor binding site (FIG.2), marked with the cat gene, was introduced upstream to the kdcA-aldH operon by the method developed by Datsenko and Wanner (Datsenko K. A. and Wanner B. L.,  Proc. Natl. Acad. Sci. USA,  2000, 97(12):6640-6645) called “λRed-dependent integration”. According to this procedure, the PCR primers P12 (SEQ ID NO: 38) and P13 (SEQ ID NO: 39), which are homologous to both the region adjacent to the kdcA gene and the region adjacent to the cat gene and the P tac  promoter in the template chromosome, were constructed. The chromosome of the  E. coli  strain MG1655 cat-P tac -lacZ (Katashkina J. I. et al., Tuning of expression level of the genes of interest located in the bacterial chromosome,  Mol. Biol . (Mosk.), 2005, 39(5):719-726) was used as a template in PCR reaction. Conditions for PCR were as follows: denaturation for 3 min at 95° C.; profile for two first cycles: 1 min at 95° C., 30 sec at 34° C., 80 sec at 72° C.; profile for the last 28 cycles: 30 sec at 95° C., 30 sec at 50° C., 80 sec at 72° C.; final step: 5 min at 72° C. The resulting 1768 bp DNA fragment 11 (SEQ ID NO: 40) was purified by “Silica Bead DNA Gel Extraction Kit” (Thermo Scientific) and used for electroporation of the strain MG1655 Δ(φ80-attB) trs5-10::kdcA-aldH containing the plasmid pKD46. The plasmid pKD46 (Datsenko K. A. and Wanner B. L.,  Proc. Natl. Acad. Sci. USA,  2000, 97(12):6640-6645) includes a 2154 bp (position numbers from 31088 to 33241) DNA fragment of phage λ (GenBank, accession No. J02459) and contains genes of the λRed homologous recombination system ((gamma, beta, exo genes) under the control of arabinose-inducible P araB  promoter. The plasmid pKD46 is necessary to integrate the DNA fragment into the bacterial chromosome. 
     Electrocompetent cells were prepared as follows: the strain MG1655 Δ(φ80-attB) trs5-10::kdcA-aldH, containing pKD46 plasmid, was grown overnight at 30° C. in LB medium containing ampicillin (100 mg/L), and the culture was diluted in 100 times with 5 mL of SOB medium (Sambrook J. et al., “Molecular Cloning: A Laboratory Manual”, 2 nd  ed., Cold Spring Harbor Laboratory Press (1989)) with ampicillin (100 mg/L) and L-arabinose (1 mM). The cells were grown with aeration (250 rpm) at 30° C. to an OD 600  of about 0.6 and then made electrocompetent by concentrating 100-fold and washing three times with ice-cold deionized H 2 O. Electroporation was performed using 200 mkL of cells and about 100 ng of DNA fragment 11 (SEQ ID NO: 40). Then, cells were incubated with 1 mL of SOC medium (Sambrook J. et al., “Molecular Cloning: A Laboratory Manual”, 2 nd  ed., Cold Spring Harbor Laboratory Press (1989)) at 37° C. for 2.5 h, placed onto the plates containing LB-medium, agar (1.5%) and chloramphenicol (20 μg/mL), and grown at 37° C. to select chloramphenicol resistant recombinants. Then, to eliminate pKD46 plasmid, one passage on L-agar with Cm (20 μg/mL) at 42° C. was performed, and the obtained individual colonies were tested for sensitivity to ampicillin. Thus, the strain MG1655 Δ(φ80-attB) trs5-10::cat-Ptac-kdcA-aldH was selected. The introduction of P tac  promoter was confirmed by PCR using primers P14 (SEQ ID NO: 41) and P6 (SEQ ID NO: 27). Conditions for PCR were as follows: denaturation for 5 at 94° C.; profile for 25 cycles: 30 sec at 94° C., 30 sec at 59° C., 1 min at 72° C.; final elongation for 7 min at 72° C. There was no DNA fragment in the reaction with DNA of the cells of the parent strain MG1655 Δ(φ80-attB) trs5-10::kdcA-aldH used as a template. DNA fragment 12 (SEQ ID NO: 42), obtained in the reaction with DNA of the cells of the strain MG1655 Δ(φ80-attB) trs5-10::cat-P tac -kdcA-aldH as a template, was 1030 bp in length. 
     At the last step, the expression cassette trs5-10::cat-P tac -kdcA-aldH was introduced into the chromosome of the strain L1190-1 (Example 1) by P1-transduction. The cells of the strain L1190-1, that harbor trs5-10::cat-Ptac-kdcA-aldH cassette, were selected on the plates containing LB-medium, agar (1.5%) and Cm (20 mg/L). Introduction of the trs5-10::cat-P tac -kdcA-aldH cassette was verified by PCR as described above. After λ-Int/Xis-mediated excision of Cm R -marker, the strain L1194-2 was obtained. Excision was confirmed by PCR using primers P15 (SEQ ID NO: 43) and P11 (SEQ ID NO: 35). DNA fragment 13 (SEQ ID NO: 44), obtained in the reaction with DNA of the cells of the parent strain L1190-1 trs5-10::cat-P tac -kdcA-aldH used as a template, was 2777 bp in length. DNA fragment 14 (SEQ ID NO: 45), obtained in the reaction with DNA of the cells of the strain L1194-2 as a template, was 1167 bp in length. 
     Example 3 
     Construction of an  E. coli  2-Methyl-Butyric Acid-Producing Strain L1201-1 
     The expression cassette P L -ilvG*M-ΔilvE::cat-DA was introduced into the strain L1194-2 (Example 2). The MG1655 strain having modification P L -ilvG*M-ΔilvE::cat-DA was constructed by the method of Red-dependent integration described above. According to this procedure, the PCR primers P16 (SEQ ID NO: 46) and P17 (SEQ ID NO: 47) homologous to both the region adjacent to the ilvE gene and the gene conferring chloramphenicol resistance in the template plasmid, were constructed. The plasmid pMW118-attL-cat-attR (Katashkina J. I. et al., Tuning of expression level of the genes of interest located in the bacterial chromosome,  Mol. Biol . (Mosk), 2005, 39(5):719-726) was used as a template in the PCR reaction. Conditions for PCR were as follows: denaturation for 3 min at 95° C.; profile for two first cycles: 1 min at 95° C., 30 sec at 34° C., 80 sec at 72° C.; profile for the last 28 cycles: 30 sec at 95° C., 30 sec at 50° C., 80 sec at 72° C.; final elongation for 5 min at 72° C. The obtained DNA fragment 15 (1713 bp) (SEQ ID NO: 48) was purified by “Silica Bead DNA Gel Extraction Kit” (Thermo Scientific) and used for electroporation of the strain MG1655 P L -SD1-ilvG*MEDA, containing the plasmid pKD46. Cm R  recombinants were selected and the deletion of the ilvE gene was verified by PCR using primers P18 (SEQ ID NO: 49) and P19 (SEQ ID NO: 50). Conditions for PCR verification were as follows: denaturation for 3 min at 95° C.; profile for the 25 cycles: 30 sec at 95° C., 30 sec at 59° C., 1 min at 72° C.; final elongation for 7 min at 72° C. DNA fragment 16 (SEQ ID NO: 51), obtained in the reaction with DNA of the cells of the parent strain MG1655 P L -SD1-ilvG*MEDA as a template, was 1354 bp in length. DNA fragment 17 (SEQ ID NO: 52), obtained in the reaction with DNA of the cells of the strain MG1655 P L -ilvG*M-ΔilvE::cat-DA as a template, was 2015 bp in length. 
     Then, the expression cassette P L -ilvG*M-ΔilvE::cat-DA was introduced into the strain L1194-2 by P1-transduction. Cm R  cells of the strain L1194-2 that harbor the cassette P L -ilvG*M-ΔilvE: :cat-DA were selected. The replacement of the cassette P L -SD1-ilvG*MEDA by the cassette P L -ilvG*M-ΔilvE::cat-DA in the strain L1194-2 was confirmed using PCR as described above. After λ-Int/Xis-mediated excision of Cm R -marker, the IlvE-deficient strain L1201-1 was obtained. Excision was confirmed by PCR using primers P18 (SEQ ID NO: 49) and P19 (SEQ ID NO: 50) as described above. DNA fragment 17 (SEQ ID NO: 52), obtained in the reaction with DNA of the cells of the parent strain L1194-2 P L -ilvG*M-ΔilvE::cat-DA as a template, was 2015 bp in length. DNA fragment 18 (SEQ ID NO: 53), obtained in the reaction with DNA of the cells of the strain L1201-1 as a template, was 405 bp in length. 
     Example 4 
     Construction of  E. coli  Strain L1201-1 ΔtyrB::cat 
     The tyrB gene deletion in the chromosome of  E. coli  MG1655 strain (ATCC 47076) was constructed by the method of Red-dependent integration described above. According to this procedure, the PCR primers P20 (SEQ ID NO: 54) and P21 (SEQ ID NO: 55) homologous to both the region adjacent to the tyrB gene and the gene conferring chloramphenicol resistance in the template plasmid, were constructed. The plasmid pMW118-attL-cat-attR was used as a template in PCR reaction. Conditions for PCR were as follows: denaturation for 3 min at 95° C.; profile for two first cycles: 1 min at 95° C., 30 sec at 34° C., 80 sec at 72° C.; profile for the last 28 cycles: 30 sec at 95° C., 30 sec at 50° C., 80 sec at 72° C.; final elongation for 5 min at 72° C. The obtained DNA fragment 19 (1713 bp) (SEQ ID NO: 56) was purified by “Silica Bead DNA Gel Extraction Kit” (Thermo Scientific) and used for electroporation of the  E. coli  strain MG1655 containing the plasmid pKD46. Cm R  recombinants were selected and the deletion of the tyrB gene marked with Cm R  gene in selected mutants was verified by PCR using primers P22 (SEQ ID NO: 57) and P23 (SEQ ID NO: 58). Conditions for PCR verification were as follows: denaturation for 3 min at 95° C.; profile for the 25 cycles: 30 sec at 95° C., 30 sec at 55° C., 1 min at 72° C.; final elongation for 7 min at 72° C. DNA fragment 20 (SEQ ID NO: 59), obtained in the reaction with DNA of the cells of the parent strain MG1655 as a template, was 1430 bp in length. DNA fragment 21 (SEQ ID NO: 60), obtained in the reaction with DNA of the cells of the strain MG1655 ΔtyrB::cat as a template, was 1894 bp in length. Thus, the strain MG1655 ΔtyrB::cat was obtained. 
     The strain MG1655 ΔtyrB::cat was used as a donor for P1-transduction of the tyrB gene deletion into the strain L1201-1 (Example 3). Cm R  recombinants were selected and the tyrB gene deletion in selected mutants was verified by PCR as described above. As a result, the strain L1201-1 ΔtyrB::cat was obtained. 
     Example 5 
     Production of 2-Methyl-Butyric Acid and Byproduct Substances using  E. coli  L1201-1 AtyrB::cat strain 
     The modified strain L1201-1 ΔtyrB::cat and the control strain L1201-1 were each cultivated at 37° C. for 6 hours in LB-medium. Then, 0.1 mL of the obtained culture was inoculated into 2 mL of a fermentation medium supplemented with Ile, Val and Leu (200 mg/L each) for the strain L1201-1 and Ile, Val, Leu, Tyr and Phe (200 mg/L each) for the strain L1201-1 ΔtyrB::cat in 20×200-mm test tubes and cultivated at 30° C. for 66 hours on a rotary shaker at 238 rpm. After cultivation, the accumulated 2-methyl-butyric acid and isobutyric acid were measured using GC (gas chromatography) analysis (Auxiliary example 1). The accumulated 3-methyl-butyric acid was measured using GC-MS (gas chromatography mass-spectrometry) analysis (Auxiliary example 2). 
     The results of three independent test-tube fermentations are shown in Table 2. As one can see from the Table 2, the modified L1201-1 ΔtyrB::cat strain was able to accumulate 3-methyl-butyric acid (3-MB) in an amount lower as compared with the control L1201-1 strain. As one can also see from the Table 2, the modified L1201-1 ΔtyrB::cat strain was able to accumulate isobutyric acid (IBA) in an amount lower as compared with the control L1201-1 strain. 
     
       
         
           
               
             
               
                 TABLE 2 
               
             
            
               
                   
               
               
                 Production of 2-methyl-butyric acid (2-MB), 3-methyl- 
               
               
                 butyric acid (3-MB), and isobutyric acid (IBA) using 
               
               
                   E. coli  strains L1201-1 and L1201-1 ΔtyrB::cat 
               
            
           
           
               
               
               
               
               
               
               
            
               
                   
                   
                   
                   
                 3-MB/ 
                   
                 IBA/ 
               
               
                   
                   
                 2-MB, 
                 3-MB, 
                 2-MB, 
                 IBA, 
                 2-MB, 
               
               
                 Strain 
                 OD 600   
                 g/L 
                 g/L 
                 % 
                 g/L 
                 % 
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
            
               
                 L1201-1 
                 10.8 
                 5.8 
                 1.33 
                 23 
                 3.9 
                 67 
               
               
                 L1201-1 
                 9.2 
                 5.6 
                 1.01 
                 18 
                 2.4 
                 42 
               
               
                 ΔtyrB::cat 
               
               
                   
               
            
           
         
       
     
     Example 6 
     Construction of  E. coli  Strain L1201-1 ΔtyrB ΔleuABCD::cat 
     The leuABCD operon deletion in the chromosome of  E. coli  MG1655 strain was constructed by the method of Red-dependent integration described above. According to this procedure, the PCR primers P24 (SEQ ID NO: 61) and P25 (SEQ ID NO: 62) homologous to both the region adjacent to the leuABCD operon and the gene conferring chloramphenicol resistance in the template plasmid, were constructed. The plasmid pMW118-attL-cat-attR was used as a template in PCR reaction. Conditions for PCR were as follows: denaturation for 3 min at 95° C.; profile for two first cycles: 1 min at 95° C., 30 sec at 34° C., 80 sec at 72° C.; profile for the last 28 cycles: 30 sec at 95° C., 30 sec at 50° C., 80 sec at 72° C.; final elongation for 5 min at 72° C. The obtained DNA fragment 22 (1713 bp) (SEQ ID NO: 63) was purified by “Silica Bead DNA Gel Extraction Kit” (Thermo Scientific) and used for electroporation of the  E. coli  strain MG1655, containing the plasmid pKD46. Cm R  recombinants were selected and the deletion of the leuABCD operon marked with Cm R  gene in selected mutants was verified by PCR using primers P26 (SEQ ID NO: 64) and P27 (SEQ ID NO: 65). Conditions for PCR verification were as follows: denaturation for 3 min at 95° C.; profile for the 25 cycles: 30 sec at 95° C., 30 sec at 55° C., 1 min at 72° C.; final elongation for 7 min at 72° C. DNA fragment 23 (SEQ ID NO: 66), obtained in the reaction with DNA of the cells of the parent strain MG1655 as a template, was 5172 bp in length. DNA fragment 24 (SEQ ID NO: 67), obtained in the reaction with DNA of the cells of the strain MG1655 ΔleuABCD::cat as a template, was 1868 bp in length. As a result, the strain MG1655 ΔleuABCD::cat was obtained. 
     The strain MG1655 ΔleuABCD::cat was used as a donor for P1-transduction of the leuABCD operon deletion into the strain L1201-1 ΔtyrB. The strain L1201-1 ΔtyrB was obtained from the strain L1201-1 ΔtyrB::cat (Example 4) by λ-Int/Xis-mediated excision of Cm R -marker. Excision was confirmed by PCR using primers P22 (SEQ ID NO: 57) and P23 (SEQ ID NO: 58). Conditions for PCR were as follows: denaturation for 3 min at 95° C.; profile for the 25 cycles: 30 sec at 95° C., 30 sec at 55° C., 1 min at 72° C.; final elongation for 7 min at 72° C. DNA fragment 21 (SEQ ID NO: 60), obtained in the reaction with DNA of the cells of the parent strain L1201-1 ΔtyrB::cat as a template, was 1894 bp in length. DNA fragment 25 (SEQ ID NO: 68), obtained in the reaction with DNA of the cells of the strain L1201-1 ΔtyrB as a template, was 284 bp in length. Cm R  recombinants were obtained and the deletion of the leuABCD operon in selected mutants was verified by PCR as described above. Thus, the strain L1201-1 ΔtyrB ΔleuABCD::cat was obtained. 
     Example 7 
     Production of 2-Methyl-Butyric Acid and a Byproduct Substance using  E. coli  L1201-1 ΔtyrB ΔleuABCD::cat strain 
     The modified strain L1201-1 ΔtyrB ΔleuABCD::cat and the control strain L1201-1 ΔtyrB::cat were each cultivated at 37° C. for 6 hours in LB-medium. Then, 0.1 mL of the obtained culture was inoculated into 2 mL of a fermentation medium supplemented with Ile, Val, Leu, Tyr and Phe (200 mg/L each) in 20×200-mm test tubes and cultivated at 30° C. for 66 hours on a rotary shaker at 238 rpm. After cultivation, the accumulated 2-methyl-butyric acid was measured using GC analysis (Auxiliary example 1). The accumulated 3-methyl-butyric acid was measured using GC-MS analysis (Auxiliary example 2). 
     The results of three independent test-tube fermentations are shown in Table 3. As one can see from the Table 3, the modified strain L1201-1 ΔtyrB ΔleuABCD::cat was able to accumulate 3-methyl-butyric acid in a lower amount as compared with the control L1201-1 strain. 
     
       
         
           
               
             
               
                 TABLE 3 
               
             
            
               
                   
               
               
                 Production of 2-methyl-butyric acid (2 MB) and 3 methyl- 
               
               
                 butyric acid (3-MB) using  E. coli  strains L1201-1 
               
               
                 ΔtyrB::cat and L1201-1 ΔtyrB ΔleuABCD::cat 
               
            
           
           
               
               
               
               
               
            
               
                   
                   
                   
                   
                 3-MB/ 
               
               
                   
                   
                 2-MB, 
                 3-MB, 
                 2-MB, 
               
               
                 Strain 
                 OD 600   
                 g/L 
                 g/L 
                 % 
               
               
                   
               
            
           
           
               
               
               
               
               
            
               
                 L1201-1 ΔtyrB::cat 
                 9.2 
                 5.6 
                 1.01 
                 18 
               
               
                 L1201-1 ΔtyrB ΔleuABCD::cat 
                 9.0 
                 4.7 
                 0.23 
                 5 
               
               
                   
               
            
           
         
       
     
     Auxiliary example 1. GC analysis of 2-methyl-butyric acid and isobutyric acid 
     Shimadzu GC-2014 Gas chromatography system equipped with a flame ionization detector (FID) was used for the analysis of 2-methyl-butyric acid and isobutyric acid in a fermentation medium. Samples were resuspended and mixed with a 1% (v/v) solution of formic acid in ethanol. Then, the samples were agitated on a vortex for 3 minutes and centrifuged at 13,000 rpm for 5 minutes. A supernatant was used for direct analysis of 2-methyl-butyric acid and isobutyric acid. Standards were dissolved directly in a mixture of ethanol and 1% (v/v) of formic acid. Calibration range was from 5 to 160 mg/L. Other parameters were as follows: 
     Column: InertCap Pure-WAX (GL science Inc.) 
     Column size: I.D. 0.25 mm, length 30 m, film thickness 0.5 μm 
     Column gradient: 65° C. (5 min hold)-5° C./min-200° C.-10° C./min-240° C. 
     Injector temperature: 240° C. 
     Injection amount: 1.0 μL 
     Injection mode split: 1:10 
     Carrier gas: He 
     Control mode: line velocity 
     Pressure: 89.0 kPa 
     Total flow: 14.0 mL/min 
     Column flow: 1.0 mL/min 
     Line velocity: 26.1 cm/sec 
     Purge flow: 3.0 mL/min 
     Detector: FID 260° C. 
     Before injection: washing with the sample, 8 μL (3 times) 
     After injection: washing with ethanol, 8 μL (3 times) 
     Auxiliary example 2. GC-MS analysis of 3-methyl-butyric acid 
     Supernatants of fermentation media were collected from two tubes, combined to 2 mL, and filtered using 0.2 μm pore size filter to remove cells. Then, 1 mL of diluted sample was used for GC-MS analysis. GC-FID analysis was accomplished on Agilent 7890A chromatograph equipped with SUPELCO β-DEX 120 column. Column size was as follows: I.D. 0.25 mm, length 30 m, film thickness 0.25 βm; limit of detection (LOD) was 0.05 mg/L. 
     While the invention has been described in detail with reference to exemplary embodiments thereof, it will be apparent to the one of ordinary skill in the art that various changes can be made, and equivalents employed, without departing from the scope of the invention. 
     INDUSTRIAL APPLICABILITY 
     The method of the present invention is useful for the production of highly pure 2-methyl-butyric acid by fermentation of a bacterium.