1. Field of the Invention
The present invention relates to a method for producing an L-amino acid such as L-cysteine. More precisely, the present invention relates to a bacterium suitable for production of an L-amino acid and a method for producing an L-amino acid utilizing such a bacterium. L-Amino acids are used in various fields, for example, as seasonings, food additives, animal feed additives, chemicals, drugs, and so forth.
2. Background Art
L-Amino acids are industrially produced by fermentation using microorganisms belonging to the genus Brevibacterium, Corynebacterium, Escherichia, or the like. In such methods, strains are used which are isolated from nature or artificial variants of such strains. Furthermore, microorganism strains can be used which are modified by recombinant DNA techniques so that the activity of a basic L-amino acid biosynthesis enzyme is increased, and so forth (EP 0643135 B, EP 0733712 B, EP 1477565 A, EP 0796912 A, EP 0837134 A, WO01/53459, EP 1170376 A, WO2005/010175, WO96/17930, WO2006/013807).
Furthermore, L-cysteine, for example, is obtained by extraction from substances containing keratin, such as hair, horns, and feathers. L-cysteine can also be obtained by converting the precursor DL-2-aminothiazoline-4-carboxylic acid using a microbial enzyme (Sano, K. et al., Appl. Environ. Microibol., 34, 806-810 (1977)).
Furthermore, L-cysteine has also been produced by fermentation utilizing a bacterium. For example, a method has been disclosed for producing L-cysteine using an Escherichia bacterium with a suppressed L-cysteine decomposition system and a serine acetyltransferase (EC 2.3.1.30, henceforth also referred to as “SAT”) with attenuated feedback inhibition by L-cysteine is attenuated (Japanese Patent Laid-open (Kokai) No. 11-155571). Furthermore, coryneform bacteria or Escherichia bacteria attenuated or deleted activity of cystathionine-β-lyase (Japanese Patent Laid-open (Kokai) No. 11-155571), tryptophanase (Japanese Patent Laid-open No. 2003-169668), or O-acetylserine sulfhydrylase B (Japanese Patent Laid-open No. 2005-245311) are examples of bacteria with an enhanced ability to produce L-cysteine by suppressing the decomposition of L-cysteine. A method for producing L-cysteine by using a bacterium in which L-cysteine metabolism is decontrolled by using a DNA sequence coding for SAT that has a specific mutation for attenuating feedback inhibition by L-cysteine is also known (National Publication of Translated Version in Japan (Kohyo) No. 2000-504926).
Furthermore, it is known that the ydeD gene which encodes the YdeD protein (Dabler et al., Mol. Microbiol., 36, 1101-1112 (2000)), and the yfiK gene which encodes the YfiK protein (Japanese Patent Laid-open No. 2004-49237), participate in secretion of the metabolic products of the cysteine pathway. Techniques are also known for enhancing the L-cysteine-producing ability by increasing expression of the mar-locus, acr-locus, cmr-locus, mex-gene, bmr-gene, or qacA-gene, which encode proteins responsible for secreting toxic substances from cells (U.S. Pat. No. 5,972,663), or emrAB, emrKY, yojIH, acrEF, bcr, or cusA genes (Japanese Patent Laid-open No. 2005-287333).
Moreover, a method has been reported for producing L-cysteine using a bacterium which overexpresses a gene coding for a protein responsible for releasing an antibiotic or a substance toxic to a bacterium directly from a cell (Japanese Patent Laid-open No. 11-56381).
Moreover, it is known that production of L-amino acids by bacteria, not only L-cysteine, can be improved by increasing expression of an L-amino acid secretion protein. It is known that, in coryneform bacteria, L-lysine production is improved by increasing expression of an L-lysine secretion carrier, named LysE (Japanese Patent Laid-open No. 2000-189180). Furthermore, for Escherichia bacteria, several membrane proteins are known to be amino acid secretion carriers. For example, it has been reported that, by increasing the copy number of a gene named rhtB, resistance to a high concentration of L-homoserine, L-threonine, L-alanine, L-valine, and L-isoleucine is improved, and the product of this gene is a secretion carrier of these L-amino acids (Japanese Patent Laid-open No. 2000-189180).
The yeaS gene belongs to the family of the aforementioned rhtB gene, the product of which is presumed to be a membrane protein, and it has been reported that, by increasing expression of this gene, resistance to a high concentration of L-threonine, L-homoserine, L-lysine, L-glutamic acid, L-histidine, L-proline, and α-aminobutyric acid is improved as compared to a control strain, and by increasing expression of this gene in an L-amino acid-producing bacterium, production of L-valine, L-isoleucine, L-alanine, L-proline, and L-histidine is improved (EP 1013765 A1).
As described above, the effect of increasing yeaS gene expression on the production of various amino acids has been investigated (Kutukova et al., FEBS Lett., 579, 4629-4634 (2005)). However, mutations of the yeaS gene which improve L-amino acid productivity have not been previously reported.