Abstract: A method is provided for obtaining an L-amino acid or nucleic acid-producing bacterium belonging to the genus Escherichia with an optimized level of expression of the gene which influences the distribution of carbon flow, such as the sucAB genes, comprising introducing into the chromosome of the bacterium a set of in vitro constructed DNA fragments which contain regulatory elements for gene expression instead of the native elements of the regulatory region of the gene, and selecting the colonies with increased L-amino acid productivity. Also, a method is provided for producing an L-amino acid, such as L-glutamic acid, L-proline, L-arginine, L-glutamine, L-leucine, using the bacterium with an optimized level of expression of the sucAB gene.

Abstract: Amino acids, such as L-glutamine, L-arginine, L-tryptophan, L-histidine and L-glutamate are produced using a bacterium belonging to the genus Escherichia harboring a mutant glutamine synthetase in which the tyrosine amino acid residue corresponding to position 397 in a wild type glutamine synthetase is replaced with any of amino acid residues, preferably with phenylalanine.