An isolated polynucleotide encodes a polypeptide comprising the amino acid sequence of SEQ ID NO: 2, with the L-aspartic acid at position 5 of the amino acid sequence replaced by another proteinogenic amino acid, and possesses citrate synthase activity. In addition, a vector comprises the polynucleotide and a bacterium comprises the vector. An isolated polynucleotide comprises a nucleotide sequence comprising, from position 1 to 39, the nucleotide sequence corresponding to position 1 to 39 of SEQ ID NO: 11, from position 40 to 105, a nucleotide sequence encoding the amino acid sequence of SEQ ID NO: 12, with each proteinogenic amino acid except L-aspartic acid being present at position 5. A method of producing an L-amino acids is also described.