PROBLEM TO BE SOLVED: To obtain a new L-lysin biosynthetic gene encoding a polypeptide having a specific amino acid sequence and homoaconitic acid hydratase activities, and useful for breeding of L-lysin-producing bacterium belonging to the genus Thermus, production of the L-lysin, or the like. SOLUTION: This new DNA fragment encodes a polypeptide having an amino acid sequence of formula I or a polypeptide having an amino acid sequence having substitution, deficiency, insertion, addition or inversion of amino acid residues in the amino acid sequence of formula I, and capable of providing a protein having an amino acid sequence of formula II and further having a homoaconitic acid hydratase activities in combination with a subunit II of the homoaconitic acid hydratase. The DNA fragment is useful for breeding of L-lysin-producing bacterium belonging to the genus Thermus, production of the L-lysin, or the like. The gene is obtained by isolating a DNA fragment complementing L-lysin requirement from a gene library of L-lysin-non-requiring Thermus thermophilus.
