To provide a heat-resistant cysteine synthetase performing the synthetic reaction of amino acids in high efficiency, a DNA encoding the enzyme, etc.

A cysteine synthetase is separated from Aeropyrum pernix belonging to ultrathermophilic archaebacteria growing even at a temperature of as high as 90-100°C and the amino acid sequence and DNA sequence of the enzyme are determined. The cysteine synthetase essentially keeps its activity after the heat-treatment at 80°C for 6 hrs. A substrate solution having high concentration and high temperature can be prepared by the use of the enzyme taking advantage of the high heat-resistance and, accordingly, amino acids, etc., can be produced in high efficiency.