GENE FOR ASPARTATE RACEMASE FROM THE SULFUR-DEPENDENT HYPERTHERMOPHILIC ARCHAEUM, DESULFUROCOCCUS STRAIN SY

Amino acid racemases are ubiquitous throughout eubacteria, However, no amino acid racemases have yet been found in eukaryotes and archaea. W e cloned a gene highly homologous to that for the aspartate racemase f rom the sulfur-dependent hyperthermophilic archaeum, Desulfurococcus s train SY, The product of the gene showed 35.2% amino acid sequence ide ntity with the aspartate racemase of Streptococcus thermophilus IAM100 64, and was also homologous to glutamate racemases around the putative catalytic cysteine residues, The encoded protein was expressed in Esc herichia coli, The recombinant protein had amino acid racemizing activ ity, which was highly specific for aspartate and increased with temper ature from 37 degrees C to 90 degrees C. Therefore, this was identifie d as the first hyperthermophilic archaeal amino acid racemase. A littl e aspartate racemizing activity was also detected in the crude extract of Desulfurococcus strain SY. The function of this aspartate racemase might be the uptake of D-aspartate formed at high temperature or the production of D-aspartate as a cell component, The fact that the amino acid racemases are distributed among both eubacteria and archaea sugg ests that endogenous D-amino acids in mammals are also synthesized by amino acid racemases.