IDENTIFICATION OF A FUNCTIONAL HOMOLOG OF THE ESCHERICHIA-COLI AND SALMONELLA-TYPHIMURIUM CYSM GENE ENCODING O-ACETYLSERINE SULFHYDRYLASE-BIN CAMPYLOBACTER-JEJUNI

The final step of L-cysteine biosynthesis in Escherichia coli and Salm onella typhimurium consists of the formation of L-cysteine from O-acet ylserine and sulfide. This reaction can be catalyzed by two enzymes, O -acetylserine sulfhydrylase A and O-acetylserine sulfhydrylase B, the former of which has been more rigorously characterized. In contrast to O-acetylserine sulfhydrylase A, O-acetylserine sulfhydrylase B is pre ferentially used for cysteine biosynthesis during anaerobic growth and is able to utilize thiosulfate as a substrate. Campylobacter jejuni i s a micro-aerophilic, Gram-negative bacterium, and a member of the eps ilon subdivision of eubacteria. We have cloned, sequenced, and express ed a gene from C. jejuni that encodes a protein of 299 aa with a calcu lated molecular mass of 32 367 Da. Complementation analysis of an E. c oli cysteine auxotroph with the pMEK34-14 recombinant plasmid containi ng a 1.2-kb insert of chromosomal DNA from C. jejuni revealed that tra nsformants were capable of growth in medium containing either sulfide or thiosulfate as sole sulfur sources. These data indicate that the cl oned C. jejuni gene is a functional homolog of the cysM gene that code s for O-acetylserine sulfhydrylase B in E. coli and S. typhimurium.