The Structural Gene for O-Acetylserine Sulfhydrylase A in Salmonella typhimurium

Mutants of Salmonella typhimurium, resistant to 1,2,4-triazole and mapping in the trzA and trzB loci, have been found to have low to virtually absent levels of O-acetylserine sulfhydrylase activity while remaining prototrophic for cysteine. Kinetic, chemical, and immunochemical studies of the highly purified enzymes from two trzA mutants prove that both strains bear mutant alleles for O-acetylserine sulfhydrylase A. A third trzA mutant contains a heat-labile enzyme. Little or no material cross-reacting with antibody to wild type O-acetylserine sulfhydrylase A could be detected in three trzB mutants or in the trzA deletion examined. Since trzA is clearly the structural gene for O-acetylserine sulfhydrylase A, we suggest that the designation, cysK, seems more appropriate for this locus. The lack of any demonstrable growth requirement for cysteine in trz mutants, which completely lack O-acetylserine sulfhydrylase A, indicates that the synthesis of cysteine from O-acetyl-l-serine and sulfide can be catalyzed by another enzyme in S. typhimurium.