Regulation of O-acetylserine sulfhydrylase B by L-cysteine in Salmonella typhimurium

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Abstract
A technique based on resistance to azaserine was used to isolate mutants lacking O-acetylserine sulfhydrylase B, 1 of 2 enzymes in S. typhimurium capable of synthesizing L-cysteine from O-acetyl-L-serine and sulfide. The mutant locus responsible for this defect was designated cysM, and genetic mapping suggests that cysM is very close to and perhaps contiguous with cysA. Strains lacking either O-acetylserine sulfhydrylase B or the 2nd sulfhydrylase, O-acetylserine sulfhydrylase A (coded for by cysK), are cysteine prototrophs, but cysK cysM double mutants required cysteine for growth. O-acetylserine sulfhydrylase B was derepressed by growth on a poor S source, and derepression was dependent upon both a functional cysB regulatory gene product and the internal inducer of the cysteine biosynthetic pathway, O-acetyl-L-serine. A cysBc strain, in which other cysteine biosynthetic enzymes cannot be fully repressed by growth on L-cysteine, was also constitutive for O-acetylserine sulfhydrylase B. Thus, O-acetylserine sulfhydrylase B is regulated by the same factors that control the expression of O-acetylserine sulfhydrylase A and other activities of the cysteine regulon. It is not clear why S. typhimurium has 2 enzymes whose physiological function appears to be to catalyze the same step of L-cysteine biosynthesis.