Regulation of l-Cysteine Biosynthesis in Salmonella typhimurium: I. EFFECTS OF GROWTH ON VARYING SULFUR SOURCES AND O-ACETYL-l-SERINE ON GENE EXPRESSION

Abstract

Abstract With the exception of serine transacetylase, the enzymes concerned with the biosynthesis of l-cysteine in Salmonella typhimurium are subject to derepression by sulfur starvation and to repression by sulfide and cysteine. Sulfate permease was not assayed in these studies. Under conditions of sulfur starvation, O-acetyl-l-serine, a direct precursor of cysteine, is necessary for derepression and is therefore an internal inducer. Sulfide and cysteine block the effect of exogenous O-acetylserine. Cys E mutants lack serine transacetylase and are pleiotropic due to their inability to synthesize the internal inducer, O-acetylserine. Cys B also appears to be regulatory, since the auxotrophs which define this region are generally repressed for most of the biosynthetic pathway even under conditions of sulfur starvation and an excess of O-acetylserine. Normally an intact cys B region, sulfur starvation, and O-acetylserine are all necessary for derepression, but certain mutations in the cys B region lead to constitutivity in the presence of sulfide or cysteine, and in the absence of O-acetylserine.