In vivo effects of anti-inducers of the cysteine regulon in Salmonella typhimurium.

Growth on readily utilizable sulfur sources reduces expression of the cysteine regulon in Salmonella typhimurium. Inhibition of serine transacetylase by cysteine and direct actions of the anti-inducers sulfide and thiosulfate are responsible for reduction of expression. In order to evaluate individual contributions of each mechanism, the inhibitory effects of Na2S and Na2S2O3 were studied in strains with or without the capacity to synthesize cysteine from these compounds, using a transcriptional fusion to the cysDNC operon. In a cysK cysM strain, although cysteine synthesis from sulfide and thiosulfate was blocked, Na2S and Na2S2O3 efficiently reduced expression of the cysDNC operon. The inhibitory effect observed in this mutant was equivalent to 70-100% of that found in a strain carrying the fusion in a wild-type context grown in the same conditions. The actions of sulfide and thiosulfate as anti-inducers seem therefore to be responsible for most of the reduction of expression caused by these agents in vivo.