The cysteine regulon of Salmonella typhimurium is positively regulated by the CysB protein and an inducer, which can be either O-acetyl-L-serine or N-acetyl-L-serine. In vivo experiments confirmed that sulfide and L-cysteine (supplied as L-cystine) interfere with induction by exogenously supplied O-acetyl-L-serine and also showed the same effects when N-acetyl-L-serine was used as an inducer. In a gel shift assay, purified CysB protein bound specifically to a 278-base-pair DNA fragment containing the S. typhimurium cysJIH promoter region. Binding occurred in the absence of inducer but did not stimulate in vitro transcription initiation, indicating that binding alone is insufficient to cause formation of a transcription initiation complex. Addition of N-acetyl-L-serine or O-acetyl-L-serine was required for transcription initiation and also stimulated binding three- to eightfold. Sulfide inhibited both transcription initiation and binding by interfering with the stimulatory effects of inducer in a competitive manner. These findings indicate that sulfide is an anti-inducer and may explain why full expression of the cysteine regulon requires sulfur limitation. L-Cysteine did not affect in vitro transcription initiation or binding of CysB protein to the cysJIH promoter region. The in vivo effects of L-cysteine may be secondary to its degradation to sulfide by the inducible enzyme cysteine desulfhydrase.