The cysP promoter of Salmonella typhimurium: characterization of two binding sites for CysB protein, studies of in vivo transcription initiation, and demonstration of the anti-inducer effects of thiosulfate.

The cysPTWA operons of Escherichia coli and Salmonella typhimurium encode components of periplasmic transport systems for sulfate and thiosulfate and are regulated as part of the cysteine regulons. In vitro transcription initiation from the cysP promoter was shown to require both CysB protein and either O-acetyl-L-serine or N-acetyl-L-serine, which act as inducers, and was inhibited by the anti-inducer sulfide. Thiosulfate was found to be even more potent than sulfide as an anti-inducer. DNase I protection experiments showed two discrete binding sites for CysB protein in the presence of N-acetyl-L-serine. CBS-P1 is located between positions -85 and -41 relative to the major transcription start site, and CBS-P2 is located between positions -19 and +25. Without N-acetyl-L-serine, the CysB protein protected the region between positions -63 and -11, which was designated CBS-P3. In gel mobility shift assays, the mobility of CysB protein-cysP promoter complexes was increased by O-acetyl-L-serine, N-Acetyl-L-serine had no effect in gel shift experiments, presumably because its anionic charge results in its rapid removal from the complex during electrophoresis. Comparison of DNA fragments differing with respect to binding site position indicated that complexes with CysB protein contain DNA that is bent somewhere between CBS-P1 and CBS-P2 and that O-acetyl-L-serine decreases DNA bending. Binding studies with fragments containing either CBS-P2 alone, CBS-P1 alone, or the entire cysP promoter region suggest a model in which the complex of bent DNA observed in the absence of O-acetyl-L-serine contains a single CysB protein molecule bound to CBS-P3. At relatively low CysB protein concentrations, O-acetyl-L-serine would cause a single CysB protein molecule to bind tightly to CBS-P1, rather than to CBS-P3, thereby decreasing DNA bending and increasing complex electrophoretic mobility. At higher CysB protein concentrations, O-acetyl-L-serine would cause a second molecule to bind at CBS-P2, giving a more slowly migrating complex.