Evidence for transcription attenuation rendering cryptic a sigmaS-dependent promoter of the osmotically regulated proU operon of Salmonella typhimurium.

The osmotically regulated proU locus in Escherichia coli has two promoters, P1 and P2, that are recognized, respectively, by the sigmaS- and sigma70-bearing RNA polymerase holoenzymes. However, the equivalent of the P1 promoter does not appear to exist in Salmonella typhimurium. We demonstrate in this study that wild-type S. typhimurium has a cryptic P1 promoter that is recognized by sigmaS RNA polymerase in vitro and that a 22-bp deletion from +63 to +84 (relative to the start site of transcription) confers sigmaS-dependent in vivo expression of a reporter gene fusion to P1. Primer extension analysis of RNA isolated from cells carrying the wild-type and mutant S. typhimurium proU constructs indicated that a primer which hybridizes proximal to +60 is able to detect P1-initiated transcripts from both constructs but a primer which hybridizes distal to +85 is able to do so only from the latter. Our results suggest that the sigmaS-controlled proU P1 promoter in S. typhimurium may be rendered cryptic because of factor-dependent transcription attenuation within a short distance downstream of the promoter start site.