EVIDENCE FOR TRANSCRIPTION ATTENUATION RENDERING CRYPTIC A SIGMA(S)-DEPENDENT PROMOTER OF THE OSMOTICALLY REGULATED PROU OPERON OF SALMONELLA-TYPHIMURIUM

The osmotically regulated proU locus in Escherichia coli has two promo ters, P1 and P2, that are recognized, respectively, by the sigma(S)- a nd sigma(70)-bearing RNA polymerase holoenzymes. However, the equivale nt of the P1 promoter does not appear to exist in Salmonella typhimuri um. We demonstrate in this study that wild-type S. typhimurium has a c ryptic P1 promoter that is recognized by sigma(S) RNA polymerase in vi tro and that a 22-bp deletion from +63 to +84 (relative to the start s ite of transcription) confers sigma(S)-dependent in vivo expression of a reporter gene fusion to P1. Primer extension analysis of RNA isolat ed from cells carrying the wild-type and mutant S. typhimurium proU co nstructs indicated that a primer which hybridizes proximal to +60 is a ble to detect P1-initiated transcripts from both constructs but a prim er which hybridizes distal to +85 is able to do so only from the latte r. Our results suggest that the sigma(S)-controlled proU P1 promoter i n S. typhimurium may be rendered cryptic because of factor-dependent t ranscription attenuation within a short distance downstream of the pro moter start site.