A TRANSCRIPTIONAL SILENCER DOWNSTREAM OF THE PROMOTER IN THE OSMOTICALLY CONTROLLED PROU OPERON OF SALMONELLA-TYPHIMURIUM

The proU operon of Salmonella typhimurium is induced by conditions of high osmolality. The cis-acting sequences that mediate osmotic control of transcription were characterized by deletion analysis. The nucleotide sequence between -60 and +274 (relative to the transcription start point) is sufficient for normal osmotic control. Deletions that removed sequences upstream of position +274 but left the promoter intact resulted in greatly increased expression from the proU promoter in the absence of osmotic stress. Thus, the transcription control region of the proU operon consists of two discrete components: (i) the promoter and (ii) a negatively acting site that overlaps the coding sequence of the first structural gene of the operon, proV. That this negative regulatory element is a transcriptional terminator or mRNA processing site was ruled out. Our results suggest that the negative regulatory element behaves as a transcriptional silencer that inhibits transcription initiation at the proU promoter in medium of low osmolality by some action at a distance. We propose several possible mechanisms for the function of this regulatory site.