Involvement of differential efficiency of transcription by E sigma(s) and E sigma(70) RNA polymerase holoenzymes in growth phase regulation of the Escherichia coli osmE promoter

Transcription of the gene osmE of Escherichia coli is inducible by elevated osmotic pressure and during the decelerating phase of growth. osmE express ion is directed by a single promoter, osmE(p). Decelerating phase induction of osmE(p) is dependent on the sigma(s) (RpoS) factor, whereas its osmotic induction is independent of sigma(s.) Purified E sigma(s) and E sigma(70) were both able to transcribe osmE(p) in vitro on supercoiled templates. In the presence of rpoD800, a mutation resulting in a thermosensitive sigma(70 ) factor, a shift to non-permissive temperature abolished induction of osmE (p) after an osmotic shock during exponential phase, but did not affect the decelerating phase induction. Point mutations affecting osmE(p) activity w ere isolated. Down-promoter mutations decreased transcription in both the p resence and the absence of sigma(s), indicating that the two forms of RNA p olymerase holoenzyme recognize very similar sequence determinants on the os mE promoter. Three up-promoter mutations brought osmE(p) closer to the cons ensus of E sigma(70)-dependent promoters. The two variant promoters exhibit ing the highest efficiency became essentially independent of sigma(s) in vi vo. Our data suggest that E sigma(s) transcribes wild-type osmE(p) with a h igher efficiency than E sigma(70.) A model in which an intrinsic differenti al recognition contributes to growth phase-dependent regulation is proposed . Generalization of this model to other sigma(s)-dependent promoters is dis cussed.