Involvement of differential efficiency of transcription by esigmas and esigma70 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 expression is directed by a single promoter, osmEp. Decelerating phase induction of osmEp is dependent on the σs (RpoS) factor, whereas its osmotic induction is independent of σs. Purified Eσs and Eσ70 were both able to transcribe osmEpin vitro on supercoiled templates. In the presence of rpoD800, a mutation resulting in a thermosensitive σ70 factor, a shift to non-permissive temperature abolished induction of osmEp after an osmotic shock during exponential phase, but did not affect the decelerating phase induction. Point mutations affecting osmEp activity were isolated. Down-promoter mutations decreased transcription in both the presence and the absence of σs, indicating that the two forms of RNA polymerase holoenzyme recognize very similar sequence determinants on the osmE promoter. Three up-promoter mutations brought osmEp closer to the consensus of Eσ70-dependent promoters. The two variant promoters exhibiting the highest efficiency became essentially independent of σsin vivo. Our data suggest that Eσs transcribes wild-type osmEp with a higher efficiency than Eσ70. A model in which an intrinsic differential recognition contributes to growth phase-dependent regulation is proposed. Generalization of this model to other σs-dependent promoters is discussed.