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 sigmas (RpoS) factor, whereas its osmotic induction is independent of sigmas. Purified Esigmas and Esigma70 were both able to transcribe osmEp in vitro on supercoiled templates. In the presence of rpoD800, a mutation resulting in a thermosensitive sigma70 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 sigmas, 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 Esigma70-dependent promoters. The two variant promoters exhibiting the highest efficiency became essentially independent of sigmas in vivo. Our data suggest that Esigmas transcribes wild-type osmEp with a higher efficiency than Esigma70. A model in which an intrinsic differential recognition contributes to growth phase-dependent regulation is proposed. Generalization of this model to other sigmas-dependent promoters is discussed.