Involvement of differential efficiency of transcription by Eσ(s) and Eσ70 RNA polymerase holoenzymes in growth phase regulation of the Escherichia coli osmE promoter

Abstract

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, osmE(p.) Decelerating phase induction of osmE(p) is dependent on the σS (RpoS) factor, whereas its osmotic induction is independent of σ(s). Purified Eσ(s) and Eσ70 were both able to transcribe osmE, in 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 osmE(p) after an osmotic shock during exponential phase, but did not affect the decelerating phase induction. Point mutations affecting osmE(p) activity were isolated. Down-promoter mutations decreased transcription in both the presence and the absence of σ(s), indicating that the two forms of RNA polymerase holoenzyme recognize very similar sequence determinants on the osmE promoter. Three up-promoter mutations brought osmE(p) closer to the consensus of Eσ70-dependent promoters. The two variant promoters exhibiting the highest efficiency became essentially independent of σ(s) in vivo. Our data suggest that Eσ(s) transcribes wild-type osmE(p) 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 as-dependent promoters is discussed.