Interplay between global regulators of Escherichia coli : effect of RpoS, Lrp and H‐NS on transcription of the gene osmC

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Abstract
The transcription of the osmC gene of Escherichia coli is regulated as a function of the phase of growth. It is induced during the decelerating phase, before entry into stationary phase. osmC expression is directed by two overlapping promoters, osmCp1 and osmCp2. osmCp2 is mainly transcribed by E‐σs, the RNA polymerase using the σs (RpoS) sigma factor, and is responsible for the growth phase regulation. Transcription from osmCp1 is independent of σs. The leucine‐responsive protein (Lrp) has been shown to bind the osmC promoter region in band shift experiments. In vivo analysis using osmC–lacZ transcriptional fusions demonstrated that Lrp affects the expression of both promoters. It represses the transcription of osmCp1 and activates the transcription of osmCp2 by E‐σs. An absence of Lrp results in an increase in the amount of RpoS during exponential growth in minimal medium. The nucleoid‐associated protein H‐NS also represses osmC transcription from both promoters. However, this happens through different mechanisms. The effect on osmCp2 is probably mediated by the increase in σs concentration in the cytoplasm of hns− mutants, while the effect on osmCp1 is independent of σs. No binding of H‐NS to the promoter region DNA could be detected, indicating that the effect on osmCp1 could also be indirect.