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

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, osmC(p1) and osmC(p2). osmC(p2) is mainly transcribed by E-sigma(s), the RNA polymerase using the sigma(s) (RpoS) sigma factor, and is responsible for the growth phase regulation. Transcription from osmC(p1) is independent of sigma(s). 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 osmC(p1) and activates the transcription of osmC(p2) by E-sigma(s). 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 osmC(p2) is probably mediated by the increase in sigma(s) concentration in the cytoplasm of hns(-) mutants, while the effect on osmC(p1) is independent of sigma(s). No binding of H-NS to the promoter region DNA could be detected, indicating that the effect on osmC(p1 )could also be indirect.