Negative regulation by RpoS: a case of sigma factor competition
A mutation in the Escherichia coli gene encoding the stationary phase-inducible sigma factor (σs, RpoS) not only abolishes transcription of some genes in stationary phase, but also causes superinduction of other stationary phase-induced genes. We have examined this phenomenon of repression by σs using as a model system the divergently transcribed stationary phase-inducible genes, uspA and uspB. uspA is transcribed by σ70-programmed RNA polymerase and is superinduced in an rpoS mutant, while uspB induction is σs dependent. The data suggest that the superinduction of uspA is caused by an increased amount of σ70 bound to RNA polymerase in the absence of the competing σs. Increasing the ability of σ70 to compete against σs by overproducing σ70 mimics the effect of an rpoS mutation by causing superinduction of σ70-dependent stationary phase-inducible genes (uspA and fadD), silencing of σs-dependent genes (uspB, bolAp1 and fadL) and inhibiting the development of σs-dependent phenotypes, such as hydrogen peroxide resistance in stationary phase. In addition, overproduction of σs markedly reduced stationary phase expression of a σ70-dependent promoter. Thus, we conclude that sigma factors compete for a limiting amount of RNA polymerase during stationary phase. The implications of this competition in the passive control of promoter activity is discussed.
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Document Type: Original Article
Affiliations: Department of Microbiology, Lund University, Sölvegatan 12, 223 62 Lund, Sweden.
Publication date: August 1, 1998