Back to log phase: σ S as a global regulator in the osmotic control of gene expression in Escherichia coli

It is now well established that the σ S subunit of RNA polymerase is a master regulator in a complex regulatory network that governs the expression of many stationary‐phase‐inducible genes in Escherichia coli . In this review, more recent findings will be summarized that demonstrate that σ S also acts as a global regulator for the osmotic control of gene expression, and actually does so in exponentially growing cells. Thus, many σ S ‐dependent genes are induced during entry into stationary phase as well as in response to osmotic upshift. K + glutamate, which accumulates in hyperosmotically stressed cells, seems to specifically stimulate the activity of σ S ‐containing RNA polymerase at σ S ‐dependent promoters. Moreover, osmotic upshift results in an elevated cellular σ S level similar to that observed in stationary‐phase cells. This increase is the result of a stimulation of rpoS translation as well as an inhibition of the turnover of σ S , which in exponentially growing non‐stressed cells is a highly unstable protein. Whereas the RNA‐binding protein HF‐I, previously known as a host factor for the replication of phage Qβ RNA, is essential for rpoS translation, the recently discovered response regulator RssB, and ClpXP protease, have been shown to be required for σ S degradation. The finding that the histone‐like protein H‐NS is also involved in the control of rpoS translation and σ S turnover, sheds new light on the function of this protein in osmoregulation. Finally, preliminary evidence suggests that additional stresses, such as heat shock and acid shock, also result in increased cellular σ S levels in exponentially growing cells. Taken together, σ S function is clearly not confined to stationary phase. Rather, σ S may be regarded as a sigma factor associated with general stress conditions.