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

It is now well established that the sigma 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 sigma S also acts as a global regulator for the osmotic control of gene expression, and actually does so in exponentially growing cells. Thus, many sigma 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 sigma S-containing RNA polymerase at sigma S-dependent promoters. Moreover, osmotic upshift results in an elevated cellular sigma 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 sigma 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 Q beta RNA, is essential for rpoS translation, the recently discovered response regulator RssB, and ClpXP protease, have been shown to be required for sigma S degradation. The finding that the histone-like protein H-NS is also involved in the control of rpoS translation and sigma 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 sigma S levels in exponentially growing cells. Taken together, sigma S function is clearly not confined to stationary phase. Rather, sigma S may be regarded as a sigma factor associated with general stress conditions.