The σ S subunit of RNA polymerase as a signal integrator and network master regulator in the general stress response in Escherichia coli

The σ^S (RpoS) subunit of RNA polymerase in Escherichia coli is a key master regulator which allows this bacterial model organism and important pathogen to adapt to and survive environmentally rough times. While hardly present in rapidly growing cells, σ^S strongly accumulates in response to many different stress conditions, partly replaces the vegetative sigma subunit in RNA polymerase and thereby reprograms this enzyme to transcribe σ^S-dependent genes (up to 10% of the E. coli genes). In this review, we summarize the extremely complex regulation of σ^S itself and multiple signal input at the level of this master regulator, we describe the way in which σ^S specifically recognizes “stress” promoters despite their similarity to vegetative promoters, and, while being far from comprehensive, we give a short overview of the far-reaching physiological impact of σ^S. With σ^S being a central and multiple signal integrator and master regulator of hundreds of genes organized in regulatory cascades and sub-networks or regulatory modules, this system also represents a key model system for analyzing complex cellular information processing and a starting point for understanding the complete regulatory network of an entire cell.