Transcriptional regulation by σ factor phosphorylation in bacteria.

A major form of transcriptional regulation in bacteria occurs through the exchange of the primary σ factor of RNA polymerase (RNAP) with an alternative extracytoplasmic function (ECF) σ factor. ECF σ factors are generally intrinsically active and are retained in an inactive state via the sequestration into σ factor-anti-σ factor complexes until their action is warranted. Here, we report a previously uncharacterized mechanism of transcriptional regulation that relies on intrinsically inactive ECF σ factors, the activation of which and interaction with the β'-subunit of RNAP depends on σ factor phosphorylation.