Structural basis for transcription activation by Crl through tethering of σ and RNA polymerase.

In bacteria, a primary σ-factor associates with the core RNA polymerase (RNAP) to control most transcription initiation, while alternative σ-factors are used to coordinate expression of additional regulons in response to environmental conditions. Many alternative σ-factors are negatively regulated by anti-σ-factors. In , , and many other γ-proteobacteria, the transcription factor Crl positively regulates the alternative σ-regulon by promoting the association of σ with RNAP without interacting with promoter DNA.