Regulation of bacterial RNA polymerase σ factor activity: a structural perspective

In bacteria, σ factors are essential for the promoter DNA-binding specificity of RNA polymerase. The σ factors themselves are regulated by anti-σ factors that bind and inhibit their cognate σ factor, and ‘appropriators’ that deploy a particular σ-associated RNA polymerase to a specific promoter class. Adding to the complexity is the regulation of anti-σ factors by both anti–anti-σ factors, which turn on σ factor activity, and co-anti-σ factors that act in concert with their partner anti-σ factor to inhibit or redirect σ activity. While σ factor structure and function are highly conserved, recent results highlight the diversity of structures and mechanisms that bacteria use to regulate σ factor activity, reflecting the diversity of environmental cues that the bacterial transcription system has evolved to respond.