Themes and variations in gene regulation by extracytoplasmic function (ECF) sigma factors

•Major themes shared by ECF sigma factors are highlighted.•The mechanisms of promoter recognition are conserved among ECF sigma factors.•Most ECF sigmas are regulated by an anti-sigma factor.•The framework of the signaling pathways is often conserved within an ECF family.•Details of signal sensing and regulation are specific to individual sigma factors.

The ECF sigma family was identified 23 years ago as a distinct group of σ70-like factors. ECF sigma factors have since emerged as a major form of bacterial signal transduction that can be grouped into over 50 phylogenetically distinct subfamilies. Advances in our understanding of these sigma factors and the signaling pathways governing their activity have elucidated conserved features as well as aspects that have evolved over time. All ECF sigma factors are predicted to share a common streamlined domain structure and mode of promoter interaction. The activity of most ECF sigma factors is controlled by an anti-sigma factor. The nature of the anti-sigma factor and the activating signaling pathways appear to be conserved within ECF families, while considerable diversity exists between different families.