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