Extra Cytoplasmic Function sigma factors, recent structural insights into promoter recognition and regulation

Bacterial transcription initiation is controlled by sigma factors, the RNA polymerase (RNAP) subunits responsive for promoter specificity. While the primary sigma factor ensures the bulk of transcription during growth, a major strategy used by bacteria to regulate gene expression consists of modifying the RNAP promoter specificity by means of alternative sigma factors. Among these factors, Extra Cytoplasmic Function sigma factors (σ(ECF)) constitute the most abundant group and are generally kept inactive by specific anti-sigma factors that are directly or indirectly sensitive to environmental stimuli. When activated by anti-sigma factor release, σ(ECF) turn on the transcription of dedicated regulons, which trigger adaptive responses for the survival of the cell. Recent structural studies have deciphered the molecular basis for σ(ECF) promoter recognition and original regulatory mechanisms.