The bacterial core RNA polymerase complex, which consists of five subunits, is sufficient for transcription elongation and termination but is unable to initiate transcription. Transcription initiation from promoter elements requires a sixth, dissociable subunit called a sigma factor, which reversibly associates with the core RNA polymerase complex to form a holoenzyme. RNA polymerase recruits alternative sigma factors as a means of switching on specific regulons. Most bacteria express a multiplicity of sigma factors. Two of these factors, Sigma70 (gene rpoD; major sigma factor) and Sigma54 (gene rpoN or ntrA) direct the transcription of a wide variety of genes. The other sigma factors, known as alternative sigma factors, are required for the transcription of specific subsets of genes.

This superfamily entry consists of a structural domain containing a helix turn helix motif. It is located in region 2 (sigma2 domain) found in several sigma factors, often in conjunction with sigma3 and sigma4 domains (INTERPRO:IPR013324). This region is present in Sigma70, Sigma28 (FliA), SigA, SigR and RpoE and it is a often a variable insert subdomain.

PFAM:PF04542, INTERPRO:IPR013325