RNA polymerase sigma factor, region 2 <p>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 [<cite idref="PUB00000061"/>]. 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.</p><p>This entry represents region 2 (sigma2 domain) found in several sigma factors, often in conjunction with sigma3 and sigma4 domains (<db_xref db="INTERPRO" dbkey="IPR013324"/>). This region is present in Sigma70 [<cite idref="PUB00000942"/>], Sigma28 (FliA), SigA [<cite idref="PUB00010042"/>], SigR and RpoE. The sigma2 domain has a conserved 4-helical core, and often a variable insert subdomain.</p>