InterPro domain: IPR013325

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 [ 1 ]. 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 entry represents region 2 (sigma2 domain) found in several sigma factors, often in conjunction with sigma3 and sigma4 domains ( IPR013324 ). This region is present in Sigma70 [ 2 ], Sigma28 (FliA), SigA [ 3 ], SigR and RpoE. The sigma2 domain has a conserved 4-helical core, and often a variable insert subdomain.

1. Structure and function of bacterial sigma factors. Annu. Rev. Biochem. 57, 839-72
2. Crystal structure of a sigma 70 subunit fragment from E. coli RNA polymerase. Cell 87, 127-36
3. Structure of the bacterial RNA polymerase promoter specificity sigma subunit. Mol. Cell 9, 527-39