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

The bacterial core RNA polymerase complex, which consists of five subunits, is sufficient for tra
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