Sigma factor

A sigma factor is a prokaryotic transcription initiation factor which binds to RNA polymerase and helps it to locate the promoter sequence in a gene. Because prokaryotic DNA is naked, the sigma factor is all that is required for RNA pol to home in on the promoter; in eukaryotes, a platform of proteins called a preinitiation complex is required instead.

Sigma factors are denoted according to their molecular weight; sigma-70 is a factor of molecular weight 70kDa.

Sigma factors have four conserved regions, which themselves are divided into subregions. Region 1 is responsible for ensuring that the sigma factor binds with a promoter only when it is complexed with RNA polymerase. Region 2.4 recognises and binds to the Pribnow box (a prokaryotic equivalent to the TATA box) and region 4.2 recognises and binds to the -35 promoter site.

Different sigma factors are induced by different environmental conditions, and this system ensures that environment-specific genes are transcribed as needed. For instance, while sigma-70 is responsible for transcribing so-called housekeeping genes, sigma-32 is turned on only in response to heat and sigma-54 only in response to nitrogen limitation. Various anti-sigma factors inhibit sigma factors to prevent transcription of the genes that they respectively stimulate.

Although it was initially believed that the sigma factor dissociates from the RNA polymerase holoenzyme following transcription initiation (in a phenomenon called 'promoter escape') it is now believed that sigma factors dissociate rather later, having an important role at least in early elongation of the transcript.