Sigma factor

A sigma factor (σ factor) is a prokaryotic transcription initiation factor that enables specific binding of RNA polymerase to gene promoters. Different sigma factors are activated in response to different environmental conditions. Every molecule of RNA polymerase contains exactly one sigma factor subunit, which in the model bacterium Escherichia coli is one of those listed below. E. coli has at least eight sigma factors; the number of sigma factors varies between bacterial species. Sigma factors are distinguished by their characteristic molecular weights. For example, σ70 refers to the sigma factor with a molecular weight of 70 kDa.

Structure

Sigma factors have four main regions that are generally conserved:

N-terminus --------------------- C-terminus1 2 3 4

The regions are further subdivided (e.g. 2 includes 2.1, 2.2, etc.)Region 1 is found only in "primary sigma factors" (RpoD, RpoS in E.coli). It is involved in ensuring the sigma factor will only bind the promoter when it is complexed with the RNA polymerase.Region 2.4 recognizes and binds to the Pribnow box.Region 4.2 recognizes and binds to the -35 promoter site.

The exception to this organization is in σ 54-type sigma factors. Proteins homologous to σ54/RpoN are functional sigma factors, but they have significantly different primary amino acid sequences.

Specialized sigma factors

Different sigma factors are activated under different environmental conditions. These specialized sigma factors bind the promoters of genes appropriate to the environmental conditions, increasing the transcription of those genes.

Sigma factors in E.coli:

σ70 (RpoD) - the "housekeeping" sigma factor, transcribes most genes in growing cells. Makes the proteins necessary to keep the cell alive.σ54 (RpoN) - the nitrogen-limitation sigma factorσ38 (RpoS) - the starvation/stationary phase sigma factorσ32 (RpoH) - the heat shock sigma factor, it is turned on when exposed to heatσ28 (RpoF) - the flagellar sigma factorσ24 (RpoE) - the extracytoplasmic/extreme heat stress sigma factorσ19 (FecI) - the ferric citrate sigma factor, regulates the fec gene for iron transport

There are also anti-sigma factors that inhibit the function of sigma factors.