Sigma (σ) factor is a small protein subunit, which associates with the core RNA polymerase, to form a holoenzyme. It recognises a specific sequence of DNA, the promoter, where the holoenzyme binds tightly to the DNA, initiating transcription. The sigma factor increases the affinity of RNA polymerase for the promoter region. After transcription has progressed about 10 bases the sigma factor is released. In E. coli the main sigma factor is σ 70, however, many others exist.
In bacteria there is a large range of sigma subunits, each directing the RNA polymerase to different promoters. The following table gives a few examples of the promoters that different sigma factors recognise.
|Sigma factor||promoter recognised|
|σ54||genes that are involved in nitrogen metabolism|
|σ24||genes involved in the dealing of misfolded proteins|
|σ28||genes that are involved in chemotaxis and motility (ability to move independatly and impulsively)|