Sigma factor

Article summary

A sigma factor (σ factor or specificity factor) is a protein needed for initiation of transcription in bacteria. It is a bacterial transcription initiation factor that enables specific binding of RNA polymerase (RNAP) to gene promoters. They are also found in plant chloroplasts as a part of the bacteria-like plastid-encoded polymerase (PEP). It is homologous to archaeal transcription factor B and to eukaryotic factor TFIIB. The specific sigma factor used to initiate transcription of a given gene will vary, depending on the gene and on the environmental signals needed to initiate transcription of that gene. Selection of promoters by RNA polymerase is dependent on the sigma factor that associates with it. The sigma factor, together with RNA polymerase, is known as the RNA polymerase holoenzyme. Every molecule of RNA polymerase holoenzyme contains exactly one sigma factor subunit, which in the model bacterium Escherichia coli is one of the seven listed below. The number of sigma factors varies among bacterial species ; for example, E. coli has seven sigma factors. Sigma factors are distinguished by their characteristic molecular weights. For example, σ 70 is the sigma factor with a molecular weight of 70 kDa.…

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