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Sigma factor
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<p>A sigma factor (σ factor or specificity factor) is a protein needed for initiation of <a href="/facts/Transcription_(biology)/bDgp9HDN">transcription</a> in <a href="/facts/Bacteria/wC8xSCsU">bacteria</a>. It is a <a href="/facts/Bacterial_transcription/RQDnjl5S">bacterial transcription</a> initiation factor that enables specific binding of <a href="/facts/RNA_polymerase/gHd0ET9R">RNA polymerase</a> (RNAP) to gene <a href="/facts/Promoter_(biology)/YDYBzLfg">promoters</a>. It is homologous to <a href="/facts/Archaeal_transcription_factor_B/DvN81KPS">archaeal transcription factor B</a> and to <a href="/facts/Eukaryote/SkwyPLMA">eukaryotic</a> factor <a href="/facts/TFIIB/t575UD5e">TFIIB</a>. The specific sigma factor used to initiate transcription of a given gene will vary, depending on the <a href="/facts/Gene/e1swwPY6">gene</a> 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. They are also found in plant <a href="/facts/Chloroplast/fo8U3yTx">chloroplasts</a> as a part of the bacteria-like plastid-encoded polymerase (PEP). </p><p>The sigma factor, together with RNA polymerase, is known as the RNA polymerase <a href="/facts/Enzyme/DSI1Fh7y">holoenzyme</a>. Every molecule of RNA polymerase holoenzyme contains exactly one sigma factor subunit, which in the model bacterium <i><a href="/facts/Escherichia_coli/nR4Gvl56">Escherichia coli</a></i> is one of those listed below. The number of sigma factors varies between bacterial species. <i>E. coli</i> has seven sigma factors. Sigma factors are distinguished by their characteristic <a href="/facts/Molecular_mass/FWueGMed">molecular weights</a>. For example, σ70 is the sigma factor with a molecular weight of 70 <a href="/facts/Dalton_(unit)/IMATU3vf">kDa</a>. </p><p>The sigma factor in the RNA polymerase holoenzyme complex is required for the initiation of transcription, although once that stage is finished, it is dissociated from the complex and the RNAP continues elongation on its own. </p>