Editing Sigma factor (section)

{{Short description|Protein needed for initiation of transcription in prokaryotes}}
A '''sigma factor''' ('''σ factor''' or '''specificity factor''') is a protein needed for initiation of [[Transcription (biology)|transcription]] in [[bacteria]].<ref name="gross">{{cite journal | vauthors = Gruber TM, Gross CA | title = Multiple sigma subunits and the partitioning of bacterial transcription space | journal = Annual Review of Microbiology | volume = 57 | pages = 441–66 | year = 2003 | pmid = 14527287 | doi = 10.1146/annurev.micro.57.030502.090913 }}</ref><ref name="Kang">{{cite journal | vauthors = Kang JG, Hahn MY, Ishihama A, Roe JH | title = Identification of sigma factors for growth phase-related promoter selectivity of RNA polymerases from Streptomyces coelicolor A3(2) | journal = Nucleic Acids Research | volume = 25 | issue = 13 | pages = 2566–73 | date = July 1997 | pmid = 9185565 | pmc = 146787 | doi = 10.1093/nar/25.13.2566 }}</ref> It is a [[bacterial transcription]] initiation factor that enables specific binding of [[RNA polymerase]] (RNAP) to gene [[promoter (biology)|promoter]]s. It is homologous to [[archaeal transcription factor B]] and to [[Eukaryote|eukaryotic]] factor [[TFIIB]].<ref>{{cite journal | vauthors = Burton SP, Burton ZF | title = The σ enigma: bacterial σ factors, archaeal TFB and eukaryotic TFIIB are homologs | journal = Transcription | volume = 5 | issue = 4 | pages = e967599 | date = 6 November 2014 | pmid = 25483602 | pmc = 4581349 | doi = 10.4161/21541264.2014.967599 }}</ref> 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.<ref>{{cite journal | vauthors = Ho TD, Ellermeier CD | title = Extra cytoplasmic function σ factor activation | journal = Current Opinion in Microbiology | volume = 15 | issue = 2 | pages = 182–8 | date = April 2012 | pmid = 22381678 | pmc = 3320685 | doi = 10.1016/j.mib.2012.01.001 }}</ref> They are also found in plant [[chloroplast]]s as a part of the bacteria-like plastid-encoded polymerase (PEP).<ref name=pmid20701995>{{cite journal | vauthors = Schweer J, Türkeri H, Kolpack A, Link G | title = Role and regulation of plastid sigma factors and their functional interactors during chloroplast transcription - recent lessons from Arabidopsis thaliana | journal = European Journal of Cell Biology | volume = 89 | issue = 12 | pages = 940–6 | date = December 2010 | pmid = 20701995 | doi = 10.1016/j.ejcb.2010.06.016 }}</ref>

The sigma factor, together with RNA polymerase, is known as the RNA polymerase [[Enzyme#Cofactors|holoenzyme]]. Every molecule of RNA polymerase holoenzyme contains exactly one sigma factor subunit, which in the model bacterium ''[[Escherichia coli]]'' is one of those listed below.  The number of sigma factors varies between bacterial species.<ref name="gross"/><ref>{{cite journal | vauthors = Sharma UK, Chatterji D | title = Transcriptional switching in Escherichia coli during stress and starvation by modulation of sigma activity | journal = FEMS Microbiology Reviews | volume = 34 | issue = 5 | pages = 646–57 | date = September 2010 | pmid = 20491934 | doi = 10.1111/j.1574-6976.2010.00223.x | doi-access = free }}</ref> ''E. coli'' has seven sigma factors.  Sigma factors are distinguished by their characteristic [[Molecular mass|molecular weights]]. For example, σ<sup>70</sup> is the sigma factor with a molecular weight of 70 [[Dalton (unit)|kDa]].

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.