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A sigma factor (σ factor) is a protein needed only for initiation of RNA synthesis. It is a bacterial transcription initiation factor that enables specific binding of RNA polymerase to gene promoters. 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. 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.〔 ''E. coli'' has seven sigma factors. Sigma factors are distinguished by their characteristic molecular weights. For example, σ70 refers to the sigma factor with a molecular weight of 70 kDa. RNA polymerase holoenzyme complex consisting of core RNA polymerase and a sigma factor executes transcription of a DNA template strand. Once initiation of RNA transcription is complete, the sigma factor can leave the complex. ==Specialized sigma factors== Different sigma factors are utilized 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) - σA - the "housekeeping" sigma factor or also called as primary sigma factor, transcribes most genes in growing cells. Every cell has a “housekeeping” sigma factor that keeps essential genes and pathways operating.〔 In the case of ''E. coli'' and other gram-negative rod-shaped bacteria, the "housekeeping" sigma factor is σ70.〔 Genes recognized by σ70 all contain similar promoter consensus sequences consisting of two parts.〔 Relative to the DNA base corresponding to the start of the RNA transcript, the consensus promoter sequences are characteristically centered at 10 and 35 nucleotides before the start of transcription (–10 and –35). *σ19 (FecI) - the ferric citrate sigma factor, regulates the fec gene for iron transport *σ24 (RpoE) - the extracytoplasmic/extreme heat stress sigma factor *σ28 (RpoF) - the flagellar sigma factor *σ32 (RpoH) - the heat shock sigma factor, it is turned on when the bacteria are exposed to heat. Due to the higher expression, the factor will bind with a high probability to the polymerase-core-enzyme. Doing so, other heatshock proteins are expressed, which enable the cell to survive higher temperatures. Some of the enzymes that are expressed upon activation of σ32 are chaperones, proteases and DNA-repair enzymes. *σ38 (RpoS) - the starvation/stationary phase sigma factor *σ54 (RpoN) - the nitrogen-limitation sigma factor There are also anti-sigma factors that inhibit the function of sigma factors and anti-anti-sigma factors that restore sigma factor function. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「sigma factor」の詳細全文を読む スポンサード リンク
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