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A sigma factor (σ factor) is a prokaryotic transcription initiation factor that enables specific binding of RNA polymerase to gene promoters. Different sigma factors are activated in response to different environmental conditions. Every molecule of RNA polymerase contains exactly one sigma factor subunit, which in the model bacterium Escherichia coli is one of those listed below. E.coli has at least eight sigma factors; the number of sigma factors varies between bacterial species. Sigma factors are distinguished by their characteristic molecular weights. For example, σ70 refers to the sigma factor with a molecular weight of 70 kDa.
Additional recommended knowledge
Sigma factors have four main regions that are generally conserved:
N-terminus --------------------- C-terminus 1 2 3 4
The regions are further subdivided (e.g. 2 includes 2.1, 2.2, etc.)
The exception to this organization is in σ54-type sigma factors. Proteins homologous to σ54/RpoN are functional sigma factors, but they have significantly different primary amino acid sequences.
Specialized sigma factors
Different sigma factors are activated 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:
There are also anti-sigma factors that inhibit the function of sigma factors.
Retention during trascription elongation
The core RNA polymerase (consisting of 2 alpha (α), 1 beta (β), 1 beta-prime (β'), and 1 omega (ω) subunits) binds a sigma factor to form a complex called the RNA polymerase holoenzyme. It was previously believed that the RNA polymerase holoenzyme initiates transcription, while the core RNA polymerase alone synthesises RNA. Thus, the accepted view was that sigma factor must dissociate upon transition from transcription initiation to transcription elongation (this transition is called "promoter escape"). This view was based on analysis of purified complexes of RNA polymerase stalled at initiation and at elongation. Finally, structural models of RNA polymerase complexes predict that as the growing RNA product becomes longer than ~10 nucleotides sigma must be "pushed out" of the holoenzyme, since there is a steric clash between RNA and a sigma domain. However, a recent study (reference *2) has shown that σ70 remains attached in complex with the core RNA polymerase, at least during early elongation. Indeed, the phenomenon of promoter-proximal stalling suggests that sigma may play a role during early elongation. All studies are consistent with the assumption that promoter escape reduces the lifetime of the sigma-core interaction from very long at initiation (too long to be measured in a typical biochemical experiment) to a shorter, measurable lifetime upon transition to elongation.
|This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Sigma_factor". A list of authors is available in Wikipedia.|