Sigma factor
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A sigma factor (σ factor) is a prokaryotic transcription initiation factor that must be part of RNA polymerase for specific binding to promoter sites on DNA. Different sigma factors are activated in response to different environmental conditions, as are listed below. 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. All 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 ploymerase is a holoenzyme. The RNA polymerase cannot act as a processive enzyme until the sigma subunit dissociates from the core enzyme (consisting of 2 alphas, beta, and beta prime subunits). The holoenzyme is what actually carries out the elongation of the mRNA.
Alternative sigma factors are important in producing different mRNA strands than those produced by the cell under normal conditions. The new mRNA will create proteins that in turn will help the cell to survive the new conditions. Sigma 38 is probably the most important.
[edit] Structure
Sigma factors have four main regions that are generally conserved:
N---------------------C 1 2 3 4
The regions are further subdivided (e.g. 2 includes 2.1, 2.2, etc.)
- Region 1 is found only in "primary sigma factors" (RpoD, RpoS in E.coli)
- Region 2.4 binds to the -10 promoter site.
- Region 4.2 binds to the -35 promoter site.
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.
[edit] Specialized Sigma Factors
Developmental responses involve transcription of genes by RNAP containing specialized sigma factors. Different sigma factors can be expressed when a cell is exposed to different conditions.
E.coli sigma factors:
- σ70 (RpoD) - the "housekeeping" sigma factor, transcribes most genes in growing cells.
- σ38 (RpoS) - the starvation/stationary phase sigma factor
- σ28 (RpoF) - the flagellar sigma factor
- σ32 (RpoH) - the heat shock sigma Factor
- σ24 (RpoE) - the extracytoplasmic stress sigma factor
- σ54 (RpoN) - the nitrogen-limitation sigma factor
- σ19 (FecI) - the ferric citrate sigma factor
[edit] References
- Gruber TM, Gross CA. (2003). Multiple sigma subunits and the partitioning of bacterial transcription space. Annu Rev Microbiol. 57, 441-66. PMID 14527287 PDFde:Sigma-Faktor
