Sigma factor

Encyclopedia : S : SI : SIG : Sigma factor

A sigma factor (σ factor) is a prokaryotic initiation enzyme factor that must be part of RNA polymerase for attachment 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 Escherichia coli is one of the ones listed below. E.coli has seven sigma factors, whereas other bacteria contain a variable number of sigma factors.

Sigma factors are important in producing different mRNA strands than what are 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.

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.

Specialized Sigma Factors

Developmental responses involve transcription of genes by RNAP containing specialized sigma factors.

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

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