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SIGMA RETENTION


  • 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 infinitely long at initiation (too long to be measured in a typical biochemical experiment) to a shorter lifetime upon transition to elongation (which can be addressed in a typical biochemical experiment).


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.


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''). It is involved in ensuring the sigma factor will only bind the promoter when it is complexed with the RNA polymerase.

  • Region 2.4 recognizes and binds to the -10 Promoter site.

  • Region 4.2 recognizes and 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. 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. Makes the proteins that are necessary to keep the cell alive.

  • σ54 (RpoN) - the nitrogen-limitation sigma factor

  • σ38 (RpoS) - the starvation/stationary phase sigma factor

  • σ32 (RpoH) - the heat shock sigma factor, it is turned on when exposed to heat

  • σ28 (RpoF) - the flagellar sigma factor

  • σ24 (RpoE) - the extracytoplasmic/extreme heat stress sigma factor

  • σ19 (FecI) - the ferric citrate sigma factor, regulates the fec gene for iron transport


There are also Anti-sigma Factors that inhibit the function of sigma factors.


REFERENCES

  • Gruber TM, Gross CA. (2003). Multiple sigma subunits and the partitioning of bacterial transcription space. ''Annu Rev Microbiol''. 57, 441-66. PMID 14527287 PDF




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