Sigma factors are usually bound to the RNA polymerase in the cell . A complete bacterial polymerase with bound sigma factor is often referred to as a holoenzyme . It consists of six subunits (α 2 ββ'ωσ). The minimal or core enzyme, on the other hand, is not bound to the σ subunit. In other contexts, however, the meaning of holoenzyme is often different.
Sigma factors have a high affinity for the Pribnow box and the -35 sequence of the promoter . This increases the probability of the polymerase holoenzyme binding to the starting point of the open reading frame of the DNA. The RNA polymerase can bind to the DNA even without sigma factor, but transcription does not occur . When the polymerase holoenzyme forms the open complex and transcription begins, the sigma factor is split off.
Depending on the environmental conditions , bacteria usually express several different sigma factors, which generally have different promoter specificities. This mediates the transcription of special genes to adapt to the environmental conditions. Two classes of sigma factors are known. A class with many representatives shows homologies to factor Sigma-70 of the bacterial species Escherichia coli . A smaller family - in most bacteria with only one representative - is homologous to the E. coli factor Sigma-54. This differs greatly from the Sigma-70 family both structurally and in terms of the mechanism of transcription initiation . Sigma factors are characterized by their molecular mass . The factor σ 70, for example, describes the sigma factor with a molecular mass of 70 kDa .
Sigma factors from E. coli :
|Sigma factor||gene||Recognition sequence (-35)||Recognition sequence (-10)||Expression|
|σ 70||rpoD||TTGACA||TATAATg||under normal conditions|
|σ 32||rpoH||CTTGAAA||CciATNT||with heat stress|
|σ 54||rpoN||CTGGCAC||TTGCA||if there is a lack of nitrogen|
|σ 28||rpoF||TAAA||GCCGATAA||Flagella expression|
|σ 38||rpoS||CCGGCG||general stress response|
|σ 19||FecI||AAGGAAAAT||Iron transport|
|σ 24||rpoE||GAACTT||TCTGA||with cell envelope stress|
- Sigma 70 is the housekeeping sigma factor of E. coli , which initiates the transcription of those genes whose gene products are needed under normal environmental conditions.
- Sigma 32 is the heat shock sigma factor in E. coli that is encoded by the rpoH gene. If the temperature in the cell increases, the factor is increasingly synthesized. Due to the increased sigma-32 concentration in the cell, the protein is more likely to bind to the polymerase core enzyme. As a result, heat shock proteins are expressed, which help the cell to survive the elevated temperatures. They include, for example, chaperones , proteases and DNA repair enzymes.
- Sigma 54 is encoded by the rpoN gene and produced when there is a nitrogen deficiency. The Sigma 54 holoenzyme activates, among other things, the expression of glutamine synthetase , which is the key enzyme for nitrogen assimilation.
- Sigma 38 is the sigma factor of the general stress response in E. coli . In the case of stress factors such as a lack of carbon, a lack of amino acids or over- acidity , Sigma 38 expression is upregulated. This is the case, for example, when a bacterial culture changes into the stationary phase. Proteins are expressed under Sigma 38 that protect the cell from harmful environmental influences.
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