A general way that prokaryotes can regulate gene expression is through the expression of alternate sigma factors. Recall that the sigma factor, or sigma subunit, of the RNA Polymerase holoenzyme is responsible for specific recognition of the promoter sequence. An RNA Polymerase without the sigma factor binds to DNA randomly.

In E. coli the sigma factor that recognizes the promoter discussed in the Transcription section is called sigma⁷⁰ based on it's molecular weight in kiloDaltons. There are additional sigma factors that are expressed only under certain conditions, such as when the cell goes into heat shock. Once the alternate sigma factors is expressed it can bind to RNA Polymerase core enzymes to form holoenzymes which can then bind to different promoters than holoenzymes with the sigma⁷⁰ factor. As a result, the expression of the heat shock sigma factor leads to the transcription of those genes with a "heat shock" promoter.

Note that the expression of an alternate sigma factor does not mean that sigma⁷⁰ is not expressed; instead both sigma factors are expressed and so there are different holoenzymes available, each transcribing a specific set of genes.

Some of the sigma factors in E. coli:

sigma⁷⁰: The general sigma factor for genes expressed during normal growth.

sigma⁵⁴: The nitrogen-limited sigma factor.

sigma³⁸: The stationary-phase or starvation sigma factor.

sigma³²: The heat-shock sigma factor for genes expressed when the cell goes into heat shock.

Prokaryotic Regulation

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