The sigma subunit of the Escherichia coli RNA polymerase holoenzyme is the specificity factor that mediates the promoter recognition and binding σ subunit of molecular weight 70 kDa. Under some conditions, some of the 70-kDa subunits are replaced by one of the other, more-specific factors. For instance, when bacteria are subjected to heat stress, some of the 70-kDa subunits are replaced by a 32-kDa subunit; when bound to σ32, RNA polymerase is directed to a specialized set of promoters with a different consensus sequence.
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