In the regulation of gene expression in prokaryotes, anti-sigma factors bind to sigma factors and inhibit their transcriptional activity. Anti-sigma factors have been found in Escherichia coli, Salmonella, and T4 bacteriophage. Anti-sigma factors are antagonists to the sigma factors, which regulate numerous cell processes including flagellar production, stress response, transport and cellular growth. For example, anti-sigma factor 70 Rsd in E. coli is present in the stationary phase and blocks the activity of sigma factor 70 which in essence shuts down gene transcription. This allows the sigma S factor to associate with RNA polymerase and direct the expression of the stationary genes.
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