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6 Bacterial Sigma Factors

Carol A. Gross, Michael Lonetto, Richard Losick


Sigma factors confer promoter specificity to bacterial core RNA polymerase (E). There are two major families of sigmas, one related in protein sequence to σ70, the primary Escherichia coli sigma, and a second related to σ54, an alternative E. coli sigma involved in transcribing nitrogen-regulated genes. We present an updated analysis of the sequence conservation among σ70-related sigmas, contrast transcription initiation at Eσ70 and Eσ54 promoters, and discuss the role of σ70-like and σ54-like sigmas in binding to core RNA polymerase and promoter recognition. The existence of alternative sigma subunits provides a way to coordinately regulate gene expression. We discuss how the synthesis, stability, and activity of alternative sigmas are regulated to achieve elaborate control of the expression of a variety of regulons in response to environmental stimuli and developmental signals.

In a groundbreaking study published in 1969, Burgess and Travers at Harvard, and Dunn and Bautz working independently at Rutgers, discovered that bacterial RNA polymerase could be split into two components (Burgess et al. 1969). Transcription of a phage T4 DNA template required not only core RNA polymerase (E), which remains bound to a phosphocellulose column, but also a new stimulatory factor called sigma (σ), which flows through the phosphocellulose column. The function of sigma was clarified in short order (Dunn and Bautz 1969; Travers and Burgess 1969). It was found to promote initiation of transcription in a catalytic manner; that is, one sigma molecule permitted many core RNA polymerases to initiate transcription (for review, see Burgess...

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