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Bacterial transcription

Sigma 54 minds the gap

Nature Reviews Microbiology volume 7, page 3 (2009)Cite this article

The main bacterial sigma factor (σ70) transcribes housekeeping genes, whereas the alternate sigma factor (σ54) transcribes a subset of genes. Despite limited amino-acid similarity between σ54 and σ70, these sigma factors bind to overlapping regions of the RNAP. However, whereas the RNAP–σ70 holoenzyme is competent for transcription, the RNAP–σ54 complex binds to promoter DNA, then remains stalled and unable to transcribe. When the activator protein PspF (an AAA+ family protein) binds to a bacterial enhancer-like sequence in the promoter and hydrolyses ATP, the RNAP–σ54 complex is converted from a 'closed' (bound but stalled) to an 'open' complex that can actively transcribe. Activation of RNAP–σ54 transcription by PspF is therefore analogous to activation of RNA polymerase II-mediated transcription in eukaryotes by the ATP-hydrolysing protein TFIIH.

Cryo–EM reconstruction was used to reveal interactions made between RNAP, σ54 and the activator protein PspF. Bose et al. compared the cryo–EM reconstruction of RNAP with RNAP–σ54, fitted into this the crystal structure of the RNAP enzyme from Thermus thermophilus and imaged RNAP complexed with truncated versions of σ54, or a form of σ54 tagged at the carboxyl terminus with nanogold, to reveal the position of the C-terminal promoter recognition domain of the sigma factor within the holoenzyme. To identify the likely position of DNA in the RNAP–σ54 complex, they inspected cryo–EM reconstructions of RNAP–σ54 and T. thermophilus holoenzyme complexed with DNA. Strikingly, the σ54 C-terminal promoter interaction domain seems to misalign promoter DNA with the active site of the RNAP.

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ORIGINAL RESEARCH PAPER

  1. Bose, D. et al. Organization of an activator-bound RNA polymerase holoenzyme. Mol. Cell 32, 337–346 (2008)

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  1. Susan Jones
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Jones, S. Sigma 54 minds the gap. Nat Rev Microbiol 7, 3 (2009). https://doi.org/10.1038/nrmicro2058

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  • DOI: https://doi.org/10.1038/nrmicro2058

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