Molecular Biology

Sigma Holds On

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Science  14 Sep 2001:
Vol. 293, Issue 5537, pp. 1955-1957
DOI: 10.1126/science.293.5537.1955e

Many decades of work have provided details of the molecular mechanism and machinery needed for prokaryotic gene expression. Bacterial genes are transcribed by the RNA polymerase enzyme, which associates with a sigma subunit during transcription initiation. Afterwards, the sigma subunit is released from the polymerase and an elongation factor, NusA, binds. NusA remains bound to the polymerase throughout transcription elongation and termination and is then removed to allow sigma to bind once again for transcription re-initiation. This is termed the “sigma cycle.”

In two recent papers, Bar-Nahum and Nudler used a purification strategy and Mukhopadhyay et al. used a fluorescence resonance energy transfer (FRET) technique to show that some populations of E. coli RNA polymerase retain s70 throughout elongation. These studies question the traditional view of bacterial transcription and the sigma cycle, as well as the suggestion that initiation and elongation are mechanistically distinct. Sigma retention may represent a regulatory mechanism used by the cell to allow multiple rounds of transcription at particular promoters. — BAP

Cell106, 443; 453 (2001).

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