Transcription Under Torsion

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Science  28 Jun 2013:
Vol. 340, Issue 6140, pp. 1580-1583
DOI: 10.1126/science.1235441

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Keeping Transcription Going

In cells, the DNA double-stranded helix (dsDNA) is mostly supercoiled—either under- or overwound. RNA polymerase (RNAP) must transcribe though this supercoiled DNA. Furthermore, the act of transcription, which involves opening the double helix and threading the separated strands through the enzyme, generates supercoiling ahead and behind the polymerase. Ma et al. (p. 1580) used single-molecule methods to measure the upstream and downstream torque forces of Escherichia coli RNAP. The upstream torque was sufficient to disrupt dsDNA structure, and the stalled RNAP could also backtrack along the DNA. Release of the torsional stress allowed RNAP to resume transcription in vitro.


In cells, RNA polymerase (RNAP) must transcribe supercoiled DNA, whose torsional state is constantly changing, but how RNAP deals with DNA supercoiling remains elusive. We report direct measurements of individual Escherichia coli RNAPs as they transcribed supercoiled DNA. We found that a resisting torque slowed RNAP and increased its pause frequency and duration. RNAP was able to generate 11 ± 4 piconewton-nanometers (mean ± standard deviation) of torque before stalling, an amount sufficient to melt DNA of arbitrary sequence and establish RNAP as a more potent torsional motor than previously known. A stalled RNAP was able to resume transcription upon torque relaxation, and transcribing RNAP was resilient to transient torque fluctuations. These results provide a quantitative framework for understanding how dynamic modification of DNA supercoiling regulates transcription.

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