Translocation by T7 RNA polymerase: A sensitively poised brownian ratchet

Qing Guo, Rui Sousa

Research output: Contribution to journalArticle

41 Scopus citations

Abstract

Studies of halted T7 RNA polymerase (T7RNAP) elongation complexes (ECs) or of T7RNAP transcription against roadblocks due to DNA-bound proteins indicate that T7RNAP translocates via a passive Brownian ratchet mechanism. Crystal structures of T7RNAP ECs suggest that translocation involves an active power-stroke. However, neither solution studies of halted or slowed T7RNAP ECs, nor crystal structures of static complexes, are necessarily relevant to how T7RNAP translocates during rapid elongation. A recent single molecule study of actively elongating T7RNAPs provides support for the Brownian ratchet mechanism. Here, we obtain additional evidence for the existence of a Brownian ratchet during active T7RNAP elongation by showing that both rapidly elongating and halted complexes are equally sensitive to pyrophosphate. Using chemical nucleases tethered to the polymerase we achieve sub-ångström resolution in measuring the average position of halted T7RNAP ECs and find that the positional equilibrium of the EC is sensitively poised between pre-translocated and post-translocated states. This may be important in maximizing the sensitivity of the polymerase to sequences that cause pausing or termination. We also confirm that a crystallographically observed disorder to order transition in a loop formed by residues 589-612 also occurs in solution and is coupled to pyrophosphate or NTP release. This transition allows the loop to make interactions with the DNA that help stabilize the laterally mobile, ligand-free EC against dissociation.

Original languageEnglish (US)
Pages (from-to)241-254
Number of pages14
JournalJournal of Molecular Biology
Volume358
Issue number1
DOIs
StatePublished - Apr 21 2006

Keywords

  • Brownian ratchet
  • T7 RNA polymerase
  • Transcription dynamics
  • Transcription elongation
  • Translocation

ASJC Scopus subject areas

  • Structural Biology
  • Molecular Biology

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