FORCE AND VELOCITY MEASURED FOR SINGLE MOLECULES OF RNA-POLYMERASE

RNA polymerase (RNAP) moves along DNA while carrying out transcription , acting as a molecular motor. Transcriptional velocities for single m olecules of Escherichia coli RNAP were measured as progressively large r forces were applied by;a feedback-controlled optical trap. The shape s of RNAP force-velocity curves are distinct from those of the motor e nzymes myosin or kinesin, and indicate that biochemical steps limiting transcription rates at Low loads do not generate movement. Modeling t he data suggests-that high Loads may halt RNAP by promoting a structur al change which moves all or part of the enzyme backwards through a co mparatively Large distance, corresponding to 5 to 10 base pairs. This contrasts with previous models that assumed force acts directly upon a single-base translocation step.