Helical filaments driven by linear molecular motors are anticipated to rotate around their axis, but rotation consistent with the helical pitch has not been observed. 14S dynein1 and non-claret disjunctional protein (ncd)2 rotated a microtubule more efficiently than expected for its helical pitch, and myosin rotated an actin filament only poorly3. For DNA-based motors such as RNA polymerase, transcription-induced supercoiling of DNA4 supports the general picture of tracking along the DNA helix5. Here we report direct and real-time optical microscopy measurements of rotation rate that are consistent with high-fidelity tracking. Single RNA polymerase molecules attached to a glass surface rotated DNA for >100 revolutions around the right-handed screw axis of the double helix with a rotary torque of >5 pNnm. This real-time observation of rotation opens the possibility of resolving individual transcription steps.
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