A Sisyphean helicase

JCB • VOLUME 171 • NUMBER 4 • 2005 582 A Sisyphean helicase ome helicases continually bang their heads against the wall, if a new report from Sua Myong, Taekjip Ha (University of Illinois, UrbanaChampaign, IL), and colleagues is any indication. The Rep helicase, the group finds, repeatedly motors along a track of DNA, hits an obstacle, and returns to square one. Not all helicases can unwind DNA, but they can all motor along it. While studying how Rep motors in a 5 direction on single-stranded (ss) DNA, Ha’s group noticed that a duplex DNA obstacle did not knock Rep off its template. Their FRET analysis instead suggested that the helicase returned to its original binding site and tried again. Unlike the 5 motoring, the return step was almost instantaneous. “At first it seemed like Rep was doing some sort of quantum tunneling,” says Ha, who is a physicist by training. But more FRET studies cleared up the situation—Rep, it seemed, was transiently bound both to S