Curie-Centre National de la Recherche Scientifique, UMR 2027,

Escherichia coli UvrD is a non-ring-shaped model helicase, displaying a 30–50 polarity in DNA unwinding. Using a transverse magnetic tweezer and DNA hairpins, we mea-sured the unwinding kinetics of UvrD at various DNA-destabilizing forces. The multiform patterns of unwinding bursts and the distributions of the off-times favour the mechanism that UvrD unwinds DNA as a dimer. The two subunits of the dimer coordinate to unwind DNA proces-sively. They can jointly switch strands and translocate backwards on the other strand to allow slow (B40 bp/s) rewinding, or unbind simultaneously to allow quick rehybridization. Partial dissociation of the dimer results in pauses in the middle of the unwinding or increases the translocation rate from B40 to B150nt/s in the middle of the rewinding. Moreover, the unwinding rate was surpris-ingly found to decrease from B45 to B10bp/s when the force is increased from 2 to 12 pN. The results lead to a strained-inchworm mechanism in which a conformational change that bends and tenses the ssDNA is required to activate the dimer