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Impediment of E. coli UvrD by DNA-destabilizing force reveals a strained-inchworm mechanism of DNA unwinding
- Original article citation: EMBO J doi:10.1038/emboj.2008.240 (2008)
- Categories: Cell & Molecular Biology and Genetics
- Recommended by: Alison Wrigley on 11/21/2008 11:05AM GMT
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Escherichia coli UvrD is a non-ring-shaped model helicase, displaying a 3'–5' polarity in DNA unwinding. Using a transverse magnetic tweezer and DNA hairpins, we measured 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 processively. They can jointly switch strands and translocate backwards on the other strand to allow slow (approx40 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 approx40 to approx150 nt/s in the middle of the rewinding. Moreover, the unwinding rate was surprisingly found to decrease from approx45 to approx10 bp/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.
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