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  • The EMBO Journal (2008) 27, 3279 - 3287
  • doi:10.1038/emboj.2008.240

Published online: 13 November 2008

Impediment of E. coli UvrD by DNA-destabilizing force reveals a strained-inchworm mechanism of DNA unwinding

Bo Sun1, Kong-Ji Wei1, Bo Zhang2, Xing-Hua Zhang1, Shuo-Xing Dou1, Ming Li1 and Xu Guang Xi2

  1. Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing, China
  2. Institut Curie-Centre National de la Recherche Scientifique, UMR 2027, Université Paris Sud-XI, Orsay, France.

Correspondence to:

Ming Li, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China. Tel.: +86 10 82649058; Fax: +86 10 82640224; E-mail: mingli@aphy.iphy.ac.cn

Xu Guang Xi, CNRS, UMR 2027, Institut Curie–Section de Recherche, Centre Universitaire, Bâtiment 110, 91405 Orsay, France. Tel.: +33 1 698 631 81; Fax: +331 698 694 29; E-mail: xu-guang.xi@curie.u-psud.fr

Received 27 June 2008; Accepted 24 October 2008

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.

  • Keywords:

    • helicase,
    • molecular motor,
    • single molecule,
    • unwinding mechanism,
    • UvrD