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J. Biol. Chem., Vol. 281, Issue 44, 33198-33205, November 3, 2006

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Mechanistic Analysis of a DNA End Processing Pathway Mediated by the Xenopus Werner Syndrome Protein^*

From the Fox Chase Cancer Center, Philadelphia, Pennsylvania 19111

The first step of homology-dependent repair of DNA double-strand breaks is the strand-specific processing of DNA ends to generate 3' single-strand tails. Despite its importance, the molecular mechanism underlying end processing is poorly understood in eukaryotic cells. We have taken a biochemical approach to investigate DNA end processing in nucleoplasmic extracts derived from the unfertilized eggs of Xenopus laevis. We found that double-strand DNA ends are specifically degraded in the 5' 3' direction in this system. The reaction consists of two steps: an ATP-dependent unwinding of double-strand ends and an ATP-independent 5' 3' degradation of single-strand tails. We also found that the Xenopus Werner syndrome protein, a member of the RecQ helicase family, plays an important role in DNA end processing. Mechanistically, Xenopus Werner syndrome protein (xWRN) is required for the unwinding of DNA ends but not for the degradation of single-strand tails. The xWRN-mediated end processing is remarkably similar to the end processing that has been proposed for the Escherichia coli RecQ helicase and RecJ single-strand nuclease, suggesting that this mechanism might be conserved in prokaryotes and eukaryotes.

Received for publication, May 25, 2006 , and in revised form, August 25, 2006.

^* This work was supported by National Institutes of Health Grant R01 GM57962-02) (to H. Y.). The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

The on-line version of this article (available at http://www.jbc.org) contains supplemental Fig. S1.

¹ To whom correspondence should be addressed: Fox Chase Cancer Center, 333 Cottman Ave., Philadelphia, PA 19111. Tel.: 215-728-2514; Fax: 215-728-3574; E-mail: Hong_Yan{at}fccc.edu.

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