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J. Biol. Chem., Vol. 283, Issue 36, 24359-24373, September 5, 2008

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Processing of G4 DNA by Dna2 Helicase/Nuclease and Replication Protein A (RPA) Provides Insights into the Mechanism of Dna2/RPA Substrate Recognition^*

From the Braun Laboratories, 147-75, California Institute of Technology, Pasadena, California 91125

The polyguanine-rich DNA sequences commonly found at telomeres and in rDNA arrays have been shown to assemble into structures known as G quadruplexes, or G4 DNA, stabilized by base-stacked G quartets, an arrangement of four hydrogen-bonded guanines. G4 DNA structures are resistant to the many helicases and nucleases that process intermediates arising in the course of DNA replication and repair. The lagging strand DNA replication protein, Dna2, has demonstrated a unique localization to telomeres and a role in de novo telomere biogenesis, prompting us to study the activities of Dna2 on G4 DNA-containing substrates. We find that yeast Dna2 binds with 25-fold higher affinity to G4 DNA formed from yeast telomere repeats than to single-stranded DNA of the same sequence. Human Dna2 also binds G4 DNAs. The helicase activities of both yeast and human Dna2 are effective in unwinding G4 DNAs. On the other hand, the nuclease activities of both yeast and human Dna2 are attenuated by the formation of G4 DNA, with the extent of inhibition depending on the topology of the G4 structure. This inhibition can be overcome by replication protein A. Replication protein A is known to stimulate the 5'- to 3'-nuclease activity of Dna2; however, we go on to show that this same protein inhibits the 3'- to 5'-exo/endonuclease activity of Dna2. These observations are discussed in terms of possible roles for Dna2 in resolving G4 secondary structures that arise during Okazaki fragment processing and telomere lengthening.

Received for publication, March 20, 2008 , and in revised form, June 13, 2008.

^* This work was supported, in whole or in part, by National Institutes of Health Grant GM087666 from the USPHS and a research grant from the Philip Morris External Research Program. 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.

¹ To whom correspondence should be addressed: Braun Laboratories 147-75, California Institute of Technology, Pasadena, CA 91125. Tel.: 626-395-6053; Fax: 626-449-0756; E-mail: jcampbel{at}its.caltech.edu.

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