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G-quadruplex structures inhibit telomerase activity and must be disrupted for telomere elongation during S phase. It has been suggested that the replication protein A (RPA) could unwind and maintain single-stranded DNA in a state amenable to the binding of telomeric components. We show here that under near-physiological in vitro conditions, human RPA is able to bind and unfold G-quadruplex structures formed from a 21mer human telomeric sequence. Analyses by native gel electrophoresis, cross-linking and fluorescence resonance energy transfer indicate the formation of both 1:1 and 2:1 complexes in which G-quadruplexes are unfolded. In addition, quadruplex opening by hRPA is much faster than observed with the complementary DNA, demonstrating that this protein efficiently unfolds G-quartets. A two-step mechanism accounting for the binding of hRPA to G-quadruplexes is proposed. These data point to the involvement of hRPA in regulation of telomere maintenance.

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