Interactions between the Werner Syndrome Helicase and DNA Polymerase δ Specifically Facilitate Copying of Tetraplex and Hairpin Structures of the d(CGG)_n Trinucleotide Repeat Sequence

Werner syndrome (WS) is an inherited disorder characterized by premature aging and genomic instability. The protein encoded by the WS gene, WRN, possesses intrinsic 3′ → 5′ DNA helicase and 3′ → 5′ DNA exonuclease activities. WRN helicase resolves alternate DNA structures including tetraplex and triplex DNA, and Holliday junctions. Thus, one function of WRN may be to unwind secondary structures that impede cellular DNA transactions. We report here that hairpin and G′2 bimolecular tetraplex structures of the fragile X expanded sequence, d(CGG)_n, effectively impede synthesis by three eukaryotic replicative DNA polymerases (pol): pol α, pol δ, and pol ε. The constraints imposed on pol δ-catalyzed synthesis are relieved, however, by WRN; WRN facilitates pol δ to traverse these template secondary structures to synthesize full-length DNA products. The alleviatory effect of WRN is limited to pol δ; neither pol a nor pol ε can traverse template d(CGG)_n hairpin and tetraplex structures in the presence of WRN. Alleviation of pausing by pol δ is observed with Escherichia coli RecQ but not with UvrD helicase, suggesting a concerted action of RecQ helicases and pol δ. Our findings suggest a possible role of WRN in rescuing pol δ-mediated replication at forks stalled by unusual DNA secondary structures.