Interactions between the Werner syndrome helicase and DNA polymerase deltaspecifically facilitate copying of tetraplex and hairpin structures of thed(CGG)(n) trinucleotide repeat sequence

Werner syndrome (WS) is an inherited disorder characterized by premature ag ing and genomic instability. The protein encoded by the WS gene, WRN, posse sses intrinsic 3' --> 5' DNA helicase and 3' --> 5' DNA exonuclease activit ies. WRN helicase resolves alternate DNA structures including tetraplex and tripler DNA, and Holliday junctions. Thus, one function of WRN may be to u nwind 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 eukar yotic replicative DNA polymerases (pol): pol alpha, pol delta, and pol epsi lon. The constraints imposed on pol delta -catalyzed synthesis are relieved , however, by WRN; WRN facilitates pol delta to traverse these template sec ondary structures to synthesize full-length DNA products. The alleviatory e ffect of WRN is limited to pol delta; neither pol alpha nor pol epsilon can traverse template d(CGG)(n) hairpin and tetraplex structures in the presen ce of WRN, Alleviation of pausing by pol delta is observed with Escherichia coli RecQ but not with UvrD helicase, suggesting a concerted action of Rec Q helicases and pol delta, Our findings suggest a possible role of WRN in r escuing pol delta -mediated replication at forks stalled by unusual DNA sec ondary structures.