Interactions between the Werner syndrome helicase and DNA polymerase delta 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 alpha, pol delta, and pol epsilon. The constraints imposed on pol delta-catalyzed synthesis are relieved, however, by WRN; WRN facilitates pol delta to traverse these template secondary structures to synthesize full-length DNA products. The alleviatory effect 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 presence of WRN. Alleviation of pausing by pol delta is observed with Escherichia coli RecQ but not with UvrD helicase, suggesting a concerted action of RecQ helicases and pol delta. Our findings suggest a possible role of WRN in rescuing pol delta-mediated replication at forks stalled by unusual DNA secondary structures.
Gottstein Memorial Cancer Research Laboratory, Departments of Pathology and Biochemistry, University of Washington, Seattle, Washington 98195, USA. laloeb@u.washington.edu
A S
AS
Kamath-Loeb
L A
LA
Loeb
E
E
Johansson
P M
PM
Burgers
M
M
Fry
D001483:Base Sequence; D004247:DNA / Q000737:chemistry* / Q000235:genetics*; D004265:DNA Helicases / Q000378:metabolism*; D004258:DNA Polymerase III / Q000378:metabolism*; D017931:DNA Primers; D005090:Exodeoxyribonucleases; D006801:Humans; D008969:Molecular Sequence Data; D009690:Nucleic Acid Conformation; D053484:RecQ Helicases; D011994:Recombinant Proteins / Q000378:metabolism; D013698:Templates, Genetic; D018911:Trinucleotide Repeats; D014898:Werner Syndrome / Q000201:enzymology* / Q000235:genetics; D000071657:Werner Syndrome Helicase
D017931:DNA Primers; D011994:Recombinant Proteins; D004247:DNA; D004258:DNA Polymerase III; D005090:Exodeoxyribonucleases; D004265:DNA Helicases; D053484:RecQ Helicases; C098785:WRN protein, human; D000071657:Werner Syndrome Helicase