Human werner syndrome DNA helicase unwinds tetrahelical structures of the fragile X syndrome repeat sequence d(CGG)n.
Formation of hairpin and tetrahelical structures by a d(CGG) trinucleotide repeat sequence is thought to cause expansion of this sequence and to engender fragile X syndrome. Here we show that human Werner syndrome DNA helicase (WRN), a member of the RecQ family of helicases, efficiently unwinds G'2 bimolecular tetraplex structures of d(CGG)7. Unwinding of d(CGG)7 by WRN requires hydrolyzable ATP and Mg2+ and is proportional to the amount of added helicase and to the time of incubation. The efficiencies of unwinding of G'2 d(CGG)7 tetraplex with 7 nucleotide-long single-stranded tails at their 3' or 5' ends are, respectively, 3.5- and 2-fold greater than that of double-stranded DNA. By contrast, WRN is unable to unwind a blunt-ended d(CGG)7 tetraplex, bimolecular tetraplex structures of a telomeric sequence 5'-d(TAGACATG(TTAGGG)2TTA)-3', or tetramolecular quadruplex forms of an IgG switch region sequence 5'-d(TACAGGGGAGCTGGGGTAGA)-3'. The ability of WRN to selectively unwind specific tetrahelices may reflect a specific role of this helicase in DNA metabolism.
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Unit of Biochemistry, Rappaport Faculty of Medicine, Technion-Israel Institute of Technology, P. O. Box 9649, Haifa 31096, Israel. mickey@tx.technion.ac.il
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D001483:Base Sequence; D004247:DNA / Q000737:chemistry; D004265:DNA Helicases / Q000737:chemistry / Q000378:metabolism*; D005600:Fragile X Syndrome / Q000235:genetics*; D006801:Humans; D009690:Nucleic Acid Conformation; D018911:Trinucleotide Repeats; D014898:Werner Syndrome / Q000201:enzymology*
D004247:DNA; D004265:DNA Helicases