The Bloom's syndrome helicase unwinds G4 DNA

BLM, the gene that is defective in Bloom's syndrome, encodes a protein homologous to RecQ subfamily helicases that functions as a 3'-5' DNA helicase in vitro. We now report that the BLM helicase can unwind G4 DNA. The BLM G4 DNA unwinding activity is ATP-dependent and requires a short 3' region of single-stranded DNA. Strikingly, G4 DNA is a preferred substrate of the BLM helicase, as measured both by efficiency of unwinding and by competition. These results suggest that G4 DNA may be a natural substrate of BLM in vivo and that the failure to unwind G4 DNA may cause the genomic instability and increased frequency of sister chromatid exchange characteristic of Bloom's syndrome.