BLM unfolds G-quadruplexes in different structural environments through different mechanisms

Mutations in the RecQ DNA helicase gene BLM give rise to Bloom’s syndrome, which is a rare autosomal recessive disorder characterized by genetic instabil-ity and cancer predisposition. BLM helicase is highly active in binding and unwinding G-quadruplexes (G4s), which are physiological targets for BLM, as re-vealed by genome-wide characterizations of gene ex-pression of cells from BS patients. With smFRET as-says, we studied the molecular mechanism of BLM-catalyzed G4 unfolding and showed that ATP is re-quired for G4 unfolding. Surprisingly, depending on the molecular environments of G4, BLM unfolds G4 through different mechanisms: unfolding G4 harbor-ing a 3′-ssDNA tail in three discrete steps with unidi-rectional translocation, and unfolding G4 connected to dsDNA by ssDNA in a repetitive manner in which BLM remains anchored at the ss/dsDNA junction, and G4 was unfolded by reeling in ssDNA. This in-dicates that one BLM molecule may unfold G4s in different molecular environments through different mechanisms