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Our data challenge Chen et al.'s interpretation of smFRET results, i.e. the repetitive unfolding of G4 with one-base translocations. We believe that the observed oscillatory curve represents the alternate binding of DHX36 to the 3' and 5'G-tetrad of G4s, rather than the repetitive unfolding between the canonical and the transformed non-canonical G4s. Noteworthily, our results reported here also call into question the previously published smFRET data of helicase-mediated G4 unfolding. Therefore,discriminating the smFRET signal of repetitive binding from that of unfolding is very important to avoid misinterpreting smFRET results., Comment: 10 pages, 4 figures, 1 Table

Chen et al. solved the crystal structure of bovine DHX36 bound to a DNA with a G-quadruplex (G4) and a single-stranded DNA segment. They believed that the mechanism they proposed may represent a general model for describing how a G4-unfolding helicase recognizes and unfolds G4 DNA. Their conclusion is interesting, however, we noticed that their linear DNA substrate (DNAMyc) that harbors a Myc-promoter-derived G4-forming sequence was directly used without pre-folding. This raises the question whether the structure they obtained really reflects DHX36-mediated G4 recognition and unfolding, or just only represents a DHX36-binding-induced quasi-folded G4 structure. By a combination of polymerase extension, DMS footprinting, stopped-flow, and smFRET assays, we obtained clear evidences that do not support their ATP-independent one-base translocation structural model. We further revealed that the oscillation of FRET signal they observed should correspond to a repetitive G4 binding, but not unfolding, by DHX36.