摘要：SummaryI-motifs are noncanonical four-stranded DNA structures formed by C-rich sequences at acidic environment with critical biofunctions. The particular pH sensitivity has inspired the development of i-motifs as pH sensors and DNA motors in nanotechnology. However, the folding and regulation mechanisms of i-motifs remain elusive. Here, using single-molecule FRET, we first show that i-motifs are more dynamic than G4s. Impressively, i-motifs display a high diversity of six folding species with slow interconversion. Further results indicate that i-motifs can be linearized by Replication protein A. More importantly, we identified a number of helicases with high specificity to i-motifs at low pH. All these helicases directly act on and efficiently resolve i-motifs into intermediates independent of ATP, although they poorly unwind G4 or duplex at low pH. Owing to the extreme sensitivity to helicases and no need for ATP, i-motif may be applied as a probe for helicase sensing bothin vitroandin vivo.Graphical abstractDisplay OmittedHighlights•I-motifs display a high diversity of six folding species with slow interconversion•Replication protein A efficiently linearizes i-motifs at acidic pH•Helicases specifically bind to and unfold i-motifs at acidic pH independent of ATP•I-motif can be used as a convenient probe for helicase sensingin vitroBiological sciences; Biochemistry; Molecular biology; Structural biology
