Distinguishing G-Quadruplexes Stabilizer and Chaperone for c-MYC Promoter G-Quadruplexes through Single-Molecule Manipulation

G-quadruplex (G4) selective stabilizing ligands can regulate c-MYC gene expression, but the kinetic basis remains unclear. Determining the effects of ligands on c-MYC promoter G4s' folding/unfolding kinetics is challenging due to the polymorphic nature of G4s and the high energy barrier to unfold c-MYC promoter G4s. Here, we used single-molecule magnetic tweezers to manipulate a duplex hairpin containing a c-MYC promoter sequence to mimic the transiently denatured duplex during transcription. We measured the effects of six commonly used G4s binding ligands on the competition between quadruplex and duplex structures, as well as the folding/unfolding kinetics of G4s. Our results revealed two distinct roles for G4s selective stabilization: CX-5461 is mainly acting as c-MYC G4s stabilizer, reducing the unfolding rate (ku) of c-MYC G4s, whereas PDS and 360A also act as G4s chaperone, accelerating the folding rates (kf) of c-MYC G4s. qRT-PCR results obtained from CA46 and Raji cell lines demonstrated that G4s stabilizing ligands can downregulate c-MYC expression, while G4s stabilizer CX-5461 exhibited the strongest c-MYC gene suppression. These results shed light on the potential of manipulating G4s' folding/unfolding kinetics by ligands for precise regulation of promoter G4-associated biological activities.