Single-Molecule FRET Investigations of Tandem Human Telomeric G-Quadruplex Structures

Abstract

Consecutive DNA G-quadruplexes, which are non-canonical nucleic acids structures, can form from long guanine-rich sequences in the genome. One such region is the single-stranded human telomeric overhang, which contains the 5’-TTAGGG-3’ motif in multiple repeats. G-quadruplex structures can serve as regulatory switches in a number of biological processes and are considered potential drug targets for cancer treatment (Balasubramanian, S. et al. Nat. Rev. Drug. Discov., 2011). Single telomeric G-quadruplex structures are highly polymorphic (Dai, J et al. Biochimie, 2008), which can influence its interaction with proteins and small molecule drugs. Here, we investigate folding and polymorphism of DNA sequences capable of forming two consecutive G-quadruplexes. We investigate the structure and dynamics of fluorophore labeled telomeric DNA sequences containing more than 37 bases able to form two consecutive G-quadruplexes using single-molecule Föster Resonance Energy Transfer (FRET), circular dichroism spectroscopy and UV-melting experiments. Our data show that various structures form under different cation conditions (K+, Na+ or Li+). G-quadruplex folding occurs in the presence of both Na+ and K+, where we observe high FRET peaks. Our data suggest formation of multiple different G-quadruplex structures under cellular like salt conditions.