Folding and structural polymorphism of G-quadruplex formed from a long telomeric sequence containing six GGG tracts

Abstract

The structure and stability of a G-quadruplex formed by a long human telomeric sequence containing five or more TTAGGG repeats are not clear yet. Using the guanine-to-inosine (G-to-I) substitution, we conducted thermodynamic studies on the structural polymorphisms of G-quadruplexes formed by the long telomeric sequences, 37htel and five G-to-I substituted sequences (I-1/2, I-1/6, I-4/5, I-4/6, and I-5/6), and investigated their folding dynamics at single-molecule level. The thermodynamic study reveals that a G-quadruplex formed from I-1/2 has a higher T_m and a larger ΔG_298K than those formed by other G-to-I substituted sequences, suggesting that a long telomeric sequence preferentially forms a thermodynamically stable G-quadruplex at the 3′ end. In addition, from changes in the hydrodynamic radius by the formation of a G-quadruplex at single-molecule level, we found that the folding reaction of 37htel may proceed through a two-state mechanism without any detectable intermediate and that the global structure, which leads to the change in molecular size, is still occurring even after the formation of a secondary structure (G-quadruplex).