Structural Diversity of G-Quadruplexes: Potassium Concentration Effect

Abstract

G-quadruplex (G4) structure, folded from Guanine-rich sequences, is a well known unique DNA secondary structure through Hoogsteen base pairing in the presence of monovalent cation. The importance of G4 structure is not only in human telomeres for protecting the ends of chromosomes, but also in several gene promoters for regulating gene expression.Here we have combined gel electrophoresis, circular dichroism, and thermal melting to study the possible coexistence of the intramolecular and intermolecular G-quadruplexes in the presence of various concentrations of potassium cation (K+). Our results showed that an appreciable amount of intermolecular G-quadruplex structures are detected in c-myc even at 1mM K+, and increases at high K+ concentration. Together with the quantification system, the amounts of intramolecular G-quadruplex structures of c-myc and bcl2 decrease as a function of K+ concentration. However, no discernible intermolecular structures of human telomeric sequences up to 150 mM K+ solution. In addition, upon late change of K+ concentration at room temperature, no appreciable exchange between intra- and intermolecular structures of blc2 is observed. Moreover, the change in melting temperature upon altering K+ concentration indicate that K+-quadruplex association is faster than the K+-quadruplex dissociation. Further thermodynamic studies based on differential scanning calorimetry and isothermal titration calorimetry measurements will be discussed.