A Thermodynamic Perspective on Potential G-Quadruplex Structures as Silencer Elements in the MYC Promoter

Multiple G-tracts within the promoter region of the c-myc oncogene may fold into various G-quadruplexes with the recruitment of different tracts and guanosine residues for the G-core assembly. Thermodynamic profiles for the folding of wild-type and representative truncated as well as mutated sequences were extracted by comprehensive DSC experiments. The unique G-quadruplex involving consecutive G-tracts II-V with formation of two one-nucleotide and one central two-nucleotide propeller loop, previously proposed to be the biologically most relevant species, was found to be the most stable fold in terms of its Gibbs free energy of formation at ambient temperatures. Its stability derives from its short propeller loops but also from the favorable type of loop residues. Whereas quadruplex folds with long propeller loops are significantly disfavored, a snap-back loop structure formed by incorporating a 3'-terminal guanosine into the empty position of a tetrad seems highly competitive based on its thermodynamic stability. However, its destabilization by extending the 3'-terminus questions the significance of such a species under in vivo conditions.