Distribution of Conformational States Adopted by DNA from the Promoter Regions of the VEGF and Bcl-2 Oncogenes

Abstract

We employed a previously described procedure, based on circular dichroism (CD) spectroscopy, to quantify the distribution of conformational states adopted by equimolar mixtures of complementary G-rich and C-rich DNA strands from the promoter regions of the VEGF and Bcl-2 oncogenes. Spectra were recorded at different pHs, concentrations of KCl, and temperatures. The temperature dependences of the fractional populations of the duplex, G-quadruplex, i-motif, and coiled conformations of each promoter were then analyzed within the framework of a thermodynamic model to obtain the enthalpy and melting temperature of each folded-to-unfolded transition involved in the equilibrium. A comparison of the conformational data on the VEGF and Bcl-2 DNA with similar results on the c-MYC DNA, which we reported previously, reveals that the distribution of conformational states depends on the specific DNA sequence and is modulated by environmental factors. Under the physiological conditions of room temperature, neutral pH, and elevated concentrations of potassium ions, the duplex conformation coexists with the G-quadruplex conformation in proportions that depend on the sequence. This observed conformational diversity has biological implications, and it further supports our previously proposed thermodynamic hypothesis of gene regulation. In that hypothesis, a specific distribution of duplex and tetraplex conformations in a promoter region is fine-tuned to maintain the healthy level of gene expression. Any deviation from a healthy distribution of conformational states may result in pathology stemming from up- or downregulation of the gene.