Selective recognition of oncogene promoter G-quadruplexes by Mg2+

Mg2+ is one of the most important cations in cells, affecting the structures and functions of the proteins and nucleic acids. It should be noted that Mg2+ is indispensable in DNA transcription, where G-quadruplex is believed to be actively involved. Therefore, it is important to investigate the influence of Mg2+ on G-quadruplex. Here we studied the effect of Mg2+ on G-quadruplex DNA with CD, FRET, EMSA, and PCR-stop assay. We found that various G-quadruplexes could be differentiated through simultaneous addition of both K+ and Mg2+, which could be used for selective identification of G-quadruplexes in promoter oncogene but not in telomere. Mg2+ at physiological relevant concentration not only greatly enhanced the thermostability of oncogene G-quadruplexes but also efficiently protected them from unfolding by their complementary strands, which revealed the great impact of Mg2+ on the equilibrium between promoter G-quadruplex and duplex DNA. The PCR-stop assay further confirmed that Mg2+ could affect gene transcription by stabilizing promoter G-quadruplex. The above studies were carried out for various G-quadruplexes of varying sequences in promoter oncogenes and telomeric region. Our results suggest that Mg2+ may be a key regulator for G-quadruplexes of oncogene promoter, which can subsequently affect the expression of related genes.

Research highlights
► The thermo stabilities of G-quadruplexes formed in oncogene promoters are greatly and selectively improved by Mg2+ of physiological concentration.
► The hybridization of promoter G-quadruplexes with their corresponding complementary strands are strongly and selectively hindered with the presence of Mg2+.
► Neither the thermo stability nor the unfolding process of telomeric G-quadruplex is sensitive to Mg2+.