RNA stem-loop to G-quadruplex equilibrium controls mature miRNA production inside the cell

Abstract

Biological role for existence of overlapping structures in RNA is possible yet remains very less explored. G-rich tracts of RNA form G-quadruplexes while GC-rich sequences prefer stem-loop structures. Equilibrium between alternate structures within RNA may occur and influence its functionality. We tested equilibrium between G-quadruplex and stem-loop structure in RNA and its effect on biological processes using pre-miRNA as a model system. Dicer enzyme recognizes canonical stem-loop structures in pre-miRNA to produce mature miRNAs. Deviation from stem-loop leads to deregulated mature miRNA levels, providing readout of existence of alternate structure per se G-quadruplex mediated structural interference in miRNA maturation. In vitro analysis using beacon and Dicer cleavage assays indicated that mature miRNA levels depend on relative amounts of K+ and Mg2+ ions suggesting an ion-dependent structural shift. Further in cellulo studies with and without TmPyP4 (RNA G-quadruplex destabilizer) demonstrated that miRNA biogenesis is modulated by G-quadruplex-stem-loop equilibrium in a subset of pre-miRNAs. Our combined analysis thus provides evidence for formation of non-canonical G-quadruplexes in competition with canonical stem-loop structure inside the cell and its effect on miRNA maturation in a comprehensive manner.