Irregular G-quadruplexes Found in the Untranslated Regions of Human mRNAs Influence Translation

G-quadruplex structures are composed of coplanar guanines and are found in both DNA and RNA. They are formed by the stacking of two or more G-quartets that are linked together by three loops. The current belief is that RNA G-quadruplexes include loops of l to 7 nucleotides in length, although recent evidence indicates that the central loop (loop 2) can be longer if loops 1 and 3 are limited to a single nucleotide each. With the objective of broadening the definition of irregular RNA G-quadruplexes, a bioinformatic search was performed to find potential G-quadruplexes located in the untranslated regions of human mRNAs (i.e. in the 5 and 3-UTRs) that contain either a long loop 1 or 3 of up to 40 nucleotides in length. RNA molecules including the potential sequences were then synthesized and examined in vitro by in-line probing for the formation of G-quadruplex structures. The sequences that adopted a G-quadruplex structure were cloned into a luciferase dual vector and examined for their ability to modulate translation in cellulo. Some irregular G-quadruplexes were observed to either promote or repress translation regardless of the position or the size of the long loop they possessed. Even if the composition of a RNA G-quadruplex is not quite completely understood, the results presented in this report clearly demonstrate that what defines a RNA G-quadruplex is much broader than what we previously believed.