RNA structures in the untranslated regions (UTRs) of mRNAs influence post-transcriptional regulation of gene expression. Much of the knowledge in this area depends on canonical double-stranded RNA elements. There has been considerable recent advancement of our understanding of guanine(G)-rich nucleic acids sequences that form four-stranded structures, called G-quadruplexes. While much of the research has been focused on DNA G-quadruplexes, there has recently been a rapid emergence of interest in RNA G-quadruplexes, particularly in the 5′-UTRs of mRNAs. Collectively, these studies suggest that RNA G-quadruplexes exist in the 5′-UTRs of many genes, including genes of clinical interest, and that such structural elements can influence translation. This review features the progresses in the study of 5′-UTR RNA G-quadruplex-mediated translational control. It covers computational analysis, cell-free, cell-based and chemical biology studies that have sought to elucidate the roles of RNA G-quadruplexes in both cap-dependent and -independent regulation of mRNA translation. We also discuss protein trans -acting factors that have been implicated and the evidence that such RNA motifs have potential as small molecule target. Finally, we close the review with a perspective on the future challenges in the field of 5′-UTR RNA G-quadruplex-mediated translation regulation.

Molecular mechanisms that regulate gene expression can occur either before or after transcription. The information for post-transcriptional regulation can lie within the sequence or structure of the RNA transcript and it has been proposed that G-quadruplex nucleic acid sequence motifs may regulate translation as well as transcription. Here, we have explored the incidence of G-quadruplex motifs in and around the untranslated regions (UTRs) of mRNA. We observed a significant strand asymmetry, consistent with a general depletion of G-quadruplex-forming RNA. We also observed a positional bias in two distinct regions, each suggestive of a specific function. We observed an excess of G-quadruplex motifs towards the 5′-ends of 5′-UTRs, supportive of a hypothesis linking 5′-UTR RNA G-quadruplexes to translational control. We then analysed the vicinity of 3′-UTRs and observed an over-representation of G-quadruplex motifs immediately after the 3′-end of genes, especially in those cases where another gene is in close proximity, suggesting that G-quadruplexes may be involved in the termination of gene transcription.

Developmental expression of the microRNA let-7 is tightly regulated in many animals, and turnover has been linked to LIN-28 and uridylation in mammals. This regulation is now shown to be conserved in Caenorhabditis elegans, and PUP-2 is shown to be a uridylase that is specifically recruited to let-7 in a LIN-28–dependent manner. The let-7 microRNA (miRNA) is an ultraconserved regulator of stem cell differentiation and developmental timing and a candidate tumor suppressor. Here we show that LIN-28 and the poly(U) polymerase PUP-2 regulate let-7 processing in Caenorhabditis elegans. We demonstrate that lin-28 is necessary and sufficient to block let-7 activity in vivo; LIN-28 directly binds let-7 pre-miRNA to prevent Dicer processing. Moreover, we have identified a poly(U) polymerase, PUP-2, which regulates the stability of LIN-28–blockaded let-7 pre-miRNA and contributes to LIN-28–dependent regulation of let-7 during development. We show that PUP-2 and LIN-28 interact directly, and that LIN-28 stimulates uridylation of let-7 pre-miRNA by PUP-2 in vitro. Our results demonstrate that LIN-28 and let-7 form an ancient regulatory switch, conserved from nematodes to humans, and provide insight into the mechanism of LIN-28 action in vivo. Uridylation by a PUP-2 ortholog might regulate let-7 and additional miRNAs in other species. Given the roles of Lin28 and let-7 in stem cell and cancer biology, we propose that such poly(U) polymerases are potential therapeutic targets.