The let-7 microRNA (miRNA) is an ultraconserved regulator of stem cell differentiation and developmental timing, and a candidate tumour suppressor. Here we show that LIN-28 and the poly(U) polymerase PUP-2 regulate let-7 processing in C. 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 orthologue 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 such poly(U) polymerases are potential therapeutic targets.

Recent progress with techniques for monitoring RNA structure in cells such as ‘DMS-Seq’ and ‘Structure-Seq’ suggests that a new era of RNA structure-function exploration is on the horizon. This will also include systematic investigation of the factors required for the structural integrity of RNA. In this context, much evidence accumulated over 50 years suggests that polyamines play important roles as modulators of RNA structure. Here, we summarize and discuss recent literature relating to the roles of these small endogenous molecules in RNA function. We have included studies directed at understanding the binding interactions of polyamines with polynucleotides, tRNA, rRNA, mRNA and ribozymes using chemical, biochemical and spectroscopic tools. In brief, polyamines bind RNA in a sequence-selective fashion and induce changes in RNA structure in context-dependent manners. In some cases the functional consequences of these interactions have been observed in cells. Most notably, polyamine-mediated effects on RNA are frequently distinct from those of divalent cations (i.e. Mg2+) confirming their roles as independent molecular entities which help drive RNA-mediated processes.

G‐quadruplexes are secondary structures formed in G‐rich sequences in DNA and RNA. Considerable research over the past three decades has led to in‐depth insight into these unusual structures in DNA. Since the more recent exploration into RNA G‐quadruplexes, such structures have demonstrated their in cellulo existence, function and roles in pathology. In comparison to Watson‐Crick‐based secondary structures, most G‐quadruplexes display highly redundant structural characteristics. However, numerous reports of G‐quadruplex motifs/structures with unique features (e.g. bulges, long loops, vacancy) have recently surfaced, expanding the repertoire of G‐quadruplex scaffolds. This review addresses G‐quadruplex formation and structure, including recent reports of non‐canonical G‐quadruplex structures. Improved methods of detection will likely further expand this collection of novel structures and ultimately change the face of quadruplex‐RNA targeting as a therapeutic strategy.