LIN-28 and the poly(U) polymerase PUP-2 regulate let-7 microRNA processing in Caenorhabditis elegans

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.