The pluripotency factor Lin28 blocks the expression of let-7 microRNAs (miRNAs) in undifferentiated cells during development and functions as an oncogene in a subset of cancers1. Lin28 binds to let-7 precursor RNAs and recruits 3′ terminal uridylyl transferases (TUTases) to selectively inhibit let-7 biogenesis2–4. Uridylated pre-let-7 is refractory to processing by Dicer and is rapidly degraded by an unknown ribonuclease5. Here we identify Dis3l2 as the 3′-5′ exonuclease responsible for the decay of uridylated pre-let-7. Biochemical reconstitution assays reveal that 3′ oligouridylation stimulates Dis3l2 activity in vitro, and knockdown of Dis3l2 in mouse embryonic stem cells leads to the stabilization of pre-let-7. Our study establishes 3′ oligouridylation as an RNA decay signal for Dis3l2 and identifies the first physiological RNA substrate of this novel exonuclease that is mutated in the Perlman syndrome of fetal overgrowth and predisposition to Wilms’ tumor6
