Uridylyltransferase that acts as a suppressor of microRNA (miRNA) biogenesis by specifically mediating the terminal uridylation of some miRNAs. Catalyzes the 3' uridylation of precursor let-7 (pre-let-7), a miRNA precursor. Uridylated pre- let-7 miRNAs fail to be processed by Dicer and undergo degradation. Degradation of pre-let-7 contributes to the maintenance of embryonic stem (ES) cells and is required for ES cells to maintain pluripotency. Does not bind RNA by itself, recruited to pre-let-7 miRNAs via its interaction with LIN28A and LIN28B (By similarity). Also catalyzes the 3' uridylation of miR- 26A, a miRNA that represses IL6 transcript, leading to abrogate IL6 transcript repression and promote cytokine expression. May also suppress Toll-like receptor-induced NF-kappa-B activity via binding to T2BP. Does not play a role in replication-dependent histone mRNA degradation (By similarity).