Tumor-suppressor let-7 pre-microRNAs (miRNAs) are regulated by terminal uridylyltransferases TUT7 and TUT4 that either promote let-7 maturation by adding a single uridine nucleotide to the pre-miRNA 3′ end or mark them for degradation by the addition of multiple uridines. Oligo-uridylation is increased in cells by enhanced TUT7/4 expression and especially by the RNA-binding pluripotency factor LIN28A. Using cryogenic electron microscopy, we captured high-resolution structures of active forms of TUT7 alone, of TUT7 plus pre-miRNA and of both TUT7 and TUT4 bound with pre-miRNA and LIN28A. Our structures reveal that pre-miRNAs engage the enzymes in fundamentally different ways depending on the presence of LIN28A, which clamps them onto the TUTs to enable processive 3′ oligo-uridylation. This study reveals the molecular basis for mono- versus oligo-uridylation by TUT7/4, as determined by the presence of LIN28A, and thus their mechanism of action in the regulation of cell fate and in cancer.

肿瘤抑制因子 let-7 pre-microRNA (miRNA) 受末端尿苷酰转移酶 TUT7 和 TUT4 调节，这些酶要么通过在 pre-miRNA 3' 端添加单个尿苷核苷酸来促进 let-7 成熟，要么通过添加多个尿苷。通过增强 TUT7/4 表达，尤其是通过 RNA 结合多能性因子 LIN28A，可以增加细胞中的寡尿苷化。使用低温电子显微镜，我们捕获了单独的 TUT7、TUT7 加 pre-miRNA 以及 TUT7 和 TUT4 与 pre-miRNA 和 LIN28A 结合的活性形式的高分辨率结构。我们的结构揭示了 pre-miRNA 以根本不同的方式与酶结合，具体取决于 LIN28A 的存在，LIN28A 将它们夹在 TUT 上以实现持续的 3' 寡尿苷化。这项研究揭示了 TUT7/4 的单尿苷化与寡尿苷化的分子基础（由 LIN28A 的存在决定），以及它们在调节细胞命运和癌症中的作用机制。