The complexity of microRNA regulation

A hallmark of mammalian embryonic development is the widespread induction of microRNA (miRNA) expression. Surprisingly, the transcription of many of these small, noncoding RNAs is unchanged through development; rather, post‐transcriptional regulatory events prevent accumulation of the mature miRNA species. Perhaps the best example of this is the inhibition of Let‐7 by the RNA binding protein Lin28. This protein blocks maturation of Let‐7, and promotes uridylation of precursor species, leading to degradation. We have expanded on this observation. We find that Lin28 blocks Let‐7 maturation in the absence of uridylation. Blockade occurs at the Drosha and Dicer steps, depending on Let‐7 family member. Let‐7 precursor uridylation does occur, but by two distinct mechanisms. Lin28 promotes uridylation of Let‐7 precursors in embryonic cells, while a Lin28 independent mechanism leads to uridylation in myriad cell types. We have developed strategies to profile precursor sequences. We find extensive uridylation of all miRNA families in a Lin28 independent manner. We have partially mapped turnover pathways for uridylated miRNA precursors. This data paints a complex picture of miRNA regulation and turnover in mammalian cells.