Many ways to generate microRNA-like small RNAs: non-canonical pathways for microRNA production

MicroRNAs (miRNAs) are an abundant class of small non-coding RNAs that collectively regulate the expression of a large number of mRNAs by either promoting destabilization or repressing translation, or both. Therefore, they play a major role in shaping the transcriptomes and proteomes of eukaryotic organisms. Typically, animal miRNAs are produced from long primary transcripts with one or more of hairpin structures by two sequential processing reactions: one by Drosha in the nucleus and the other by Dicer in the cytoplasm. However, deviations from this paradigm have been observed: subclasses of miRNAs, which only partially meet the classical definition of a miRNA, are derived by alternative biogenesis pathways, thereby providing an additional level of complexity to miRNA-dependent regulation of gene expression.