MicroRNA Biogenesis via Splicing and Exosome-Mediated Trimming in Drosophila

microRNAs (miRNAs) are similar to 22 nucleotide regulatory RNAs derived from hairpins generated either by Drosha cleavage (canonical substrates) or by splicing and debranching of short introns (mirtrons). The 5' end of the highly conserved Drosophila mirtron-like locus mir-1017 is coincident with the splice donor, but a substantial tail separates its hairpin from the 3'splice acceptor. Genetic and biochemical studies define a biogenesis pathway involving splicing, lariat debranching, and RNA exosome-mediated trimming, followed by conventional dicing and loading into AGO1 to yield a miRNA that can repress seed-matched targets. Analysis of cloned small RNAs yielded six additional candidate 3' tailed mirtrons in D. melanogaster. Altogether, these data reveal an unexpected role for the exosome in the biogenesis of miRNAs from hybrid mirtron substrates.