Intronic microRNA precursors that bypass Drosha processing.
MicroRNAs (miRNAs) are approximately 22-nucleotide endogenous RNAs that often repress the expression of complementary messenger RNAs. In animals, miRNAs derive from characteristic hairpins in primary transcripts through two sequential RNase III-mediated cleavages; Drosha cleaves near the base of the stem to liberate a approximately 60-nucleotide pre-miRNA hairpin, then Dicer cleaves near the loop to generate a miRNA:miRNA* duplex. From that duplex, the mature miRNA is incorporated into the silencing complex. Here we identify an alternative pathway for miRNA biogenesis, in which certain debranched introns mimic the structural features of pre-miRNAs to enter the miRNA-processing pathway without Drosha-mediated cleavage. We call these pre-miRNAs/introns 'mirtrons', and have identified 14 mirtrons in Drosophila melanogaster and another four in Caenorhabditis elegans (including the reclassification of mir-62). Some of these have been selectively maintained during evolution with patterns of sequence conservation suggesting important regulatory functions in the animal. The abundance of introns comparable in size to pre-miRNAs appears to have created a context favourable for the emergence of mirtrons in flies and nematodes. This suggests that other lineages with many similarly sized introns probably also have mirtrons, and that the mirtron pathway could have provided an early avenue for the emergence of miRNAs before the advent of Drosha.
SciCrunch is a data sharing and display platform. Anyone can create a custom portal where they can select searchable subsets of hundreds of data sources, brand their web pages and create their community. SciCrunch will push data updates automatically to all portals on a weekly basis. User communities can also add their own data to scicrunch, however this is not currently a free service.