No abstract

No abstract

No abstract

During microRNA (miRNA) biogenesis, one strand of a ∼21-22-nucleotide RNA duplex is preferentially selected for entry into a silencing complex. The other strand, known as the miRNA* species, has typically been assumed to be a carrier strand. Here we show that, although Drosophila melanogaster miRNA* species are less abundant than their partners, they are often present at physiologically relevant levels and can associate with Argonaute proteins. Comparative genomic analyses revealed that >40% of miRNA* sequences resist nucleotide divergence across Drosophilid evolution, and at least half of these well-conserved miRNA* species select for conserved 3′ untranslated region seed matches well above background noise. Finally, we validated the inhibitory activity of miRNA* species in both cultured cells and transgenic animals. These data broaden the reach of the miRNA regulatory network and suggest an important mechanism that diversifies miRNA function during evolution.miRNAs are an abundant class of ∼21-22-nucleotide (nt) RNAs that typically function as posttranscriptional repressors of gene activity 1,2 . The biogenesis of animal miRNAs involves stepwise processing of precursor transcripts containing hairpin structures. Canonical primary miRNA transcripts are cleaved in the nucleus by the RNase III enzyme Drosha, releasing ∼60-80-nt pre-miRNA hairpins 3 . In addition, splicing and debranching of short hairpin introns termed 'mirtrons' can directly generate pre-miRNA-like hairpins 4-6 . In both cases, the hairpins are exported to the cytoplasm and cleaved by the RNase III enzyme Dicer, resulting in a ∼21-nt miRNA duplex 7-10 . Although both strands of miRNA duplexes are necessarily produced in equal amounts by transcription, their accumulation is asymmetric at steady state. The convention is to refer to the more abundant product of a pre-miRNA or mirtron hairpin as the miRNA and its rarer partner as a miRNA* species 11 .The function of miRNA strands is evident from the preferential conservation of 7-nt sequences in target transcripts with Watson-Crick complementarity to positions 2-8 of mature miRNAs (the 'seed' region). Although other features influence target-site efficacy, miRNA seed matches are often necessary and sufficient for target regulation 12-15 and are the basis of most genomewide predictions of miRNA regulatory sites 16-18 . Such studies conclude that most animal genes are either actively regulated by one or more miRNAs or actively avoid the acquisition © 2008 Nature Publishing Group Correspondence should be addressed to E.C.L. (E-mail: laie@mskcc.org 17,19 . The reach of the miRNA regulatory network may in fact be larger, depending on the extent to which additional miRNA genes remain to be discovered, the extent to which noncanonical target sites are functional, and the extent to which nonconserved sites are relevant in vivo 20 .The nonrandom nature of miRNA strand selection was posited to reflect an active process that minimizes the population of silencing complexes with illegitimate miRNA* speci...

No abstract

No abstract

An obligate intermediate during microRNA (miRNA) biogenesis is an~22-nucleotide RNA duplex, from which the mature miRNA is preferentially incorporated into a silencing complex. Its partner miRNA* species is generally regarded as a passenger RNA, whose regulatory capacity has not been systematically examined in vertebrates. Our bioinformatic analyses demonstrate that a substantial fraction of miRNA* species are stringently conserved over vertebrate evolution, collectively exhibit greatest conservation in their seed regions, and define complementary motifs whose conservation across vertebrate 39-UTR evolution is statistically significant. Functional tests of 22 miRNA expression constructs revealed that a majority could repress both miRNA and miRNA* perfect match reporters, and the ratio of miRNA:miRNA* sensor repression was correlated with the endogenous ratio of miRNA:miRNA* reads. Analysis of microarray data provided transcriptome-wide evidence for the regulation of seedmatched targets for both mature and star strand species of several miRNAs relevant to oncogenesis, including mir-17, mir-34a, and mir-19. Finally, 39-UTR sensor assays and mutagenesis tests confirmed direct repression of five miR-19* targets via star seed sites. Overall, our data demonstrate that miRNA* species have demonstrable impact on vertebrate regulatory networks and should be taken into account in studies of miRNA functions and their contribution to disease states.

To prevent catastrophic incidents, the manufacturer's guidelines should be followed carefully because they are known to result in the safe performance of MRI examinations of patients with neurostimulation systems used for DBS.