Résumé
MicroRNAs (miRNAs) are short (~21–23 nt) regulatory RNAs that direct repression of their mRNA targets. The miRNA “seed”—nucleotides 2 through 7—establishes miRNA target specificity, because small silencing RNAs bind their targets through this region [1-5]. Accurate processing of the miRNA 5′ end is thought to be under strong selective pressure [6, 7], as a shift by just one nucleotide in the 5′ end of a miRNA would alter its seed sequence, redefining its repertoire of targets (Figure 1). Animal miRNAs are produced by the sequential cleavage of partially double-stranded precursor RNAs by the RNase III endonucleases Drosha, which acts in the nucleus to convert primary miRNA transcripts into pre-miRNAs, and Dicer, which cleaves the pre-miRNA in the cytoplasm to generate a transitory double-stranded intermediate comprising the mature miRNA paired to its partially complementary miRNA* strand [8, 9]. Here, we report that in flies, the 5′ end of a miRNA and of its miRNA* strand is typically more precisely defined than the 3′ ends of either the miRNA or its miRNA*. Surprisingly, the 5′ ends of both miRNA and miRNA* sequences present in mature Argonaute2 (Ago2) complexes are more precisely defined than in the total small RNA population. Our data imply that either many miRNA* sequences are under evolutionary pressure to maintain their seed sequences—that is, they have cellular or exogenous RNA targets—or that secondary constraints such as the sequence requirements for loading small RNAs into functional Argonaute protein complexes narrow the range of miRNA and miRNA* 5′ ends that accumulate in flies.