Five Questions about Viruses and MicroRNAs

MicroRNAs (miRNAs) are ,22-nt regulatory RNAs expressed by all multicellular eukaryotes [1]. Humans encode .700 miRNAs and similar numbers are likely to exist in other mammalian species. Almost all cellular miRNAs are initially transcribed by RNA polymerase II (Pol II) as part of a long, capped, polyadenylated primary miRNA (pri-miRNA) precursor. The miRNA forms part of one arm of an RNA stem-loop that consists of an ,32-bp imperfect stem flanked by unstructured RNA sequences. This stem-loop is recognized by the nuclear RNase III enzyme Drosha, which cleaves the stem to liberate an ,60-nt pre-miRNA hairpin. The pre-miRNA is then transported to the cytoplasm where it is cleaved by a second RNase III enzyme, called Dicer, which removes the terminal loop to generate the miRNA duplex intermediate. One strand of this duplex is incorporated into the RNA-induced silencing complex (RISC), where it acts as a guide RNA to direct RISC to complementary mRNA species [1]. Depending on the level of complementarity, RISC can either cleave bound mRNAs and/or inhibit their translation. Inhibition of mRNA translation generally requires full complementarity of the mRNA to nucleotides 2 through 7 or 8 from the miRNA 59 end—the miRNA seed region. The primary, and possibly sole, function of mammalian miRNAs is therefore to act as specific post-transcriptional inhibitors of mRNA function.