Beginning to understand microRNA function

MicroRNAs (miRNAs) are ~22 nt small RNAs expressed by plants, animals, viruses and at least one unicellular organism, the green alga, Chlamydomonas reinhardtii 1. Most miRNAs are transcribed as primary miRNAs (pri-miRNAs) by RNA polymerase II, although a few are transcribed by RNA polymerase III. In animals, pri-miRNAs are converted to mature miRNAs by two successive endonucleolytic cleavages 2. The pri-miRNA is first cut in the nucleus by Drosha, a ribonuclease III (RNase III) enzyme, acting with its double-stranded RNA-binding domain (dsRBD) protein partner, called DGCR8 in vertebrates and Pasha in invertebrates, into an ~70 nt stem loop, the precursor miRNA (pre-miRNA). After its export to cytoplasm by Exportin 5, the pre-miRNA is cut into mature miRNA by a second RNase III enzyme, Dicer, which partners in mammals with one of two dsRBD proteins—TRBP (HIV-1 tar RNA-binding protein) or PACT, or in Drosophila melanogaster with the dsRBD protein Loquacious (Loqs). The mature miRNA is then loaded into an effector complex, the RNA-induced silencing complex (RISC), whose core component is always a member of the Argonaute (Ago) family of RNA-guided RNA regulatory proteins 3. Recently, an alternative processing pathway was identified for a distinct sub-group of miRNAs in Drosophila and C. elegans 4, 5. These miRNAs exploit the pre-mRNA splicing machinery to generate a pre-miRNA directly, bypassing the processing of a pri-miRNA by Drosha. For these miRNAs, the pre-miRNA is at once the precursor of a mature miRNA and a compact, fully functional intron, hence the name, 'mirtrons'.