Virus meets RNAi

Introduction Small rnas are important regulators of gene expression that operate through a range of rna-silencing mechanisms. of these, rna interference (rnai)—gene silencing through small-interfering rnas (sirnas) derived from exogenous double-stranded rna (dsrna)—and the microrna (mirna) pathway—gene silencing guided by small rnas encoded in the genome—are the best known. the discovery that dsrna induces rnai (Fire et al, 1998) and the appreciation of its antiviral activity revolutionized our view of viruses in many ways. the European Science Foundation (ESF)– European Molecular Biology organization (EMBo) Symposium on antiviral applications of rna interference brought together about 50 scientists working at the interface between virology and rnai. Here, i review the main issues that were discussed at the meeting. RNAi as an antiviral response in plants and insects rnai provides an antiviral defence mechanism in plants, nematodes and insects. the cleavage of viral dsrna by the ribonuclease Dicer generates viral sirnas (v-sirnas), which are incorporated into the rna-induced silencing complex (riSc) and direct riSc activity onto messenger rnas (mrnas) in a sequence-specific manner. recognition of a complementary sequence then triggers the endonucleic cleavage (slicer activity) or translational inhibition of the viral target rnas by an argonaute family member (Fig 1). in plants, rnai acts as a systemic antiviral defence against cytoplasmic rna and nuclear Dna viruses (Ding & Voinnet, 2007). Arabidopsis thaliana encodes four Dicer-like genes (Dcls) that exhibit both specialization and redundancy. rna viruses are mainly targeted by Dcl4 and by the redundant activity of Dcl2. all four Dcls are involved in targeting nuclear Dna viruses (geminiviruses and the pararetrovirus cauliflower mosaic virus (caMV; Ding & Voinnet, 2007). Substrates for Dicer include dsrna replication intermediates and local base-paired rna structures of cytoplasmic rna viruses (Hamilton & Baulcombe, 1999; Molnar et al, 2005), or overlapping transcripts of geminiviruses (chellappan et al, 2004). t. Hohn (Basel, Switzerland) reported that v-sirnas map across the genome of caMV, with up to 1,000-fold enrichment in hotspots in the structured translational leader sequence. as a counter-defence, many, if not all, plant viruses encode viral suppressors of rna silencing (VSrs). the discovery that viruses might even encode multiple VSrs underlines the selection pressure exerted by rna silencing (Ding & Voinnet, 2007). J. garcia (Madrid, Spain) discussed the example of cucumber vein yellowing virus—a potyvirus that lost the typical silencing suppressor, helper component-proteinase (Hcpro), through a recombination event, but evolved VSr activity in another protein, p1b, which arose through gene duplication (Valli et al, 2008). rnai also mediates antiviral defence in Drosophila melanogaster. Flies that are defective in rnai—Dicer-2 (Dcr-2), R2D2 and Argonaute-2 (Ago-2) null mutants—are hypersensitive to the (+) stranded rna viruses, flock house virus (FHV), cricket paralysis virus (crpV) and Drosophila c virus (DcV; galiana-arnoux et al, 2006; van rij et al, 2006; Wang et al, 2006). Ago-2 and R2D2 mutants are also hypersensitive to the dsrna virus Drosophila X virus but, paradoxically, Dcr-2 mutants are not (zambon et al, 2006). J.-l. imler (Strasbourg, France) reported that the (–) stranded Virus meets RNAi