FUNCTIONSOFTHE 3′-UNTRANSLATED REGIONSOF POSITIVE STRAND RNA VIRAL GENOMES

Positive strand RNA viral genomes are unique in the viral world in serving a dual role as mRNA and replicon. Since the origin of the minus-strand RNA replication intermediate is at the 3′-end of the genome, the 3′-untranslated region (UTR) clearly plays a role in viral RNA replication. The messenger role of this same RNA likely places functional demands on the 3′-UTR to serve roles typical of cellular mRNAs, including the regulation of RNA stability and translation. Current understanding indicates varied roles for positive strand RNA viral 3′-UTRs, with the dominant roles differing between viruses. Three case studies are discussed: turnip yellow mosaic virus RNA, whose 3′ tRNA mimicry is thought to negatively regulate minus strand synthesis; brome mosaic virus, whose 3′-UTR contains a unique promoter element directing minus strand synthesis; and tobacco mosaic virus, whose 3′-UTR contains an enhancer of translational expression.