Synthesis of subgenomic RNAs by positive-strand RNA viruses.

f b w t ( t g s p t n p Many RNA viruses encode more than one gene on a single genomic RNA. Yet only the first gene, or open reading frame (ORF), on a normal eukaryotic mRNA is translated. Thus, downstream genes on viral genomes are expressed either via novel translational events or, more commonly, by deployment of subgenomic mRNAs (sgRNAs). Subgenomic RNAs of positive-strand viruses have the same 39 ends as genomic RNA, but have deleions at the 59 ends to bring the 59 end of the RNA in proximity with the start codon of downstream (on genomic RNA) ORFs. Because replication is required for sgRNA synthesis, the RNA-dependent RNA polymerase (RdRp) is always translated first, directly from genomic RNA of positive-strand RNA viruses. sgRNAs express products needed during intermediate and late stages of infection, such as structural or movement proteins. Taxa of (1) sense viruses that produce sgRNAs inlude animal viruses in the order Nidovirales (Coronairidae and Arteriviridae families) and the families Togairidae, Caliciviridae, Nodaviridae, and Astroviridae, plant iruses of the Luteoviridae, Bromoviridae, Tombusviridae, nd Closteroviridae families, and the Tobravirus, Carlairus, Tymovirus, Potexvirus, Hordeivirus, Tobamovirus, obemovirus, and Furovirus genera. Subgenomic RNA ynthesis has been studied more in plant viruses than in nimal viruses. This is probably because a greater perentage of all plant viruses make sgRNAs and also ecause plant viruses, with smaller genomes and highly fficient replication, are often more amenable than anial viruses to studies of RNA replication mechanisms, specially in cell-free extracts. The similarities between