Organization and expression of single-stranded positive-sense RNA plant viruses

Abstract

A majonty of plant viruses have positive-sense RNA as their genome. These viruses are dassified as mono-, bi- or tripartite based on the presence of 1, 2, or 3 RNA segments as their genomes. The constraints imposed by eukaryotic protein synthesizing machinery for translation of only the first 5' ORF as well as the need for optimal utilization of the limited genetic material necessitate these viruses to evolve different strategies to express their genes. These strategies include termination at a non-conventional stop signal, read through of leaky termination codon, polyprotein proccssing, frame shifting  generation of subgenomic mRNAs and lnternal initiation. Despite wide variation in particle morphology, host plant, vector transmission, etc., an analysis of the genomic sequence and strategies of expression in these viruses has led to their classification into a limited number of supergroups. Many internal relationships not envisaged earlier have emerged and this has helped in understanding the molecular mechanisms of viral replication. translocation, etc. Determination of genomic sequences of a large number of viruses will help in further strengthening the established evolutionary relationships among various groups of plant viruses.