Striking similarities in amino acid sequence among nonstructural proteins encoded by RNA viruses that have dissimilar genomic organization.

The plant viruses alfalfa mosaic virus (AMV) and brome mosaic virus (BMV) each divide their genetic information among three RNAs while tobacco mosaic virus (TMV) contains a single genomic RNA. Amino acid sequence comparisons suggest that the single proteins encoded by AMV RNA 1 and BMV RNA 1 and by AMV RNA 2 and BMV RNA 2 are related to the NH2-terminal two-thirds and the COOH-terminal one-third, respectively, of the largest protein encoded by TMV. Separating these two domains in the TMV RNA sequence is an amber termination codon, whose partial suppression allows translation of the downstream domain. Many of the residues that the TMV read-through domain and the segmented plant viruses have in common are also conserved in a read-through domain found in the nonstructural polyprotein of the animal alphaviruses Sindbis and Middelburg. We suggest that, despite substantial differences in gene organization and expression, all of these viruses use related proteins for common functions in RNA replication. Reassortment of functional modules of coding and regulatory sequence from preexisting viral or cellular sources, perhaps via RNA recombination, may be an important mechanism in RNA virus evolution.