Studies on encapsidated viroid-like RNA. IV. Requirement for infectivity and specificity of two RNA components from velvet tobacco mottle virus.

Neither the virus-like RNA (RNA 1) nor the viroid-like RNA (RNA 2) of velvet tobacco mottle virus (VTMoV) is capable of independent replication in Nicotiana clevelandii. The function of RNA 2 for the RNA 1 replication could not be fulfilled by conventional viroids such as those of chrysanthemum stunt, citrus exocortis, or the viroid-like RNAs associated with avocado sunblotch and cadang-cadang disease of coconut. Similarly, RNA 2 failed to replicate when coinoculated with RNAs of conventional viruses such as those of southern bean mosaic, sowbane mosaic, galinsoga mosaic, red clover necrotic mosaic, and carnation mottle, all of which possess genomes with molecular weights similar to those of VTMoV RNA 1. The interdependence of the two VTMoV RNAs for biological activity was so highly specific that the function of VTMoV RNA 2 could not be replaced by the viroid-like RNA 2 from solanum nodiflorum mottle virus (SNMV), a closely related virus, for the replication of VTMoV RNA 1. Similarly, SNMV RNA 2 could not be replaced by VTMoV RNA 2 for the replication of SNMV RNA 1. These data support the view that RNA 2 cannot be a satellite RNA or an independent viroid. It seems that viruses such as VTMoV may have originated from associations of conventional single-stranded RNA viruses and viroids or, alternatively, may represent a stage in the evolution of viroids from viruses.