The infectivities of turnip yellow mosaic virus genomes with altered tRNA mimicry are not dependent on compensating mutations in the viral replication protein

Five highly infectious turnip yellow mosaic virus (TYMV) genomes with seque nce changes in their 3'-terminal regions that result in altered aminoacylat ion and eEF1A binding have been studied. These genomes were derived from cl oned parental RNAs of low infectivity by sequential passaging in plants. Th ree of these genomes that are incapable of aminoacylation have been reporte d previously (J. B. Goodwin, J. M. Skuzeski, and T. W. Dreher, Virology 230 :113-124, 1997), We now demonstrate by subcloning the 3' untranslated regio ns into wild-type TYMV RNA that the high infectivities and replication rate s of these genomes compared to their progenitors are mostly due to a small number of mutations acquired in the 3' tRNA-like structure during passaging , Mutations in other parts of the genome, including the replication protein coding region, are not required for high infectivity but probably do play a role in optimizing viral amplification and spread in plants, Two other TY MV RNA variants of suboptimal infectivities, one that accepts methionine in stead of the usual valine and one that interacts less tightly with eEF1A, w ere sequentially passaged to produce highly infectious genomes, The improve d infectivities of these RNAs were not associated with increased replicatio n in protoplasts, and no mutations were acquired in their 3' tRNA-like stru ctures. Complete sequencing of one genome identified two mutations that res ult in amino acid changes in the movement protein gene, suggesting that imp roved infectivity may be a function of improved viral dissemination in plan ts. Our results show that the wild-type TYMV replication proteins are able to amplify genomes with 3' termini of variable sequence and tRNA mimicry. T hese and previous results have led to a model in which the binding of eEF1A to the 3' end to antagonize minus-strand initiation is a major role of the tRNA-like structure.