Turnip yellow mosaic virus RNAs with anticodon loop substitutions that result in decreased valylation fail to replicate efficiently.

Single and multiple nucleotide substitutions have been introduced into the anticodon loop of the tRNA-like structure of turnip yellow mosaic virus (TYMV) genomic RNA. We studied the effects of these mutations on in vitro valylation and on replication in Chinese cabbage protoplasts and plants. Only those mutants capable of efficient and complete valylation showed efficient replication in protoplasts and gave rise to systemic symptoms in whole plants. Mutants that accepted valine inefficiently (in some cases Vmax/Km values were less than 10(-3) relative to wild-type values) replicated to levels 200- to 500-fold below wild-type levels in protoplasts (estimated on the basis of coat protein and genomic RNA levels). These mutants could not support systemic spread in plants. In one plant inoculated with TYMC-A55 RNA, which replicates poorly in protoplasts, systemic symptoms developed after a delay. The reversion in replication was accompanied by improved valine acceptance and the appearance of a U57 second-site mutation. Our results indicate a correlation between valine acceptance activity and viral yield. Possible roles for valylation are discussed, and the results are compared with those of similar studies with brome mosaic virus which suggested that tyrosylation is not crucial for brome mosaic virus replication (T. W. Dreher, A. L. N. Rao, and T. C. Hall, J. Mol. Biol. 206:425-438, 1989).