Point mutations in the turnip crinkle virus capsid protein affect the symptoms expressed by Nicotiana benthamiana

In an effort to determine the biological function(s) of the capsid protein protruding domains unique to the plant carmo- and tombusviruses, we constructed turnip crinkle virus (TCV) mutants in which tandem, in-frame translation terminators replaced the first two codons of the five-amino acid hinge between the shell and the protruding domains of the TCV capsid protein. One of the mutants replicated in inoculated leaves and protoplasts without detectable accumulation of capsid protein. The mutant lacked the capacity to move systemically in Brassica campestris and Nicotiana benthamiana. After 8 weeks, revertant virions that had regained the capacity to move systemically were purified and found to have sense codons at the positions of the introduced translation terminators. One of the revertants, with amino acid substitutions in the hinge, elicited milder symptoms than those elicited by the wild-type virus, and another elicited more severe symptoms. Oligonucleotide-directed mutagenesis was used to show that the hinge mutations were sufficient to elicit the milder, but not the more severe, symptom syndrome. Single amino acid substitutions were also shown to be sufficient to elicit the milder, but not the more severe, symptoms.