Regeneration of a functional RNA virus genome by recombination between deletion mutants and requirement for cowpea chlorotic mottle virus 3a and coat genes for systemic infection.

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Abstract
RNAs 1 and 2 of the tripartite cowpea chlorotic mottle virus (CCMV) genome are sufficient for RNA replication in protoplasts, whereas systemic infection of cowpea plants additionally requires RNA3, which encodes the 3a noncapsid protein and coat protein. By using biologically active CCMV cDNA clones, we find that deletions in either RNA3 gene block systemic infection. Thus, though some plant RNA viruses are able to spread systemically without encapsidation, both the coat and 3a genes are required for systemic infection of cowpeas by CCMV. When plants were coinoculated with CCMV RNAs 1 and 2 and both the 3a and coat deletion mutants of RNA3, 30-60% rapidly developed systemic infection. Progeny RNA recovered from systemically infected leaves in such infections contained neither of the starting deletion mutants but rather a single full-length RNA3 component with both genes intact. Nucleotide substitutions introduced into the coat protein deletion mutant as an artificial marker were recovered in the full-length progeny RNA, confirming its recombinant nature. Intermolecular RNA recombination in planta can, therefore, rescue a complete infectious genome from coinoculated mutants independently disabled for systemic spread. These results have implications for the repair of defective genomes produced by frequent natural replication errors, the possible emergence of newly adapted RNA viruses upon coinfection of new hosts, and further studies of RNA virus recombination.