Insertion and deletion mutagenesis of the two virion-sense genes, V1 and V2, of maize streak virus (MSV) prevents symptomatic infections following -mediated ‘agroinoculation’ of maize seedlings. These genes code for an 10900 protein and for coat protein, respectively. Mutants containing insertions or deletions in the coat protein gene, V2, were able to replicate to low levels, producing dsDNA although virion ssDNA was not detected and symptoms were not observed. Hence, unlike the bipartite geminiviruses, MSV requires coat protein to produce symptomatic systemic infection. Mutations in gene V1 which considerably shortened the 10900 protein (V1 gene) resulted either in low levels of replication, in which all the DNA forms associated with a wild-type infection were produced, or in no infection, in which case coat protein production may also have been affected. A V1 mutant generated with 11 of the 14 N-terminal amino acids altered, was viable and produced symptoms typical of a wild-type infection. Infectivity, assessed by replication and symptom expression, was restored by co-inoculating constructs containing single mutations in different open reading frames, thus rescue can occur by trans-complementation of gene products. The experiments showed that the mutations did not affect the nucleotide sequence requirements for replication and that in all cases intermolecular recombination eventually resulted in dominant wild-type virus.


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