Functional Analysis of Cymbidium Ringspot Virus Genome

Genomic RNA of cymbidium ringspot tombusvirus (CyRSV) contains five large open reading frames (ORFs). The two 5′ proximal ORFs encode proteins of 33 and 92 kDa and the three 3′ proximal ORFs encode proteins of 41,22, and 19 kDa. In addition, a small reading frame encoding a protein of 4 kDa was recently observed by computer analysis of the 3′ nontranslated region of CyRSV and other tombusvirus RNAs (ORF 6). The function of putative gene products was investigated with the use of several mutants. Mutants in ORFs 1 and 2 were not viable indicating that both 33- and 92-kDa proteins are required for replication. Deletion of a large portion of the coat protein gene, encoding a 41-kDa protein, did not prevent replication of viral RNA and cell-to-cell movement, but interfered severely with long-distance translocation. No virus particles or viral RNA could be detected in inoculated or upper leaves of plants inoculated with transcripts obtained from mutants not expressing the 22-kDa protein. However, replication and encapsidation occurred normally in inoculated protoplasts indicating that this gene product is a transport protein. Conversely, no role could be assigned to putative gene products of ORF 5 (19-kDa protein) and QRF 6. It was also shown that factors other than gene expression can influence the replication of RNA mutants, probably due to instability of RNA molecules.