EXPRESSION OF HOMOLOGOUS AND HETEROLOGOUS VIRAL COAT PROTEIN-ENCODING GENES USING RECOMBINANT DI RNA FROM CYMBIDIUM RINGSPOT TOMBUSVIRUS

Defective interfering (DI) RNA of cymbidium ringspot tombusvirus (CyRSV) was tested as a potential RNA vector. The coat protein-encoding gene (CP) of the same virus or of the unrelated tomato aspermy cucumovirus (TAV) was inserted in a biologically active clone of CyRSV DI-3 RNA. Both homologous and heterologous CP genes were inserted into DI-3 cDNA clone in two different positions. The CyRSV CP was expressed only in the leaves which were inoculated with DI-3CPWt plus CP-less mutant helper virus (Delta ABgII). Chimaeric DI RNA carrying a heterologous CP gene (DI-3HCPtav) was able to replicate and express the inserted TAV CP in the presence of a wild-type (wt) helper genome. Both constructs, which were stable and active for gene expression, carried the inserted CP genes in the same position, between the A and B blocks of DI-3 RNA. Other constructs in which the CP were cloned between the B and C blocks of DI-3 RNA were not able to direct the translation of the encoded CP. The expression level of CP derived from recombinant DI RNA was lower relative to expression of CP from wt virus infection.