A novel naturally occurring hybrid gene encoded by a plant RNA virus facilitates long distance virus movement.

We recently identified a new cucumovirus-specific gene (2b) which is encoded by RNA 2 of the cucumber mosaic cucumovirus (CMV) tripartite RNA genome and whose coding sequence overlaps the C-terminal 69 codons of ORF 2a encoding the RNA polymerase protein. We have now found that although a CMV mutant lacking ORF 2b accumulated in the inoculated cotyledons of cucumber plants, it was unable to spread systemically, demonstrating involvement of 2b in long distance movement. The same mutant infected tobacco systemically with a much reduced virulence and delayed appearance of symptoms, indicating that 2b may contribute to long distance movement in this host. Deletion of the overlapping C-terminal part of ORF 2a did not change infectivity of the mutant in either host species, ruling out 2a mutation as the reason for the change of phenotype. Further infectivity studies with mutants containing partial deletions in ORF 2b further supported the conclusion that 2b encodes a host-specific long distance movement function. Sequence analysis revealed that 2b may represent a novel naturally occurring hybrid gene important to the evolutionary formation of the cucumovirus group and that it could provide a genetic basis for the wide host range of these viruses.