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 e ncoded by RNA 2 of the cucumber mosaic cucumovirus (CMV) tripartite RN A genome and whose coding sequence overlaps the C-terminal 69 codons o f ORF 2a encoding the RNA polymerase protein. We have now found that a lthough a CMV mutant lacking ORF 2b accumulated in the inoculated coty ledons of cucumber plants, it was unable to spread systemically, demon strating involvement of 2b in long distance movement, The same mutant infected tobacco systemically with a much reduced virulence and delaye d appearance of symptoms, indicating that 2b may contribute to long di stance movement in this host, Deletion of the overlapping C-terminal p art 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 phenot ype, Further infectivity studies with mutants containing partial delet ions 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 occuring hybrid gene important to the evolutionary formation of the cucumovirus group and that it co uld provide a genetic basis for the wide host range of these viruses.