Characterization of defective RNAs derived from RNA 3 of the Fny strain of cucumber mosaic cucumovirus

Two defective RNAs (designated D RNA 3α and D RNA 3β) were found to be associated with the Fny strain of cucumber mosaic cucumovirus but not with the Sny strain after serial passages in a tobacco host. The D RNAs were derived from RNA 3 by single, in-frame deletions within the 3a open reading frame. A full-length cDNA clone from which biologically active transcripts can be produced in vitro has been constructed for D RNA 3β. This transcript can be replicated in tobacco plants infected with subgroup I and II cucumber mosaic cucumovirus strains and with peanut stunt cucumovirus. Translation of D RNA 3β in vitro produced a 20-kDa peptide, which was consistent with the predicted coding capacity of the deleted 3a open reading frame. D RNA 3β was also associated with polyribosomes isolated from infected tobacco plants. The presence of the D RNAs had no apparent effect upon helper virus yield or symptom production.