A subpopulation of RNA 1 of Cucumber mosaic virus contains 3′ termini originating from RNAs 2 or 3

Tobacco plants transgenic for RNA 1 of Cucumber mosaic virus and inoculated with transcript of RNAs 2 and 3 regenerated viral RNA 1 from the transgenic mRNA, and the plants became systemically infected by the reconstituted virus. cDNA fragments corresponding to the 3' non-coding region (NCR) of viral RNA 1 were amplified, cloned and sequenced. In some clones the termini of the 3' NCR corresponded to those of viral RNAs 2 or 3. This suggested that in some cases RNA 1 may have been regenerated during replication by a template switching mechanism between the inoculated transcript RNAs and the mRNA, However, encapsidated, recombinant RNA 1 with the 3' NCR ends originating from RNAs 2 or 3 also was found in virus samples that had been passaged exclusively through nontransgenic plants. Thus, these chimeras occur naturally due to recombination between wild-type viral RNAs, and they are found encapsidated in low, but detectable amounts.