Recombination in a plant virus: template-switching in cauliflower mosaic virus.

A hybrid plasmid, containing tandemly arranged pieces of two different but well-defined cauliflower mosaic virus (CaMV) genomes, was used to study the mechanism by which infectious viral DNA can escape from transforming DNA. Systemic viral infection followed inoculation of Brassica plants with a strain of Agrobacterium tumefaciens containing the hybrid plasmid in its T-DNA. Restriction mapping of uncloned viral DNA from these plants, and sequencing of relevant portions of cloned viral DNA, showed that the majority of viral progeny were probably descendants of DNA produced by transcription/reverse transcription of the viral genome, thus providing further evidence for the hypothesis that this process is normally involved in viral replication. The reverse transcriptase enzyme, which is thought to undergo an intramolecular template switch during viral replication, is shown to move very close to the 5' end of the terminal repeat on the 35S RNA molecule before switching templates. The remaining minority of viral genomes can best be explained as arising from products of recombination between homologous regions of CaMV DNA.