Recombination sites in cauliflower mosaic virus DNAs: implications for mechanisms of recombination.

Pairs of mutant cauliflower mosaic virus (CaMV) DNAs readily recombine in plants. Five plasmid clones of CaMV DNAs resulting from infection of turnips with pairs of mutant DNAs from DNAs resulting from infection of turnips with pairs of mutant DNAs from different isolates were obtained. Restriction analysis and nucleotide sequencing identified deletions in two cloned recombinants, VR1249 and VR244B. The sequence missing in the former was consistent with its deletion by splicing of an RNA intermediate. These DNAs were not infectious in turnips. VR1243, VR244A, and VR246 induced in turnips disease symptoms that were mixtures of those produced by the parental isolates. Junctions between sequences of the parental isolates were identified by restriction fragment analysis. Three cloned chimeras resulted from multiple recombination events. Nucleotide sequencing identified more precisely the junctions in the five cloned chimeras and in three chimeras previously characterized. Consistent with a model in which reverse transcription plays a major role in generating recombinants, six chimeras had junctions at or near the site for initiation of DNA(-) strand synthesis, three had junctions near the initiation site of 35 S RNA transcription, and one junction was found near the initiation site of 19 S mRNA transcription. Junctions were also found in regions not bearing any obvious relation to DNA (-) strand synthesis by reverse transcription, suggesting that recombination of double-stranded DNAs may also generate CaMV DNA recombinants.