Nonmutant forms of the avirulent satellite D of turnip crinkle virus are produced following inoculation of plants with mutant forms synthesized in vitro

The avirulent satellite RNA D (sat D) of turnip crinkle virus (TCV) has been cloned as a copy DNA. RNA transcripts synthesized in vitro from the cloned cDNA are infectious when coinoculated with RNA transcripts of the cloned TCV genome. Sat D is the smallest and most likely the progenitor of the other TCV satellites and, in contrast to the virulent satellite RNA C (sat C), does not intensify viral symptoms. Mutant forms of sat D including internal deletions up to 50 bases yielded sat D in infected plants. However, sat D mutants were not recovered in mutant form, but reverted to normal size and sequence in infected turnip plants. Mutations at a site with homology to the catalytic strand of self-cleaving sequences in certain viroids and satellites appeared to confer virulence on sat D in that test plants showed severe crinkling and stunting normally associated with sat C. However, sat C appeared along with a restored form of sat D in the progeny RNAs of these severely infected plants. Sat C was presumably generated by recombination between sat D and the TCV genome. In contrast, when plants were inoculated with transcripts containing the equivalent mutations in sat C, sat C was recovered from infected plants in mutant form. These findings demonstrate the tendency for mutant forms of the avirulent satellite, sat D, to revert, but raise questions about the source of information used in the reversion process.