Turnip yellow mosaic virus variants produced from DNA clones encoding their genomes

Full-length dsDNA clones that encode the genomes of two Australian turnip yellow mosaic isolates, TYMV-BL and TYMV-CL have been constructed. These clones were transcribed to give 6.3 kb capped ssRNA which infects Chinese cabbages to give symptoms indistinguishable from those produced by the parental viruses. Extensions of up to 26 nucleotides at the 3' end of the TYMV-BL clone delay infections, but virus particles isolated from these plants 4 weeks after inoculation contain RNA with the original TYMV-BL 3' terminus. A 90 nucleotide-long portion of the virion protein gene of TYMV-BL was replaced by a synthetic 90-mer primer with 16 nucleotide changes to decrease the large cytosine content (34-42%) characteristic of tymovirus genomic RNA. No reversion of any of the mutated nucleotides to cytosine occurred during 7 passages in Chinese cabbage. Hybrids between the TYMV-BL and TYMV-CL clones were also constructed, by exchanging various portions of the genome. However, it was not possible to determine definitively which part of the viral genome is responsible for the more severe symptoms caused by TYMV-BL as the hybrids gave intermediate symptoms.