UNUSUAL SEQUENCE RELATIONSHIPS BETWEEN 2 ISOLATES OF CITRUS TRISTEZA VIRUS

The complete, 19226 nt sequence of the RNA genome from VT, a seedling yellows strain of citrus tristeza virus (CTV), was determined and foun d to have a genome organization identical with that of the previously determined CTV-T36 isolate, except that ORF 1 of CTV-VT was 70 nt shor ter due to two widely separated 18 nt deletions. Sequence comparison o f CN-VT and CTV-T36 revealed approximately 89% identity throughout the ten 3' ORFs, but only 60-70% identity throughout ORF 1, The 5' nontra nslated regions were only 60% identical whereas the 3' nontranslated r egions were 97% identical, The transition between regions of similarit y and deviation was gradual, suggesting that the sequence similarities and differences compared to CTV-T36 were unlikely to have arisen from a recent recombination event between a close T36 relative and a dista ntly related CTV isolate, This is the first attempt to compare in deta il the variation between the genomes of two strains of a member of the closterovirus group, The observed deviation between the large RNA gen omes of the two CTV strains is greater than that among different virus es of most other groups, raising the question of how to define the tax onomy of these viruses.