COMPARATIVE SEQUENCE-ANALYSIS OF THE COAT PROTEINS OF BIOLOGICALLY DISTINCT CITRUS-TRISTEZA CLOSTEROVIRUS ISOLATES

The genome of citrus tristeza closterovirus (CTV) consists of a 20 kb single-stranded RNA encapsidated in a 2000 nm long flexuous particle. Double-stranded (replicative form) RNAs purified from CTV-infected tis sue were used to prepare complementary DNA libraries that involved ini tial first-strand cDNA synthesis followed by selective amplification o f the coat protein gene. CTV-specific antisera were used to select clo nes expressing the coat protein. The coat protein genes of seven Flori da and four exotic isolates that differ in their biological properties were cloned and sequenced. The gene is 669 base pairs long and encode s a 223 amino acid protein. There was a greater than 80% homology at b oth nucleotide and amino acid levels among all the isolates examined. However, comparisons showed that each isolate was found to have severa l unique amino acid residues. Several blocks of amino acid residues we re conserved among all the isolates. A cluster dendrogram showed great er similarities among groups of mild and severe Florida isolates that, differed significantly from those of the geographically distinct, exo tic CTV isolates.