Genome structure of horseradish curly top virus and phylogenetic relationships among geminiviruses
Klute, Kimberly A.
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The nucleotide sequence (3080 nt.) of the geminivirus horseradish curly top virus (HrCTV) was determined and compared to other gemini viruses using dot matrix analyses and phylogenetic methods. Sequence similarities among full-length viral genomes were determined using dot matrix plots. Phylogenetic relationships among HrCTV and other geminiviruses were inferred for individual viral proteins using parsimony, UPGMA, and Fitch methods. HrCTV encodes three virion-sense ORFs (VI, V2, and V3), which are similar to the corresponding ORFs of the subgroup II gemini virus beet curly top virus (BCTV). The genome of HrCTV encodes three complementary-sense ORFs (Cl, C2 and C4), and lacks a C3 ORF, which is conserved among all other subgroup II and HI geminiviruses characterized to date. Although the HrCTV C2 protein was most closely related to the C2 protein of BCTV, the HrCTV C2 protein has diverged considerably relative to strains of BCTV. To further characterize the HrCTV C2 ORF, a frameshift mutation was introduced into the HrCTV C2 ORF and the mutant genome was inoculated onto shepherd's purse plants. The HrCTV ORF C2 mutant was not infectious, demonstrating that ORF C2 is required for infectivity. Phylogenetic analyses indicated that the HrCTV Cl protein was most closely related to the subgroup III geminivirus, squash leaf curl virus (SLCV). These analyses indicate that HrCTV is a recombinant virus, in which SLCV and BCTV show the closest phylogenetic relationships with HrCTV.