Genome characterization and genetic diversity of beet curly top Iran virus: a geminivirus with a novel nonanucleotide

Beet curly top Iran virus (BCTIV) was previously reported as a distinct curtovirus in Iran. Complete nucleotide sequences of three BCTIV isolates, one each from central, southern, and south eastern Iran were determined to be 2844, 2844, and 2845 nt long, respectively. BCTIV shared highest nucleotide sequence identity (52.3%) with Spinach curly top virus (SpCTV) and lowest identity (46.6%) with Horseradish curly top virus (HrCTV). The BCTIV genome comprises three virion-sense (V1, V2, and V3) and two complementary-sense (C1 and C2) ORFs. ORFs C3 and C4 were not found in BCTIV genome. Based on a comparison of nucleotide sequence identity of individual genes, the three virion-sense ORFs were 72.7–79.9% related to the corresponding ORFs of curtoviruses, whereas no significant relationship was found between the C1 and C2 ORFs of BCTIV and curtoviruses. These two ORFs, however, were only distantly related with those of mastreviruses. Similar to the latter viruses, the BCTIV genome comprises two intergenic regions. The BCTIV large intergenic region included a sequence capable of forming a stem loop structure and a novel nonanucleotide (TAAGATT/CC) with a unique nick site. Phylogenetic analysis using deduced amino acid sequence of individual ORFs revealed that the V2 and V3 ORFs are monophyletic and the V1 ORF is classified with the related ORF of curtoviruses. Whereas the two complementary-sense ORFs are grouped with those of mastreviruses. Computer-based prediction suggested that BCTIV has a chimeric genome which may have arisen by a recombination event involving curto- and mastrevirus ancestors. Percent nucleotide sequence identities of the coat protein gene of ten isolates of BCTIV, collected from a wide range of geographical regions in Iran, varied from 87.1 to 99.9, with the isolates being distributed between two subgroups. Based on biological and molecular properties, BCTIV is proposed as a new member of the genus Curtovirus.