The Nucleotide Sequence and Genomic Organization of Citrus Leaf Blotch Virus: Candidate Type Species for a New Virus Genus

Abstract

The complete nucleotide sequence of Citrus leaf blotch virus (CLBV) was determined. CLBV genomic RNA (gRNA) has 8747 nt, excluding the 3'-terminal poly(A) tail, and contains three open reading frames (ORFs) and untranslated regions (UTR) of 73 and 541 nucleotides at the 5' and 3' termini, respectively. ORF1 potentially encodes a 227.4-kDa polypeptide, which has methyltransferase, papain-like protease, helicase, and RNA-dependent RNA polymerase motifs. ORF2 encodes a 40.2-kDa polypeptide containing a motif characteristic of cell-to-cell movement proteins. The 40.7-kDa polypeptide encoded by ORF3 was identified as the coat protein. The genome organization of CLBV resembles that of viruses in the genus Trichovirus, but they differ in various aspects: (i) in trichoviruses ORF2 overlaps ORFs 1 and 3, whereas in CLBV, ORFs 2 and 3 are separated and ORFs 1 and 2 overlap in one nucleotide; (ii) CLBV gRNA and CP are larger than those of trichoviruses; and (iii) the CLBV 3' UTR is larger than that of trichoviruses. Phylogenetic comparisons based on CP amino acid signatures clearly separates CLBV from trichoviruses. Also contrasting with trichoviruses, CLBV could not be transmitted to Chenopodium quinoa Willd. Considering these singularities, we propose that CLBV should be included in a new virus genus.