POPULATIONS OF CITRUS TRISTEZA VIRUS CONTAIN SMALLER-THAN-FULL-LENGTHPARTICLES WHICH ENCAPSIDATE SUB-GENOMIC RNA MOLECULES

Plants infected by citrus tristeza virus (CTV) contain, in addition to the 2000 nm full-length thread-like virions, a heterogeneous populati on of smaller particles. The CTV particles and RNA extracts from purif ied CTV preparations were fractionated by sucrose gradient centrifugat ion and the RNA molecules from different fractions translated in a ret iculocyte translation system. Fractions containing predominantly an RN A band of approximately 3.2 kb directed the synthesis of CTV coat prot ein (CP), which in SDS-PAGE had an estimated molecular mass of 28 kDa. Three additional poly-peptides, with estimated sizes of 21 kDa, 23 kD a and 27 kDa, were translated from a range of RNA molecules smaller th an 3.2 kb. Hybridization with cDNA to the CP gene (CTV-CPG) and with a 350 base clone complementary to the 3' and 5' termini of the genes fo r CTV p20 and p23.5, respectively, indicated that preparations of CTV particles contain, in addition to the genomic (20kb) RNA, two sub-geno mic RNA molecules of 3.2 kb and 2.4 kb and probably also two smaller m olecules of 1.6 kb and 0.9 kb. Only the 3.2 kb RNA, its corresponding dsRNA molecule and populations of larger RNAs, including the 20kb geno mic RNA, hybridized with a CTV-CPG probe, thus conflicting with our pr evious assignment of the CTV-CPG to the 0.8 kbp dsRNA. Based on these results we propose that distinct populations of CTV particles encapsid ate smaller RNAs which were formed as a nested set of subgenomic RNAs. Sequence analysis of 2540 nucleotides downstream to CTV-CPG of strain VT revealed four open reading frames ( ORFs) potentially encoding, in the 5' to 3' direction, 18 kDa (p18), 13 kDa (p13), 20 kDa (p20) and 23.5 kDa (23.5) proteins. The CTV-VT ORFs showed variable but usually close levels of homology with the corresponding ORFs of CTV-T3 6 from Florida.