Analysis of the replicative form (RF) RNAs of three viruses assigned to the Tombusvirus group-tomato bushy stunt virus (TBSV), carnation ringspot (CRSV), and turnip crinkle (TCV) viruses-indicated that all three have different replicative strategies. Viral specific double-stranded (ds)RNAs were isolated from infected plant tissues by phenol extraction and cellulose chromatography and analyzed by polyacrylamide gel electrophoresis. A single RF species with a molecular weight of 3.0 x 106 daltons, twofold that of the viral single-stranded RNA, was isolated from TCV-infected plants. Two major RF species of 3.0 and 1.0 × 106 daltons and a minor one with 1.2 x 106 daltons were isolated from CRSV-infected tissues corresponding to the three genomic RNAs of this multicomponent virus. TBSV-infected tissues yielded two replicative intermediate RNA species in addition to two dsRNAs with molecular weights of 3.2 and 1.5 x 106 daltons. Hybridization experiments indicated that both dsRNAs were TBSV-specific. It is proposed that the 1.5 × 106 dalton dsRNA represents the replicative form of a subgenomic fragment of the TBSV RNA with a postulated MW of 0.75 × 106 daltons. A model for the replication of TBSV is suggested and its possible significance discussed.
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