Electron microscopic and molecular characterization of turnip vein-clearing virus.

We recently isolated turnip vein-clearing virus (TVCV), a tobamovirus which causes vein clearing in Brassica rapa (turnip) and a mosaic in Nicotiana tabacum (tobacco). We present an electron microscopic and molecular characterization of TVCV. Viral particles from lower epidermis peel contained rod-shaped viral particles, typical of tobamoviruses. Viral RNA extracted from infected turnip leaves was used as template for cDNA synthesis prior to cloning in a plasmid vector. Inserts of selected cDNA clones were sequenced to obtain the nucleotide sequence of the 126 K replicase component. The nucleotide and predicted amino acid sequences were 56 to 59% identical to those of most other sequenced tobamoviruses. The least related sequence, that of cucumber green mottle mosaic virus, was more related to the TVCV lineage than it was to those of the other sequenced tobamoviruses. UV spectroscopy suggested a tryptophan content characteristic of the ribgrass mosaic virus (RMV) group. Fragmentation of the TVCV coat protein by cyanogen bromide treatment produced a profile of fragments indistinguishable from those generated from the coat protein of RMV. Thus, while symptoms of TVCV infection on Nicotiana tabacum cv. Samsun and Nicotiana clevelandii differ from those reported for RMV, TVCV appears to be closely related to RMV.