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 N icotiana tabacum (tobacco). We present an electron microscopic and mol ecular characterization of TVCV. Viral particles from lower epidermis peel contained rod-shaped viral particles, typical of tobamoviruses. V iral RNA extracted from infected turnip leaves was used as template fo r cDNA synthesis prior to cloning in a plasmid vector. Inserts of sele cted cDNA clones were sequenced to obtain the nucleotide sequence of t he 126 K replicase component. The nucleotide and predicted amino acid sequences were 56 to 59% identical to those of most other sequenced to bamoviruses. The least related sequence, that of cucumber green mottle mosaic virus, was more related to the TVCV lineage than it was to tho se of the other sequenced tobamoviruses. UV spectroscopy suggested a t ryptophan content characteristic of the ribgrass mosaic virus (RMV) gr oup. Fragmentation of the TVCV coat protein by cyanogen bromide treatm ent produced a profile of fragments indistinguishable from those gener ated from the coat protein of RMV. Thus, while symptoms of TVCV infect ion on Nicotiana tabacum cv. Samsun and Nicotiana clevelandii differ f rom those reported for RMV, TVCV appears to be closely related to RMV.