Evidence that tobacco mosaic virus particles disassemble contranslationally in vivo.

The possibility that a cotranslational disassembly mechanism, similar to that observed when pH 8-treated tobacco mosaic virus (TMV) particles are incubated in an in vitro translation system [T. M. A. Wilson. (1984), Virology 137, 255-265], may be involved in the early stages of virus infection was investigated. Extracts of tobacco leaf epidermal cells, collected between 10 and 70 min after inoculation with 32P- and [3H]leucine-labeled TMV, contained material which had higher buoyant densities in Cs2SO4 gradients and higher 32P:3H ratios than did virus particles. The material sedimented to positions similar to those of in vitro-prepared complexes of partially stripped virus particles and ribosomes and to those of the in vivo-produced complexes of TMV rodlets and in vivo-labeled, nascent polypeptides that formed after inoculation with unlabeled, untreated TMV. In the electron microscope, some of this material resembled the complexes observed in the in vitro translation system. Experiments in which TMV or partially stripped TMV was mixed with epidermal cells from mock-inoculated leaves indicated that the material did not arise by dissociation of virus particles. nor by binding of subcellular components to partially uncoated TMV, during extraction and analysis. These observations provide evidence of the occurrence of a cotranslational disassembly mechanism during the early stages of infection with TMV.