Timing of the synthesis of empty shells and minor nucleoproteins in relation to turnip yellow mosaic virus synthesis in Brassica protoplasts.

Following inoculation of Chinese cabbage protoplasts in vitro with turnip yellow mosaic virus, the development of characteristic changes in the chloroplasts was significantly asynchronous when observed by light or electron microscopy. In fully infected protoplasts there were only about 25% as many peripheral vesicles (the site of virus RNA synthesis) as in infected cells in the intact leaf. The yield of virus per infected protoplast was only about 1/10 th that found for an equivalent population of infected palisade cells from inoculated leaves. The production of empty protein shells was not reduced as much. Approximately equal amounts of virus nucleoprotein and empty protein shells were produced at all stages of the virus growth curve. However, if [35S]methionine was supplied when virus replication was well established, empty protein shells became labeled at an initial rate almost 10 times that of virus. Thus empty shells and virus must be assembled from differently located "pools" of protein subunits. The minor virus nucleoprotein fraction known to contain a substantial proportion of coat mRNA appears to be assembled earlier than the other minor nucleoprotein components, empty protein shells, or virus.