Coat protein is required for infectivity of tobacco streak virus: Biological equivalence of the coat proteins of tobacco streak and alfalfa mosaic viruses

Abstract

The four nucleoprotein components of tobacco streak virus (TSV) were purified by zonal centrifugation. Each component contains mainly one RNA species. Infectivity studies indicate that the three fastest-sedimenting nucleoproteins are required for viral replication. A mixture of the three largest RNA's however, is not infectious but can be activated by the viral coat protein or by the smallest TSV-RNA. The same situation exists in alfalfa mosaic virus (AMV). The coat proteins of TSV and AMV, which are rather different as judged by serology and tryptic digestion products, cannot only activate their own genome but also that of each other. The nucleoprotein components of the two viruses, however, could not be substituted for each other. TSV-RNA is as efficient as AMV-RNA in withdrawing protein subunits from AMV nucleoprotein. However, TSV nucleoprotein could not be uncoated by its own RNA or by AMV-RNA. Nevertheless, the affinity of TSV-RNA for AMV coat protein and the functional equivalence of the two coat proteins point to a possible common origin of these two viruses.