The objectives of the proposed research are to understand the interactions which occur in a synergistic viral infection. Proposed experiments focus on the interaction of two unrelated (+)-stranded RNA viruses of plants, potato virus X (PVX) and potato virus Y (PVY). In mixed tobacco infections with these two viruses the accumulation of PVX is greatly increased over that in singly infected plants, while that of PVY is unchanged. The immediate goal of the work is to determine the level at which the increased accumulation of PVX is regulated. The steady state levels, time course of accumulation and rates of synthesis of coat protein and various viral RNAs will be measured in singly and doubly infected plant protoplasts. Later aims are to pinpoint the specific sequences on the PVY and PVX genomes which are important for the increased accumulation of PVX in co- infected cells. These experiments will use transgenic plants expressing subsets of the PVY genome, as well as genetically engineered PVX mutants. Recent studies have shown similarities in diverse plant and animal viruses based on genome organization,mechanisms of gene expression, and sequence homology of virally encoded, nonstructural proteins. These results suggest that elucidation of molecular mechanisms for virus interactions obtained in one system may well apply to a diverse group of other virus systems. This work should give insight into a fundamental biological phenomenon and, particularly if the results are found to apply to a number of viral systems, may be of economic importance.