FIG. 4.

BDV-infected cells are resistent to superinfection. C6 cell lines persistently infected with either strain He/80 (lanes 5 to 7) or strain V (lanes 9 to 11) were incubated with various doses (100, 500, and 2,500 FFU) of partially purified BDV strain V (lanes 5 to 7) or with various doses (40, 200, and 4,000 FFU) of partially purified BDV strain He/80 (lanes 9 to 11). To control for the quality of the virus stocks, uninfected C6 cells were infected with 100 FFU of strain V (lane 4) or with 40 FFU of strain He/80 (lane 8). Three weeks postinfection, when almost all cells in the control cultures were expressing viral antigen, samples of total RNA (10 μg) from the various cultures were analyzed by RPA as for Fig. 1, using high concentrations of RNase A. Undigested RPA probe (lane 1). No specific products were observed when the RPA was carried out with 10 μg of tRNA (lane 2) or 10 μg of total RNA from uninfected C6 cells (lane 3).