Three different virus-specific particles were identified in stocks of SV which had been serially passaged without dilution through BHK21 cells. These particles had densities of 1.16, 1.20, and 1.22 g/cc in sucrose gradients prepared in D2O. The 1.20 g/cc particle was infective, contained 42 S RNA, and was the only species present in stocks prepared by infecting BHK21 cells with plaque-purified virus. The 1.16 g/cc particle was not infective and contained heterogeneous RNA species. No additional intracellular viral RNA species were produced in BHK21 cells coinfected with the 1.16 g/cc particle and unpassaged virus compared to cells infected with unpassaged virus alone. The 1.22 g/cc particle was not infective although it contained an RNA molecule which migrated identically with the RNA of infective virions in polyacrylamide-agarose gels. Cells coinfected with the 1.22 g/cc particle and unpassaged SV synthesized 12 S double-stranded RNA and small, single-stranded RNA species not present when cells were infected with only unpassaged SV. RNA synthesis by the 1.22 g/cc particles, however, appeared to require the presence of infective virions. The 1.22 g/cc particle also interfered with RNA synthesis by unpassaged SV, and, therefore, can be designated a defective-interfering particle.
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