Studies on a defective variant of simian virus 40 that is substituted with DNA sequences derived from monkey DNA. I. Origin, properties, and purification.

Serial passage of plaque-purified simian virus 40 at high multiplicity in monkey kidney cells produces defective virus. A particular series of such passages, in which the defective genomes contain substitutions by host (monkey) DNA sequences, is described here. The suspensions of virus obtained during the passaging are shown to be defective by the usual criteria, namely, a decreased yield of plaque-forming units compared to the yield from infection with wild type virus. It is shown that the rate of viral DNA synthesis in cells infected with passaged virus, which includes the synthesis of the host-substituted variants as well as that of remaining wild type virus, can also be used as a measure of the defectiveness of the yields of the passages. For this purpose, conditions are described under which the rate of viral DNA synthesis in infected cells is proportional to the multiplicity of infection as calculated from plaque-forming units of virus. This proportionality holds both for wild type, plaque-purified virus, and for lysates containing defective variants. Further, the rate of DNA synthesis was used to study the course of infection during serial passaging. Interference with the synthesis of wild type viral DNA by the defective lysates was also demonstrated in this manner. The nature of the viral genomes synthesized in one particular passage of the series under study was investigated. Genomes of at least three types were present: (a) wild type SV40 genomes; (b) defective genomes that are somewhat shorter than wild type DNA and give patterns of DNA fragments upon digestion with restriction endonucleases Hind11 and III that differ markedly from the pattern typical of wild type DNA; and (c) defective genomes that give still another pattern of DNA fragments upon digestion with Hind11 and III, that have lost the single cleavage site for restriction endonuclease EcoRI that is typical of wild type SV40 DNA, and that contain sequences derived from monkey DNA.