Infectious DNA of spleen necrosis virus is integrated at a single site in the DNA of chronically infected chicken fibroblasts.

The infectious DNAs of a number of avian leukosis-sarcoma and reticuloendotheliosis viruses were digested with six nucleotide-specific restriction endonucleases, and the digests were tested for infectivity. All of the enzymes inactivated the viral infectivities except for EcoRI, which did not inactivate the infectivity of the DNA of two of the reticuloendotheliosis viruses, spleen necrosis and chick syncytial viruses. The infectious DNA of spleen necrosis virus after digestion with EcoRI had a buoyant density in CsCl solution greater than the density of the high-molecular-weight infectious viral DNA. The infectious EcoRI-digested spleen necrosis virus DNA from chronically infected chicken cells was uniform in size, 10 megadaltons, which indicated a single site of integration. The infectious EcoRI-digested spleen necrosis virus DNA from acutely infected cells was heterogeneous in size, ranging from 8-14 megadaltons, which indicated multiple sites of integration. These results are consistent with the hypothesis that cells that integrate infectious spleen necrosis virus DNA at a single site survive and multiply, whereas cells that integrate infectious viral DNA at additional sites either die or selectively lose or inactivate the DNA in the additional sites.