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.