Electron microscopic studies of circular DNA in mouse embryo fibroblasts infected by Rauscher leukemia virus

Using electron microscopy, a closed circular form of DNA (4.3 mum in contour length) was detected in the nucleus of mouse embryo fibroblasts 2.5 h after infection by Rauscher murine leukemia virus. These circles were distinguishable from mitochondrial DNA by various criteria, including size, absence of secondary features, and resistance to EcoRI endonuclease.