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

Abstract
Using EM, a closed circular form of DNA (4.3 .mu.m in contour length) [presumed to be a late intermediate in the formation of integrated proviral DNA] 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 of EcoRI [restriction] endonuclease.