Mouse mammary tumor virus DNA in infected rat cells: characterization of unintegrated forms.

Mouse mammary tumor virus (MMTV) DNA in chronically infected rat hepatoma cells is maintained in both the integrated and unintegrated state. Fractionation of DNA by the procedure of Hirt (1967) as well as by sedimentation through alkaline sucrose suggests that about two thirds of the viral DNA is associated with high molecular weight cell DNA. The remainder of the viral DNA is unintegrated and is present primarily as linear or open circular duplexes consisting of a genome-length strand complementary to the viral RNA ("minus" strand) and "plus" strands of subgenomic length. Approximately 20% of the unintegrated MMTV DNA is present as double-stranded, covently closed circles (form I) with a molecular weight of 6 X 10(6) daltons. Form I viral DNA is found primarily in the nucleus, whereas the open forms are both nuclear and cytoplasmic.