Mapping of linear and circular forms of mouse mammary tumor virus DNA with restriction endonucleases: evidence for a large specific deletion occurring at high frequency during circularization.

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Abstract
Rat hepatoma cells infected with mouse mammary tumor virus contain multiple forms of unintegrated viral DNA when grown in the presence of glucocorticoids. Using the DNA transfer procedure of Southern [SalI, HpaI, HindIII, BamHI, KpnI, XhoI, XbaI, EcoRI, BglII and PstZ], restriction endonuclease fragment maps of these forms of viral DNA were prepared. The maps indicate that: the major species of viral DNA is a linear molecule of 5.9 .times. 106 MW located in the cytoplasm; the nuclei contain covalently closed circular viral DNA of 2 distinct sizes (5.1 .times. 106 and 5.9 .times. 106 MW) besides linear molecules (5.9 .times. 106 MW); the linear molecule has specific termini; there is extensive homology between regions at or near termini of the linear molecule; the predominant form of circular DNA lacks 1.2 kilobase pairs present in both the larger circular molecule and the linear molecule; and the sequences deleted from the majority of the circular DNA molecules are located at the ends of the linear DNA that are joined during circularization.