Restriction endonuclease cleavage of linear and closed circular murine leukemia viral DNAs: discovery of a smaller circular form.

Both linear (form III) and closed circular (form I) viral DNAs obtained from mouse cells infected with Moloney murine leukemia virus were cleaved by Sal I, Sma I, Bam HI and Pst I restriction endonucleases. DNA fragments generated by these cleavages were ordered with respect to the 5' and 3' ends of the RNA genome by several techniques, including comparisons of the DNA fragments from cleavages of the linear and closed circular forms, double digestions using different combinations of enzymes and the use of an RNA probe specific for the 3' end. DNA from Hirt extractions of infected cells yielded a discrete species of linear viral DNA whose size was determined by agarose gel electrophoresis to be 5.7 x 10(6) daltons. In the course of characterizing the closed circular DNA, we observed two form I DNA molecules. The larger molecule was the same size as the linear DNA. The second molecule migrated faster on agarose gels and was the predominant species of the two closed circular DNAs. Using the restriction endonuclease maps which we derived, we demonstrate that this novel form I DNA is a smaller homogeneous species of viral DNA, missing about 600 nucleotides found in the linear and larger closed circular DNA molecules. We have localized the site of this missing DNA piece to be at either one or both ends of the linear viral DNA.