Restriction endonuclease mapping of unintegrated viral DNA of B- and N-tropic BALB/c murine leukemia virus

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Abstract
Unintegrated linear and closed circular DNA of B- and N-tropic endogenous BALB/c murine leukemic virus (MuLV) were extracted from newly infected mouse cells and cleaved with EcoRI, XhoI, PvuI, HindIII, SalI, XbaI, KpnI, SmaI and PstI restriction endonucleases. The DNA fragments were separated by electrophoresis and analyzed by the Southern blot hybridization procedure. EcoRI did not cleave the 2 genomes. A physical map of 15 cleavage sites on B- and N-tropic genomes was constructed with the other restriction endonucleases. Identical cleavage sites of B- and N-tropic MuLV DNA were found with all these enzymes. The N-tropic linear genome lacked about 75 base pairs at each end of the molecule. PstI, KpnI and SmaI recognize a cleavage site at both ends of the linear molecules. Sequences derived from the 5'' end of the RNA genome were found in the third left end of the linear DNA and at its extreme right-end terminus, suggesting the presence of redundant sequences. Two species of closed circular viral DNA were observed. The larger species has the same size as the linear molecule and appears to be a circularized form of linear DNA. The smaller species contains sequences common to the linear and the larger circular viral DNA but seems to be deleted from sequences present at either one or both ends of the linear DNA. The general structure of the linear and circular DNA species of these B- and N-tropic endogenous BALB/c MuLV appears analogous to the structure found with other retroviruses.