Characterization of endonuclease activities in Moloney murine leukemia virus and its replication-defective mutants

To study Moloney murine leukemia virus (M-MuLV) proteins associated with the integration of proviral DNA into the host chromosome, we isolated endonuclease activities from purified virion preparations of the wild type and two of its replication mutants. A major endonuclease activity was identified in virions of M-MuLV; the enzyme catalyzed nicks in double-stranded DNA in the presence of either Mn²⁺ or Mg²⁺ and was stimulated by ATP. The endonuclease nicked DNA adjacent to all four nucleotides with some preference for G and C. The same enzyme, and in comparable amounts, was isolated from two virus replication mutants: dl2905, deficient in the processing of Pr65(gag) and Pr200(gag-pol), and dl5401, deficient for the virus integration function. In the process of these experiments, the residual reverse transcriptase in mutant dl2905 was shown to be the mature size, implying that the uncleaved precursor lacks enzymatic activity. It appears that the major endonuclease activity found in virions of M-MuLV is not encoded by either the gag or pol genes.