Endodeoxyribonuclease activity associated with Rauscher murine leukemia virus

Preparations of purified Rauscher murine leukemia virus were found to contain an endodeoxyribonuclease after disruption of the virus with nonionic detergents. The enzyme makes single-strand breaks in linear or covalently closed circular phage double-stranded DNA molecules. The enzyme was partially purified by ion-exchange chromatography on DEAE- and carboxymethyl-Sepharose columns followed by electrophoresis in DNA-containing polyacrylamide gels. The enzyme was separated from reverse transcriptase (p80pol), and the final endonuclease preparation contained no detectable reverse transcriptase activity. The DEAE-Sepharose column-purified endonuclease activity contained a polypeptide of about 40,000 Mr that we term p40. Peptide mapping experiments demonstrated that p40 shares methionine-labeled tryptic peptides with Pr200gag-pol and Pr135pol. Six major methionine-labeled tryptic peptides derived from p40 were found in Pr200gag-pol, but only five of these were detected in Pr135pol. The four core proteins (p30, p15, pp12, and p10) and p80pol plus p40 account for most, but not all, of the peptide sequences of Pr200gag-pol. The endonuclease-associated p40 is similar in size and precursor origin to the avian retrovirus-coded endonuclease (p32). In view of these similarities to the avian p32 endonuclease and its association with partially purified Rauscher murine leukemia virus-associated endonuclease preparations, we propose that p40 is the Rauscher murine leukemia virus-coded endonuclease.