Purification and properties of a mammalian endonuclease showing site-specific cleavage of DNA

An endonuclease activity has been identified from a variety of mammalian sources which cleaves native viral DNAs (adenovirus-2, SV40) and mammalian repetitive DNA to yield specific, double-stranded DNA segments when the cleavage products are analyzed by gel electrophoresis. The enzyme has a pH optimum of 7.5 and shows an absolute requirement for Mg super(2+) or Mn super(2+) and reducing agents in the reaction buffer. It is strongly inhibited by salt and stimulated by glycerol or dimethyl sulfoxide. By glycerol gradient analysis it has a sedimentation coefficient of 4.5 (65,000 daltons of globular) and it may exist as a dimer of 6.4 S (130,000 daltons). The enzyme liberates 3'-OH and 5'-P termini in its cleavage of mammalian viral and repetitive DNAs and there is a clear nonrandomness in the 5'-terminal nucleotides: purines and 5'-pG in particular predominate.