ENDONUCLEASE-G FROM MAMMALIAN NUCLEI IS IDENTICAL TO THE MAJOR ENDONUCLEASE OF MITOCHONDRIA

Two Mg2+-dependent DNA endonucleases have been isolated from mammalian cells which have a strong preference to nick within long tracts of gu anine residues in vitro. One endonuclease activity is mitochondrial (m t). The other endonuclease, called Endonuclease G, is associated with isolated nuclei, and is released when the nuclear chromatin is exposed to moderate ionic strength. Our laboratory has previously purified th e mt endonuclease to near homogeneity from mitochondria of bovine hear t and reported the enzyme to be a homodimer of a approximate to 29 kDa polypeptide [Cummings, O. W. at al. (1987) J. Biol. Chem., 262, 2005- 2015]. Although the purified mt endonuclease will extensively fragment M13 viral ssDNA and plasmid dsDNAs in vitro, the enzyme displays an u nusually strong preference to nick within a (dG)12:(dC)12 sequence tra ct which resides just upstream from the origin of DNA replication in t he mitochondrial genome. The nuclear Endonuclease G first identified f rom its selective targeting of several (dG)n:(dC)n tracts in vitro (wh ere N=3-29), was subsequently purified from calf thymus nuclei and sho wn to be a homodimer of a approximate to 26-kDa polypeptide [Cote, J. at al. (1989) J. Biol. Chem., 264, 3301-3310]. In the present study, w e find that Endonuclease G partially purified from calf thymus nuclei will extensively degrade both viral ss- and dsDNAs in vitro, and that the enzyme possesses biochemical properties and specificity for nucleo tide sequences in DNA that are strongly related or identical to those of the mt endonuclease. These findings and the discovery of sequence i dentity between the proteins strengthen the conclusion that the nuclea r Endonuclease G is the same enzyme as the mt endonuclease.