Endonuclease activities from a permanently established mouse cell line that act upon DNA damaged by ultraviolet light, acid and osmium tetroxide.

The activity of damage-dependent endonuclease in mouse plasmacytoma cells (line MPC-11) has been studied using damaged phi X 174 RFI DNA as substrate. The DNA was treated with ultraviolet light, acid, or osmium tetroxide to introduce different types of lesions. Ultraviolet light-damaged DNA was cleaved at approx. 1.1 sites per 35 thymine-containing dimers by the extract, which indicates no specificity towards this type of lesion. The acid-treated DNA, which contains apurinic sites, was enzymatically broken in every alkalilabile site and this strongly suggests the presence of an apurinic-specific endonuclease activity in the nuclear extract. The activity which acts on ultraviolet-irradiated DNA and that which acts on acid-treated DNA have different specificities as shown by their salt requirements and the extent to which they are stimulated by magnesium. While the ultraviolet-endonuclease activity was very little affected by reducing the KCl concentration, the apurinic-specific activity was almost completely abolished. Osmium tetroxide renders the DNA an excellent substrate for endonucleolytic activity in the mouse cell extract. The response to KCl and MgCl2 of the osmium tetroxide-specific endonuclease activity is qualitatively similar to that of the endonuclease activity, which acts on ultraviolet-irradiated DNA. Treatment of DNA with osmium tetroxide is known to produce 5,6-dihydroxydihydrothymine which is a minor photoproduct in DNA after irradiation, suggesting that the ultraviolet-specific endonuclease activity acts upon this lesion.