111 Biochimiea et Biophysica Acta, 520 (1978) 111--121 © Elsevier/North-Holland Biomedical Press BBA 99219 ENDONUCLEASE ACTIVITIES FROM A PERMANENTLY ESTABLISHED MOUSE CELL LINE THAT ACT UPON DNA DAMAGED BY ULTRAVIOLET LIGHT, ACID AND OSMIUM TETROXIDE INGOLF F. NES and JON NISSEN-MEYER Department of Biochemistry, University of Bergen, Y~rstadveien 19, 5000 Bergen (Norway) (Received December 6th, 1977) Summary The activity of damage-dependent endonuclease in mouse plasmacytoma cells (line MPC-11) has been studied using damaged ¢ X 174 RFI DNA as sub- strate. 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 alkali- labile site and this strongly suggests the presence of an apurinic-specific endo- nuclease 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 require- ments and the extent to which they are stimulated by magnesium. While the ultraviolet-endonuclease activity was very little affected by reducing the KC1 concentration, the apurinic-specific activity was almost completely abolished. Osmium tetroxide renders the DNA an excellent substrate for endonucleo- lytic activity in the mouse cell extract. The response to KC1 and MgC12 of the osmium tetroxide-specific endonuclease activity is qualitatively similar to that of the endonuclease activity, which acts on ultraviolet-irradiated DNA. Treat- ment of DNA with osmium tetroxide is known to produce 5,6-dihydroxy- dihydrothymine which is a minor photoproduct in DNA after irradiation, suggesting that the ultraviolet-specific endonuclease activity acts upon this lesion. Abbreviation: RFI is the replicative form of DNA which is circular, double-stranded and covalenfly closed.