Purification and characterization of UV endonucleases I and II from murine plasmacytoma cells

Three endonucleases from murine plasmacytoma cells that specifically nick DNA which was heavily irradiated with ultraviolet (UV) light were resolved by Sephacryl S-200 column chromatography. Two of these, UV endonucleases I and II, were purified extensively. UV endonuclease I appears to be a monomeric protein with a molecular mass of 43 kDa; UV endonuclease II has an S value of 2.9 S, with a corresponding molecular mass estimated at 28 kDa. Both enzymes act as a class I AP endonuclease, cleaving phosphodiester bonds via a β-elimination mechanism, so as to form an unsaturated deoxyribose at the 3' terminus. Both have thymine glycol DNA glycosylase activity and their substrate specificities generally appear to be overlapping but not identical. UV endonuclease I acts on both supercoiled and relaxed DNAs, whereas II acts only on supercoiled DNA. Both enzymes are active in EDTA, but have different optima for salt, pH, and Triton X-100. Each enzyme is also present in cultured diploid human fibroblasts.