Partial purification and characterization of the major AP endonuclease from Chlamydomonas reinhardi

The major AP endonuclease from Chlamydomonas reinhardi has been partially purified and characterized. The enzyme has a molecular weight of about 38 000 as measured by molecular sieving. There is an absolute requirement for a divalent cation, with magnesium being better than manganese. The activity is stimulated by dithiothreitol and Triton X-100. The activity is sensitive to ionic strength, as 50 mM NaCl or KCl results in 70% inhibition. The enzyme is specific for apurinic and apyrimidinic (AP) sites and does not cleave DNA that has been damaged by ultraviolet light, methyl methanesulfonate, osmium tetroxide or sodium bisulfite. There is no deficiency in the AP endonuclease activity in extracts prepared from two mutants of Chlamydomonas that are sensitive to both ultraviolet light and methyl methanesulfonate. There was no evidence for induction of AP endonuclease after exposure of the cells to methyl methanesulfonate.