Endonuclease II, apurinic acid endonuclease, and exonuclease III

An endonuclease of Escherichia coli active on a DNA treated with methylmethane sulfonate has been separated from an endonuclease active on depurinated sites. The former enzyme is disignated here as endonuclease II, while the latter enzyme is designated as apurinic acid endonuclease. Endonuclease II is also active on DNA treated with methylnitrosourea, 7-bromomethyl-12-methylbenz[a]anthracene, and gamma-irradiation. A third fraction which contains activities for both depurinated and alkylated sites needs further study. Endonuclease II, molecular weight 33,000, has been purified 12,500-fold and does not have exonuclease III activity. Apurinic acid endonuclease, molecular weight 31,500, has been purified 11,000-fold and does not have exonuclease III activity. Exonuclease III, molecular weight 26,000, has been purified 2300-fold and does not have endonucleolytic activity at depurinated reduced sites or at alkylated sites in DNA. Therefore, these are three separate proteins. Exonuclease III can produce, presumably by its exonucleolytic activity, double-strand breaks in heavily alkylated DNA under conditions where it does not make single-strand endonucleolytic breaks at either depurinated-reduced or alkylated sites.