Endonuclease II, apurinic acid endonuclease, and exonuclease III.

Abstract

An endonuclease of Escherichia coli active on a DNA treated with methylmethane sulfonate was separated from an endonuclease active on depurinated sites. The former enzyme is designated 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 3rd fraction which contains activities for depurinated and alkylated sites needs further study. Endonuclease II, MW 33,000, was purified 12,500-fold and does not have exonuclease III activity. Apurinic acid endonuclease, MW 31,500, was purified 11,000-fold and does not have exonuclease III activity. Exonuclease III, MW 26,000, was purified 2300-fold and does not have endonucleolytic activity at depurinated reduced sites or at alkylated sites in DNA. Thus, these are 3 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 depurinated-reduced or alkylated sites.