Properties of the Main Endonuclease Specific for Apurinic Sites of Escherichia coli (Endonuclease VI)

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Abstract
The main endonuclease for apurinic sites of E. coli (endonuclease VI) has no action on normal strands, either in double-stranded or single-stranded DNA, or on alkylated sites. The enzyme has an optimum pH at 8.5, is inhibited by EDTA and needs Mg2+ for its activity; it has a half-life of 7 min at 40.degree. C. A purified preparation of endonuclease VI, free of endonuclease II activity, contained exonuclease III; the 2 activities (endonuclease VI and exonuclease III) copurified and were inactivated with the same half-lives at 40.degree. C. Endonuclease VI cuts the DNA strands on the 5'' side of the apurinic sites giving a 3''-OH and a 5''-phosphate, and exonuclease III, working afterwards, leaves the apurinic site in the DNA molecule; this apurinic site can subsequently be removed by DNA polymerase I. The details of the excision of apurinic sites in vitro from DNA by endonuclease VI/exonuclease III, DNA polymerase I and ligase, are described; exonuclease III probably works as an antiligase to facilitate the DNA repair.

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