AN ENDONUCLEASE ACTIVITY OF ESCHERICHIA-COLI THAT SPECIFICALLY REMOVES 8-HYDROXYGUANINE RESIDUES FROM DNA

An enzyme that specifically removes an 8-hydroxyguanine (8-OH-Gua) residue in DNA has been purified from Escherichia coli. To assay the enzymatic activity, a synthetic double-stranded DNA (dsDNA) containing 8-OH-Gua at a defined position was used as a substrate. The substrate DNA was simultaneously cleaved at 2 sites, i.e., the phosphodiester bonds 5' and 3' to 8-OH-Gua, leaving a phosphate at each of the neighboring deoxynucleosides. The cleavage was observed only in dsDNA, but not with single-stranded DNA containing 8-OH-Gua. This enzyme showed almost no activity on DNAs containing other kinds of modified bases such as 8-hydroxyadenine, O6-methylguanine and N7-methylguanine. Also DNAs containing mismatches (A/G or C/T) were not cleaved. Studies on several other properties of this enzyme indicate that it differs from endonucleases previously isolated from E. coli, indicating that it is likely to be an endonuclease which specifically recognizes 8-OH-Gua in dsDNA.