Release of 7-methyIguanine residues whose imidazole rings have been opened from damaged DNA by a DNA glycosylase from Escherichia coli

Abstract

Double-stranded DNA containing 7-methylguanine residues whose imidazole rings have been opened, i.e. 2, 6-diamino-4-hydroxy-5—N-methylformamidopyrimidine residues, may be prepared by treatment of DNA with dimethyl sulphate followed by prolonged incubation at pH 11.4. These substituted formamidopyrimidine residues are actively removed from DNA by a DNA glycosylase present in E.coli cell extracts. The enzyme shows no apparent cofactor requirement and has a molecular weight of about 30 000. The release of ring-opened 7-methylguanine residues is due to a previously unrecognized activity, different from the three known E.coli DNA glycosylases that release uracil, 3-methyladenine, and hypoxanthine from DNA. This enzyme may serve to repair a major secondary alkylation product in DNA. In addition, it may remove non-methylated purines, whose imidazole rings have been opened, from X-irradiated DNA.