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The OGG1 gene of Saccharomyces cerevisiae codes for a DNA glycosylase that excises 7,8-dihydro-8- oxoguanine (8-OxoG) and 2,6-diamino-4-hydroxy-5- N -methylformamidopyrimidine (Fapy) from damaged DNA. In this paper, we have analysed the substrate specificity and the catalytic mechanism of the Ogg1 protein acting on DNA subtrates containing 8-OxoG residues or apurinic/apyrimidinic (AP) sites. The Ogg1 protein displays a marked preference for DNA duplexes containing 8-OxoG placed opposite a cytosine, the rank order for excision of 8-OxoG and cleavage efficiencies being 8-OxoG/C >8-OxoG/T >>8-OxoG/G and 8-OxoG/A. The cleavage of the DNA strand implies the excision of 8-OxoG followed by abeta-elimination reaction at the 3'-side of the resulting AP site. The Ogg1 protein efficiently cleaves a DNA duplex where a preformed AP site is placed opposite a cytosine (AP/C). In contrast, AP/T, AP/A or AP/G substrates are incised with a very low efficiency. Furthermore, cleavage of 8-OxoG/C or AP/C substrates implies the formation of a reaction intermediate that is converted into a stable covalent adduct in the presence of sodium borohydre (NaBH4). Therefore, the Ogg1 protein is a eukaryotic DNA glycosylase/AP lyase. Sequence homology searches reveal that Ogg1 probably shares a common ancestor gene with the endonuclease III of Escherichia coli. A consensus sequence indicates a highly conserved lysine residue, K120 of endonuclease III or K241 of Ogg1, respectively. Mutations of K241 to Gln (K241Q) and Arg (K241R) have been obtained after site directed mutagenesis of OGG1. Mutation K241Q completely abolishes DNA glycosylase activity and covalent complex formation in the presence of NaBH4. However, the K241Q mutant still binds DNA duplexes containing 8-OxoG/C. In contrast, K241R mutation results in a catalytically active form of Ogg1. These results strongly suggest that the free amino group of Lys241 is involved in the catalytic mechanism of the Ogg1 protein.