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Repair of the ubiquitous mutagenic lesion 7,8-dihydro-8-oxoguanine (8-oxoG) is initiated in eukaryotes by DNA glycosylases/lyases, such as yeast Ogg1, that do not share significant sequence identity with their prokaryotic counterparts, typified by Escherichia coli MutM (Fpg) protein. The unexpected presence of a functional mutM orthologue in the model plant Arabidopsis thaliana has brought into question the existence of functional OGG1 orthologues in plants. We report here the cDNA cloning, expression and functional characterization of AtOGG1, an Arabidopsis thaliana gene widely expressed in different plant tissues which encodes a 40.3 kDa protein with significant sequence identity to yeast and human Ogg1 proteins. Purified AtOgg1 enzyme specifically cleaves duplex DNA containing an 8-OxoG:C mispair, and the repair reaction proceeds through an imine intermediate

Reactive oxygen species (ROS) are ubiquitous DNA-damaging agents, and the repair of oxidative DNA lesions is essential to prevent mutations and cell death. Escherichia coli endonuclease III is the prototype repair enzyme for removal of oxidized pyrimidines from DNA. A database homology search identified a genomic sequence in Arabidopsis thaliana encoding a predicted protein with sequence similarity to E. coli endonuclease III. We cloned, sequenced and expressed the corresponding cDNA, which encodes a 39.1 kDa protein containing several sequence motifs conserved in endonuclease III homologues, including an iron-sulfur cluster domain and critical residues at the active site. The protein, designated AtNTH1, was over-expressed in E. coli and purified to apparent homogeneity. AtNTH1 exhibits DNA-glycosylase activity on different types of DNA substrates with pyrimidine damage, being able to release both urea and thymine glycol from double-stranded polydeoxyribonucleotides. The enzyme also possesses an apurinic/apyrimidinic lyase activity on UV- and gamma-irradiated DNA substrates. The AtNTH1 gene contains 10 introns and 11 exons and is widely expressed in different plant tissues. Our results suggest that AtNTH1 is a structural and functional homologue of endonuclease III and probably plays a major role in plant defence against oxidative DNA damage.