Bifunctional DNA N-glycosylase with associated apurinic/apyrimidinic (AP) lyase function that catalyzes the first step in base excision repair (BER), the primary repair pathway for the repair of oxidative DNA damage. The DNA N-glycosylase activity releases the damaged DNA base from DNA by cleaving the N-glycosidic bond, leaving an AP site. The AP-lyase activity cleaves the phosphodiester bond 3' to the AP site by a beta-elimination. Primarily recognizes and repairs oxidative base damage of pyrimidines, but also purine-derived lesions, alkylation damage and cytosine photoproducts generated by UV irradiation as well as abasic sites. Has also 8-oxoguanine DNA glycosylase activity. The AP lyase can incise AP sites opposite all four bases. May also play a role in the regulation of mtDNA copy number by introducing a double-stranded break (DSB) at the mtDNA replication origin ori5, initiating the rolling-circle mtDNA replication. 2'-deoxyribonucleotide-(2'-deoxyribose 5'-phosphate)-2'-deoxyribonucleotide-DNA = a 3'-end 2'-deoxyribonucleotide-(2,3-dehydro-2,3-deoxyribose 5'-phosphate)-DNA + a 5'-end 5'-monophospho-2'-deoxyribonucleoside-DNA + H(+) Relocalizes to organelles containing elevated oxidative DNA damage. By oxidizing agents. Monosumoylated. Sumoylation is associated with targeting of NTG1 to nuclei containing oxidative DNA damage. Greatly increases spontaneous and hydrogen peroxide-induced mutation frequency. Causes mitochondrial genome instability. Suppresses mitochondrial point mutation rates, frameshifts and recombination rates, probably because NTG1 can generate mutagenic intermediates in yeast mitochondrial DNA. Does not possess a consensus sequence for a C-terminal iron-sulfur center typical of all other endonuclease III homologs. Belongs to the Nth/MutY family.