ACTION OF MITOCHONDRIAL-DNA POLYMERASE-GAMMA AT SITES OF BASE LOSS OROXIDATIVE DAMAGE

Mitochondrial DNA is subject to oxidative damage generating 7,8-dihydr o-8-oxo-2'-deoxyguanosine (8-oxo-dG) residues and to spontaneous or in duced base loss generating abasic sites, Synthetic oligonucleotides co ntaining these lesions were prepared and used as templates to determin e their effects on the action of Xenopus Laevis DNA polymerase gamma, An analogue of an abasic site in DNA, tetrahydrofuran, was found to in hibit elongation by DNA polymerase gamma, When the DNA polymerase was able to complete translesional synthesis, a dA residue was incorporate d opposite the abasic site, In contrast, elongation by DNA polymerase gamma was not inhibited by an 8-oxo-dG residue in the template strand. The polymerase inserted dA opposite 8-oxo-dG in approximately 27% of the extended products, The effects of these lesions on the 3' --> 5' e xonuclease proofreading activity of DNA polymerase gamma were also inv estigated, The 3' --> 5' exonuclease activity excised any of the four normal bases positioned opposite either a tetrahydrofuran residue or 8 -oxo-dG, suggesting that proofreading may not play a major role in avo iding misincorporation at abasic sites or 8-oxo-dG residues in the tem plate, Thus, both of these lesions have the prospect of causing high r ates of mutation during mtDNA replication.