Influence of C5-Substituents on Repair of O4-Methyl Adducts of Pyrimidines by O6-Alkylguanine DNA Alkyltransferases

The DNA repair protein O-6-alkylguanine-DNA alkyltransferase (AGT), found in numerous organisms, can remove methyl groups from the O-6- and O-4-atoms of 2 '-deoxyguanosine and thymidine in DNA. AGT variants demonstrate different repair efficiencies towards these lesions. To understand the influence of C5 nucleobase substituents on O-4-methyl removal by AGTs, DNA duplexes containing 5-chloro-, 5-bromo- 5-iodo- and 5-trifluoromethyl-O-4-methyl-2 '-deoxyuridine were studied. UV thermal denaturation revealed a stability reduction of 11 degrees C for the O-4-methyl halogen series and 5-trifluoromethyl analog relative to their controls. For the 5-chloro analog efficient repair was observed by human and E.coli AGTs. For the larger halogens (5-bromo and 5-iodo) and 5-trifluoromethyl analog, human AGT showed moderate repair of the O-4-methyl adduct. E.coli OGT and Ada-C readily repaired most adducts with reduced efficiency for the larger groups, except C5-iodo. These results suggest electronic contributions and favourable interactions of the C5-halogens within the AGT active site contribute to efficient dealkylation.