Three homologs are all required for efficient repair of spontaneous DNA damage in Deinococcus radiodurans

is a bacterium that can survive extreme DNA damage. To understand the role of endonuclease III (Nth) in oxidative repair and mutagenesis, we constructed single, double and triple mutants. The mutants showed no significant difference with wild type in both IR resistance and H2O2 resistance. We characterized these strains with regard to mutation rates and mutation spectrum using the /Rif(r) system. The Rif(r) frequency of mutant MK1 (a-(3)) was twofold higher than that of wild type. The triple mutant of (ME3)generated a mutation frequency 34.4-fold, and a mutation rate 13.8-fold higher than the wild type. All strains demonstrated specific mutational hotspots. Each single mutant had higher spontaneous mutation frequency than wild type at base substitution (G:C -> A:T). The mutational response was further increased in the double and triple mutants. The higher mutation rate and mutational response in ME3 suggested that the three homologs had non-overlapped and overlapped substrate spectrum in endogenous oxidative DNA repair.