(nei) Mutants of Escherichia coli K-12

The nth and nei genes of Escherichia coli affect the production of endonuclease III and endonuclease VIII, respectively, glycosylases/apurinic lyases that attack DNA damaged by oxidizing agents. Here, we provide evidence that oxidative lethal lesions are repaired by both endonuclease III and endonuclease VIII and that spontaneous mutagenic lesions are repaired mainly by endonuclease III. Endonuclease III (endo III) has an apurinic (AP) lyase ac-tivity at apurinic/apyrimidinic sites in DNA and has a DNA glycosylase activity that excises oxidative pyrimidine residues such as thymine glycol (2, 5, 8, 11, 14). Escherichia coli mutants defective in endo III (nth mutants) were found to exhibit nor-mal resistance to DNA-damaging agents such as hydrogen peroxide and X rays (7). Since thymine glycol has been shown to be a lethal lesion in vitro (6, 12, 13) and in vivo (10, 15, 17), and a mutagenic lesion in vitro (4), the possibility existed that alternative pathways for the repair of these damages are present in E. coli. The availability of nth mutants facilitated th