Excision of formamidopyrimidine lesions by endonucleases III and VIII is not a major DNA repair pathway in Escherichia coli.

Proper maintenance of the genome is of great importance. Consequently, damaged nucleotides are repaired through redundant pathways. We considered whether the genome is protected from formamidopyrimidine nucleosides (Fapy*dA, Fapy*dG) via a pathway distinct from the Escherichia coli guanine oxidation system. The formamidopyrimidines are produced in significant quantities in DNA as a result of oxidative stress and are efficiently excised by formamidopyrimidine DNA glycosylase. Previous reports suggest that the formamidopyrimidine nucleosides are substrates for endonucleases III and VIII, enzymes that are typically associated with pyrimidine lesion repair in E.coli. We investigated the possibility that Endo III and/or Endo VIII play a role in formamidopyrimidine nucleoside repair by examining Fapy*dA and Fapy*dG excision opposite all four native 2'-deoxyribonucleotides. Endo VIII excises both lesions more efficiently than does Endo III, but the enzymes exhibit similar selectivity with respect to their action on duplexes containing the formamidopyrimidines opposite native deoxyribonucleotides. Fapy*dA is removed more rapidly than Fapy*dG, and duplexes containing purine nucleotides opposite the lesions are superior substrates compared with those containing formamidopyrimidine-pyrimidine base pairs. This dependence upon opposing nucleotide indicates that Endo III and Endo VIII do not serve as back up enzymes to formamidopyrimidine DNA glycosylase in the repair of formamidopyrimidines. When considered in conjunction with cellular studies [J. O. Blaisdell, Z. Hatahet and S. S. Wallace (1999) J. Bacteriol., 181, 6396-6402], these results also suggest that Endo III and Endo VIII do not protect E.coli against possible mutations attributable to formamidopyrimidine lesions.