Genetic evidence for the nature, and excision repair, of DNA lesions resulting from incorporation of 5-bromouracil

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Abstract
Escherichia coli mutants defective in DNA uracil N-glycosidase (ung −) or endonuclease VI active against apurinic/apyrimidinic sites in DNA (xthA−) exhibit enhanced sensitivity towards 5-bromodeoxyuridine relative to the wild type strain, pointing to involvement of these enzymes in repair of bromouracil-induced lesions in DNA. Mutants defective in DNA polymerase I, either in polymerizing activity (polAl−) or (5′→3′)-exonuclease activity (polA107−) exhibit unusually high sensitivity (including marked lethality) in the presence of 5-bromodeoxyuridine. The results indicate that DNA polymerase I, and its associated (5′–3′)-exonuclease activity, are involved in repair of bromouracil-induced lesions and are not readily replaced, if at all, by DNA polymerases II and III. Thermosensitive mutant in DNA ligase gene (lig ts7) shows high sensitivity towards 5-bromodeoxyuridine at 42°C indicating the role of the enzyme in repair of bromouracil-induced lesions in DNA. Involvement of DNA uracil N-glycosidase, and endonuclease active against apurinic/apyrimidinic sites in recognition and repair of 5-bromouracil-induced damage permits of some inferences regarding the nature of this damage (lesions), in particular dehalogenation of incorporated bromouracil to uracil residues.