DNA repair and sequence context affect 1O2-induced mutagenesis in bacteria

Electronic excited molecular oxygen (singlet oxygen, O-1(2)) is known to damage DNA, yielding mutations. In this work, the mutagenicity induced by O-1(2) in a defined sequence of DNA was investigated after replication in Escherichia coli mutants deficient for nucleotide and base excision DNA repair pathways. For this purpose a plasmid containing a O-1(2)-damaged 14 base oligonucleotide was introduced into E. coli by transfection and mutations were screened by hybridization with an oligonucleotide with the original sequence. Mutagenesis was observed in all strains tested, but it was especially high in the BH20 (fpg), AYM57 (fpg mutY) and AYM84 (fpg mutY uvrC? strains. The frequency of mutants in the fpg mutY strain was higher than in the triple mutant fpg mutY uvrC, suggesting that activity of the UvrABC excinuclease can favor the mutagenesis of these lesions. Additionally, most of the mutations were G -->T and G -->C transversions, but this was dependent on the position of the guanine in the sequence and on repair deficiency in the host bacteria. Thus, the kind of repair and the mutagenesis associated with O-1(2)-induced DNA damage are linked to the context of the damaged sequence.