Lethal and mutagenic effects of 8-methoxypsoralen-induced lesions on plasmid DNA

The genotoxic effect of 8-methoxypsoralen damages (monoadducts and crosslinks) on plasmid DNA was studied. pBR322 DNA was treated with several concentrations of 8-methoxypsoralen plus fixed UVA light irradiation. After transformation into E. coli cells with different repair capacities (uvrA, recA and wild-type), plasmid survival and mutagenesis in ampicillin- and tetracycline-resistant genes were analysed. Results showed that crosslinks were extremely lethal in all 3 strains; indeed, it seemed that they were not repaired even in proficient bacteria. Monoadducts were also found to be lethal although they were removed to some extent by the excision-repair pathway (uvrA-dependent). Damaged plasmid DNA appeared to induce mutagenic repair, but only in the wild-type strain. In order to study the influence of the SOS response on plasmid recovery, preirradiation of the host cells was also performed. Preirradiation of the uvrA or wild-type strains significantly increased plasmid recovery. Consistent with the expectations of SOS repair, no effect was observed in preirradiated recA cells. Plasmid recovery in the excision-deficient strain was mainly achieved by the mutagenic repair of some fraction of the lesions, probably monoadducts. The greatest increase in plasmid recovery was found in the wild-type strain. This likely involved the repair of monoadducts and some fraction of the crosslinks. We conclude that repair in preirradiated repair-proficient cells is carried out mainly by an error-free pathway, suggesting enhancement of the excision repair promoted by the induction of SOS functions.