Ectopic Expression of DNA Repair Enzymes Modulates Survival following Ultraviolet Irradiation Challenge

In Escherichia coli, the nucleotide excision repair (NER) pathway removes ultraviolet (UV) light-induced cyclobutane pyrimidine dimers (CPDs) and 6-4 dipyrimidine photoproducts (6-4 PPs). Activation of alternative repair pathways, such as base excision repair (BER) and nucleotide incision repair (NIR), is inoperative because this organism lacks both the necessary BER DNA glycosylase and NIR UV endonuclease to initiate repair of these lesions. To determine if initiation of either pathway would enhance survival to biologically-relevant UV irradiation, the BER and NIR pathways were activated by expression of Chlorella virus-1 pyrimidine dimer glycosylase (cv-pdg) and Schizosaccharomyces pombe UV endonuclease (UVDE), respectively. The substrate specificity of cv-pdg includes CPDs and ring-fragmented purines, 4,6-diamino-5-formamidopyrimidine and 2,6-diamino-4-hydroxy-5-formamidopyrimidine, but not 6-4 PPs. In contrast, while UVDE incises DNA containing CPDs and 6-4 PPs, it was not previously known if the substrate specificity of UVDE included DNA containing ring-fragmented purines. Mass spectrometry was used to establish that these oxidatively-induced lesions were not substrates for UVDE. Expression of either cv-pdg or UVDE in NER-deficient E. coli significantly enhanced survival following UVB irradiation, but not to the levels of wild type (WT) cells. Survival of NER-proficient, homologous recombination-deficient cells could also be significantly enhanced by expression of either enzyme, suggesting that in response to UVB exposure, interactions between NER and activated BER or NIR pathways could be additive. Further, expression of cv-pdg or UVDE in WT E. coli enhanced survival following solar-simulated light (SSL) exposures.