Efficient Formation of the Tandem Thymine Glycol/8-Oxo-7,8-dihydroguanine Lesion in Isolated DNA and the Mutagenic and Cytotoxic Properties of the Tandem Lesions in Escherichia coli Cells

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Abstract
Reactive oxygen species can induce the formation of not only single-nucleobase lesions, which have been extensively studied, but also tandem lesions. Herein, we report a high frequency of formation of a type of tandem lesion, where two commonly observed oxidatively induced single-nucleobase lesions, that is, thymidine glycol (Tg) and 8-oxo-7,8-dihydro-2′-deoxyguanosine (8-oxodG), are vicinal to each other in calf thymus DNA upon exposure to Cu(II)/ascorbate along with H2O2 or γ-rays. We further explored how the tandem lesions perturb the efficiency and fidelity of DNA replication by assessing the replication products formed from the propagation, in Escherichia coli cells, of the single-stranded pYMV1 shuttle vectors containing two tandem lesions [5′-(8-oxodG)-Tg-3′ and 5′-Tg-(8-oxodG)-3′] or an isolated Tg or 8-oxodG. The bypass efficiencies for the two tandem lesions were approximately one-half of those for the two isolated single-nucleobase lesions. The presence of an adjacent Tg could lead to significant increases in G→T transversion at the 8-oxodG site as compared to that of a single 8-oxodG lesion; the frequencies of G→T mutation were approximately 18, 32, and 28% for 8-oxodG that is isolated, in 5′-(8-oxodG)-Tg-3′ and in 5′-Tg-(8-oxodG)-3′, respectively. Moreover, both pol IV and pol V are involved, in part, in bypassing the Tg, either present alone or as part of the tandem lesions, in E. coli cells. Together, our results support that complex lesions could exert greater cytotoxic and mutagenic effects than when the composing individual lesions are present alone.

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