In nature, organisms are exposed to chronic low-dose ultraviolet light (CLUV) as opposed to the acute high doses common to laboratory experiments. Analysis of the cellular response to acute high-dose exposure has delineated the importance of direct DNA repair by the nucleotide excision repair pathway1 and for checkpoint-induced cell cycle arrest in promoting cell survival2. Here we examine the response of yeast cells to CLUV and identify a key role for the RAD6–RAD18–RAD5 error-free postreplication repair (RAD6 error-free PRR) pathway3,4 in promoting cell growth and survival. We show that loss of the RAD6 error-free PRR pathway results in DNA-damage-checkpoint-induced G2 arrest in CLUV-exposed cells, whereas wild-type and nucleotide-excision-repair-deficient cells are largely unaffected. Cell cycle arrest in the absence of the RAD6 error-free PRR pathway was not caused by a repair defect or by the accumulation of ultraviolet-induced photoproducts. Notably, we observed increased replication protein A (RPA)– and Rad52–yellow fluorescent protein foci5 in the CLUV-exposed rad18Δ cells and demonstrated that Rad52-mediated homologous recombination is required for the viability of the rad18Δ cells after release from CLUV-induced G2 arrest. These and other data presented suggest that, in response to environmental levels of ultraviolet exposure, the RAD6 error-free PRR pathway promotes replication of damaged templates without the generation of extensive single-stranded DNA regions. Thus, the error-free PRR pathway is specifically important during chronic low-dose ultraviolet exposure to prevent counter-productive DNA checkpoint activation and allow cells to proliferate normally.
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We thank R. Rothstein and H. Araki for strains; T. Matsunaga for anti-thymine dimer monoclonal antibody (designated TDM2 antibody); and T. Ohya for technical assistance. This work was supported by the Sumitomo foundation, by Grants-in-Aid for Scientific Research from the Ministry of Education, Culture, Sports, Science and Technology of Japan, and by the MRC (UK) grant G0600233.
Author Contributions T.H. designed the study. T.H. and Y.K. performed the experiments. H.I. coordinated the study. T.H., H.I. and A.M.C. analysed data and wrote the paper.
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Hishida, T., Kubota, Y., Carr, A. et al. RAD6–RAD18–RAD5-pathway-dependent tolerance to chronic low-dose ultraviolet light. Nature 457, 612–615 (2009). https://doi.org/10.1038/nature07580
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