Endonuclease III and endonuclease IV protect Escherichia coli from the lethal and mutagenic effects of near-UV irradiation David M. Serafini and Herb E. Schellhorn Abstract: In contrast to the DNA damage caused by far-UV (λ < 290 nm), near-UV (290 < λ < 400 nm) induced DNA damage is partially oxygen dependent, suggesting the involvement of reactive oxygen species. To test the hypothesis that enzymes that protect cells from oxidative DNA damage are also involved in preventing near-UV mediated DNA damage, isogenic strains deficient in one or more of exonuclease III (xthA), endonuclease IV (nfo), and endonuclease III (nth) were exposed to increasing levels of far-UV and near-UV. All strains, with the exception of the nth single mutant, were found to be hypersensitive to the lethal effects of near-UV relative to a wild-type strain. A triple mutant strain (nth nfo xthA) exhibited the greatest sensitivity to near-UV-mediated lethality. The triple mutant was more sensitive than the nfo xthA double mutant to the lethal effects of near-UV, but not far-UV. A forward mutation assay also revealed a significantly increased sensitivity for the triple mutant compared to the nfo xthA deficient strain in the presence of near-UV. However, the triple mutant was no more sensitive to the mutagenic effects of far-UV than a nfo xthA double mutant. These data suggest that exonuclease III, endonuclease IV, and endonuclease III are important in protection against near-UV-induced DNA damage. Key words: near-UV, UVA, DNA damage, DNA repair. Résumé : Contrairement au dommage causé à l’ADN par les rayons UV lointains (λ < 290 nm), le dommage relié aux UV rapprochés (290 < λ < 400 nm) est partiellement dépendant de l’oxygène, ce qui implique donc des espèces capables de réagir avec l’oxygène. Pour vérifier l’hypothèse voulant que les enzymes qui protègent les cellules d’un endommagement de l’ADN par oxydation aient aussi un rôle de protection contre les dommages causé par les UV rapprochés, des souches isogéniques privées d’une ou de plus d’une des enzymes exonucléase III (xthA), endonucléase IV (nfo) et endonucléase III (nth) ont été exposées à des doses croissantes de rayons UV lointains et rapprochés. Toutes les souches, à l’exception d’un mutant nth se sont montrées hypersensibles à l’effet létal de l’UV rapproché comparativement à une souche de type sauvage. La plus grande sensibilité à l’effet létal des rayons UV rapprochés a été observé chez un triple mutant (nth nfa xthA). Ce triple mutant était plus sensible qu’un double mutant nfo xthA à l’effet létal des UV rapprochés mais pas des UV lointains. Une expérience antérieure de mutation a aussi révélé une augmentation significative de sensibilité chez le triple mutant comparativement à la souche déficiente nfo xthA en présence d’un rayonnement UV rapproché. Le triple mutant était par contre plus sensible aux effets mutagènes de l’UV lointain que le double mutant nfo xthA. Ces résultats confirment que les enzymes exonucléase III, endonucléase IV et endonucléase III sont importantes dans la protection de l’ADN contre les dommages causés par les rayons UV rapprochés. Mots clés : UV rapproché, UVA, endommagement de l’ADN, réparation de l’ADN. [Traduit par la Rédaction] Notes 637 Solar ultraviolet radiation is comprised of three parts. UVC, or far-UV, is the designation for wavelengths between 100 and 290 nm. Longer wavelengths are classified as either UVB (290 < λ < 320 nm) or UVA (320 < λ < 400 nm), which are collectively referred to as near-UV. Since far-UV is effectively filtered out by stratospheric ozone, wave- lengths in the near-UV range are probably of greater biolog- ical relevance. Recent declines in the levels of protective ozone, which can be attributed to the use of fossil fuels and chlorofluorocarbon-containing products, have resulted in an increase in surface near-UV (Kerr and McElroy 1993). Near-UV can, like far-UV, induce the formation of both cyclobutane pyrimidine dimers and pyrimidine-pyrimidone (6-4) photoproducts (Matsunaga et al. 1991). However, near- UV is distinct from far-UV in that much of the DNA dam- age induced by the former is oxygen dependent and similar to that caused by oxidative insult (Hattori-Nakakuki et al. 1994). Enzymes known to be important in the repair of far- UV and oxidative DNA damage may, therefore, also be part of the cell’s near-UV repair repertoire. Consistent with this idea, UvrABC excinuclease and formamidopyrimidine Can. J. Microbiol. 45: 632–637 (1999) © 1999 NRC Canada 632 Received October 14, 1998. Revision received March 9, 1999. Accepted March 10, 1999. D.M. Serafini and H.E. Schellhorn. 1 Department of Biology, McMaster University, Hamilton, ON L8S 4K1, Canada. 1 Author to whom all correspondence should be addressed (e-mail: schell@mcmaster.ca).