Endonuclease III and endonuclease IV protect Escherichia coli from the lethal and mutagenic effects of near-UV irradiation.

In contrast to the DNA damage caused by far-UV (lambda < 290 nm), near-UV (290 < lambda < 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.