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.

Near ultraviolet radiation (UVA and UVB) causes a formamidopyrimidine glycosylase-dependent increase in G to T transversions.

In contrast to far-UV (< 290 nm) DNA damage, a large fraction of the DNA damage caused by near-UV is oxygen-dependent, suggesting the involvement of reactive oxygen species (ROS). The oxidized base 8-oxo-7,8-dihydroguanine (GO) is characteristic of ROS-induced DNA damage and is removed by Fapy (formamidopyrimidine) glycosylase. We have recently shown that Escherichia coli strains deficient in Fapy glycosylase (fpg) are hypersensitive to the lethal effects of UVA but not far-UV (UVC), suggesting lesions recognized by this enzyme may be important premutagenic or lethal lesions generated by near-UV radiation. In this study, we have found that while the far-UV-induced mutation rates of Fapy-deficient and wild-type strains were similar, near-UV (UVA and UVB) was hypermutagenic to a Fapy-deficient strain, causing a dose-dependent increase in induced mutation relative to wild type (up to five-fold at 200 kJ/m2). Using a plasmid back mutation assay, the predominant near-UV-induced mutations in both wild-type and Fapy-deficient strains were found to be C-->T transitions and G -->T transversions. The former is probably due to replicative bypass of pyrimidine dimers or (6-4) photoproducts that are known to be generated by near-UV, whereas the latter may be due to mispairing of GO lesions with adenine during replication. Consistent with this, the frequency of near-UV-induced G-->T transversions was 16-fold higher in a Fapy-deficient strain than a wild-type strain.