ALKALINE ELUTION STUDIES OF HEMATOPORPHYRIN-DERIVATIVE PHOTOSENSITIZED DNA DAMAGE AND REPAIR IN CHINESE HAMSTER OVARY CELLS

Abstract We have used alkaline elution to study DNA damage produced by the photosensitizer hematoporphyrin derivative (HPD) in cultured Chinese hamster cells. Dosimetry was performed by measuring fluence and calculating photon absorption by intracellular HPD. HPD photosensitization causes DNA strand breakage. These breaks are repaired by the cell, although their fractional rate of repair is smaller than that for X-ray induced strand breaks at equivalent levels of strand breakage. The combined DNA polymerase inhibitors cytosine arabinoside and hydroxyurea suppress the repair of HPD-photosensitized breaks more strongly than they suppress repair of X-ray induced breaks. Addition of novobiocin to the aforementioned inhibitors causes almost total suppression of photosensitized break repair. A nucleotide excision repair system with inhibitor susceptibility similar to that of the system which removes pyrimidine dimers thus does not act upon HPD-photosensitized damage. The repair rate and inhibitor sensitivity findings together suggest biologically important differences in the chemical nature of X-ray induced and HPD-photosensitized strand breaks. In addition to strand breaks, HPD photosensitization produces covalent DNA-protein crosslinks, some of which persist through at least 90 min incubation, but which are repaired within 180 min.     

USE OF METABOLIC INHIBITORS TO INVESTIGATE THE EXCISION REPAIR OF PYRIMIDINE DIMERS AND NON-DIMER DNA DAMAGES INDUCED IN HUMAN AND ICR 2A FROG CELLS BY SOLAR ULTRAVIOLET RADIATION

Abstract— ICR 2A frog and normal human skin fibroblasts were exposed to either 5 J/m² of 254 nm UV or 50 kJ/m² of the Mylar-filtered solar UV wavelengths produced by a fluorescent sunlamp. Following these approximately equitoxic treatments, cells were incubated in medium containing the DNA synthesis inhibitors hydroxyurea (HU) and 1–β-D-arabinofuranosyl cytosine (ara C) for 0–20 min (human fibroblasts) or 0–4 h (frog cells) to accumulate DNA breaks resulting from enzymatic incision during excision repair. It was found that breaks were formed in human cells at about a 200-f-old higher rate compared with the ICR 2A cells indicating a relatively low capacity for excision repair in the frog cells. In addition, the rate of DNA break formation in solar UV-irradiated cells was only one-third of the level detected in 254 nm-irradiated cells. This result is consistent with the conclusion that the pathway(s) involved in the repair of solar UV-induced DNA damages differs from the repair of lesions produced in cells exposed to 254 nm UV.     

DNA DAMAGE AND REPAIR IN HUMAN CELLS EXPOSED TO SUNLIGHT

Cultured human cells were treated with direct sunlight under conditions which minimised the hypertonic, hyperthermic and fixative effects of solar radiation. Sunlight produced similar levels of DNA strand breaks as equitoxic 254 nm UV in two fibroblast strains and a melanoma cell line, but DNA repair synthesis and inhibition of semiconservative DNA synthesis and of DNA chain elongation were significantly less for sunlight-exposed cells. DNA breaks induced by sunlight were removed more rapidly. Thus, the repair of solar damage differs considerably from 254 nm UV repair. Glass-filtered sunlight (> 320 nm) was not toxic to cells and did not induce repair synthesis but gave a low level of short-lived DNA breaks and some inhibition of DNA chain elongation; thymidine uptake was enhanced. Filtered sunlight slightly enhanced UV-induced repair synthesis and UV toxicity; photoreactivation of UV damage was not found. Attempts to transform human fibroblasts using sunlight, with or without phorbol ester, were unsuccessful.     

Lack of correlation between DNA strand breakage and p53 protein levels in human fibroblast strains exposed to ultraviolet lights

The contribution of DNA strand breaks accumulating in the course of nucleotide excision repair to upregulation of the p53 tumor suppressor protein was investigated in human dermal fibroblast strains after treatment with 254 nm ultraviolet (UV) light. For this purpose, fibroblast cultures were exposed to UV and incubated for 3 h in the presence or absence of l-beta-D-arabinofuranosylcytosine (araC) and/or hydroxyurea (HU), and then assayed for DNA strand breakage and p53 protein levels. As expected from previous studies, incubation of normal and ataxia telangiectasia (AT) fibroblasts with araC and HU after UV irradiation resulted in an accumulation of DNA strand breaks. Such araC/HU-accumulated strand breaks (reflecting nonligated repair-incision events) following UV irradiation were not detected in xeroderma pigmentosum (XP) fibroblast strains belonging to complementation groups A and G. Western blot analysis revealed that normal fibroblasts exhibited little upregulation of p53 (approximately 1.2-fold) when incubated without araC after 5 J/m2 irradiation, but showed significant (three-fold) upregulation of p53 when incubated with araC after irradiation. AraC is known to inhibit nucleotide excision repair at both the damage removal and repair resynthesis steps. Therefore, the potentiation of UV-induced upregulation of p53 evoked by araC in normal cells may be a consequence of either persistent bulky DNA lesions or persistent incision-associated DNA strand breaks. To distinguish between these two possibilities, we determined p53 induction in AT fibroblasts (which do not upregulate p53 in response to DNA strand breakage) and in XP fibroblasts (which do not exhibit incision-associated breaks after UV irradiation). The p53 response after treatment with 5 J/m2 UV and incubation with araC was similar in AT, XPA, XPG and normal fibroblasts. In addition, exposure of XPA and XPG fibroblasts to UV (5, 10 or 20 J/m2) followed by incubation without araC resulted in a strong upregulation of p53. We further demonstrated that HU, an inhibitor of replicative DNA synthesis (but not of nucleotide excision repair), had no significant impact on p53 protein levels in UV irradiated and unirradiated human fibroblasts. We conclude that upregulation of p53 at early times after exposure of diploid human fibroblasts to UV light is triggered by persistent bulky DNA lesions, and that incision-associated DNA strand breaks accumulating in the course of nucleotide excision repair and breaks arising as a result of inhibition of DNA replication contribute little (if anything) to upregulation of p53.     

EFFECT OF POLYAMINE DEPLETION ON DNA DAMAGE and REPAIR FOLLOWING UV IRRADIATION OF HeLa CELLS

Treatment of HeLa cells with the polyamine biosynthesis inhibitors, methylglyoxal bis(guanylhydrazone) (MGBG), difluoromethylornithine (DFMO) or a combination of the two, resulted in reduction in cellular polyamine levels. Analysis of UV light-induced DNA damage and repair in these polyamine depleted cells revealed distinct differences in the repair process relative to that seen in cells possessing a normal polyamine complement. Initial yield of thymine dimers and rate of removal of these lesions from cellular DNA appeared normal in polyamine-depleted cells. However, depleted cells exhibited retarded sealing of DNA strand breaks resulting from cellular repair processes, reduced repair synthesis and an increased sensitivity to UV killing. Incision at damaged sites was not affected since ara-C repair-dependent breaks accumulated in a normal fashion. Molecular analysis of inhibited repair sites by exonuclease III and T4 DNA ligase probes suggest that the strand interruptions consist of gaps rather than ligatable nicks, consistent with an interpretation of the repair defect being at the gap-filling stage rather than the ligation step. Observed patterns of differential polyamine depletion by DFMO and MGBG, and partial reversal of repair inhibition by polyamine supplementation, suggests that polyamine depletion per se, rather than some secondary effect of inhibitor treatment, is responsible for the inhibition of repair.     

Only complete rejoining of DNA strand breaks after UVC allows K562 cell proliferation and DMSO induction of erythropoiesis

DNA strand breaks are early intermediates of the repair of UVC-induced DNA damage, however, since they severely impair cellular activities, their presence should be limited in time. In this study, the effects of incomplete repair of UVC-induced DNA strand breaks are investigated on K562 cell growth and the induction of erythroid differentiation by addition of DMSO to the cell culture medium. The kinetics were followed after UV irradiation by single cell gel electrophoresis, and in total cell population by alkaline or neutral agarose gel electrophoresis. Shortly after exposure, an extensive fragmentation occurred in DNA; DNA double strand breaks were negatively correlated with recovery time for DNA integrity. DNA damage induced by UVC 9J/m2 rapidly triggered necrosis in a large fraction of irradiated K562 cells, and only 40% of treated cells resumed growth at a very low rate within 24h of culture. The addition of DMSO to the culture medium of cells 15min after UVC, when DNA strand break repair was not yet complete, produced apoptosis in >70% of surviving cells, as determined by TUNEL assay. Conversely, if DMSO was added when the resealing of DNA strand breaks was complete, surviving K562 cells retained full growth capacity, and their progeny underwent erythroid differentiation with normal levels of erythroid proteins, delta-aminolevulinic acid dehydrase and hemoglobin. This study shows that the extent of DNA strand break repair influences cell proliferation and the DMSO induced erythroid program, and the same UVC dose can have opposite effects depending on cellular status.     

INDUCTION OF DNA-PROTEIN CROSS-LINKS IN CHINESE HAMSTER CELLS BY THE PHOTODYNAMIC ACTION OF CHLOROALUMINUM PHTHALOCYANINE AND VISIBLE LIGHT

Abstract— Chloroaluminum phthalocyanine (CAPC) is an efficient photosensitizer for the inactivation of Chinese hamster V79 cells. In order to investigate possible molecular mechanisms in the photo-dynamic action of CAPC and visible light, the induction and repair rate of two classes of DNA lesions have been determined, i.e. DNA single-strand breaks and DNA-protein cross-links. In cells pretreated with 1 μ.M CAPC, a fluence of 12 kJ/m² of red light (>600 nm) kills approximately 50% of the cells and induces 3 to 3.5 Gy-equivalents of single-strand breaks. The repair of these breaks was slower than the repair of single-strand breaks induced by -irradiation. The photodynamic action of CAPC also induces a large number of DNA-protein cross-links which, in contrast to -radiation-induced DNA-protein cross-links, do not appear to be repaired during 4 h of post-treatment incubation in fresh medium. These studies suggest that DNA may be an important target for the cytotoxicity of CAPC + red light.     

PHOTODYNAMIC DAMAGE AND ITS REPAIR IN Vibrio cholerae

Abstract— Repair of photodynamic damage induced by acriflavine and visible light has been examined in three strains of Vibrio cholerae differing in their capabilities to repair ultraviolet (UV) light induced DN A damage. Excision repair deficient wild type cells of strain 154 are more sensitive to photodynamic treatment compared to repair proficient cells of strain 569B. However, no difference in their capabilities to repair of damage following photodynamic treatment can be detected. No single-strand breaks in the irradiated cell DNA are observed when the cell survival is more than 10%. Single-strand breaks observed at cell survival less than 5% are not dark repairable even in excision repair proficient wild type cells. Repair of membrane damage can partially account for the recovery observed at low doses. In contrast, radiation-sensitive mutant 569B_s cells which lack both excision and medium-dependent dark repair for UV-lesions are most efficient in repairing damage induced by photodynamic treatment.     

IS DNA TOPOISOMERASE INVOLVED IN THE UV EXCISION REPAIR PROCESS? NEW EVIDENCE FROM STUDIES WITH DNA INTERCALATING AND NON-INTERCALATING ANTITUMOR AGENTS

Abstract— The effects of selected DNA intercalating and non-intercalating drugs on the UV excision repair process in human fibroblasts have been examined. 9-Amino acridine, acridine orange, quinacrine, doxorubicin (adriamycin), ethidium bromide and actinomycin-D all inhibited the removal of pyrimidine dimers from cellular DNA by inhibiting the incision process as monitored by the nick translation assay and by an endonuclease-sensitive site assay. These agents also partially inhibited incision by the M. luteus endonuclease in an in vitro system. This is the only class of compounds tested to date that appears to block this early step of repair in mammalian cells. The DNA topoisomerase inhibitors, m -amsacrine and VP-16 (etoposide) and the bacterial gyrase inhibitors nalidixic acid and oxolinic acid were shown not to inhibit UV repair. As shown previously, however, novobiocin does block dimer removal and we show here that it is a potent inhibitor of the M. luteus UV endonuclease. While it has recently been demonstrated that many DNA intercalating agents block the strand-passing activity of DNA topoisomerase II giving rise to protein associated DNA strand breaks, the finding that the specific inhibitors of topoisomerase, m -AMSA and VP-16, do not inhibit repair, even though they block this strand passing activity, strongly suggests that inhibition of DNA topoisomerase is not associated with inhibition of DNA repair.     

INDUCTION OF DNA STRAND BREAKS IN NORMAL HUMAN FIBROBLASTS EXPOSED TO MONOCHROMATIC ULTRAVIOLET AND VISIBLE WAVELENGTHS IN THE 240–546 nm RANGE

Abstract— The induction of DNA single-strand breaks in normal human fibroblasts exposed to monochromatic wavelengths from 240–546 nm was measured by the alkaline elution assay. The cells were irradiated at 1°C to prevent both repair of induced breaks and formation of enzymatically induced breaks through excision repair. The cultures were also washed with and irradiated while suspended in phosphate buffered saline to prevent the formation of DNA damaging photoproducts from medium components. The action spectrum for DNA strand breakage was found to exhibit one peak at 265 nm, consistent with DNA absorption, and a second peak at 450 nm. The normalized action spectrum in the visible is similar to the normalized absorption spectrum for riboflavin, a known photosensitizing agent, implicating this molecule as the absorbing chromophore.     

INHIBITION OF DNA REPAIR SYNTHESIS BY SUNLIGHT

Abstract— DNA repair synthesis as determined by thymidine incorporation in the presence of hydroxyurea reached a much lower maximum level after solar compared with UVC exposure in five human melanoma cell lines, in HeLa cells, and in two human fibroblast strains. This finding was confirmed by determination of unscheduled DNA synthesis where both the number of labelled nuclei and grain count per nucleus were lower in sun-exposed cells. In a cloned human melanoma line (MM253cl), glass-filtered sunlight inhibited UVC repair synthesis, and solar UVB alone induced a higher level of repair synthesis than either complete sun or solar UVA plus solar UVB. The fluence response of filtered sunlight for inhibition of UVB (sunlamp) and UVC showed that most inhibition was obtained at low fluences (5-10 min), further exposure giving a plateau at 40% of the original level. Ultraviolet C and sunlight inactivated adenovirus 5 giving F ₀ values for virus survival 40-fold higher than for cell survival. Replication of either UVC- or solar-irradiated virus was not affected by prior irradiation of cells with glass-filtered sunlight. Stathmokinetic analysis of cell cycle progression by DNA flow cytometry showed that UVC and sunlamp UVB retarded cell movement from the G1 and S phases whereas equitoxic sunlight and glass-filtered sunlight (nontoxic) had no effect. These results indicate that solar UVA at low environmental fluences partially inhibits UVB repair synthesis in a range of human cell types but does not affect the replication of a UVB- or UVC-damaged virus when applied to the genome alone or to the host cell.     

From green to red – To more dead? Autofluorescent proteins as photosensitizers

Inactive compounds like autofluorescent proteins can absorb visible daylight (around 500–700 nm) and can emit active electrons producing reactive oxygen species (ROS) leading to an increase in photokilling processes in bacteria. The endogenously originated ROS create single strand breaks in the cells DNA. These various types of breaks can be partially repaired by different cellular repair systems but a high number of breaks leads to cell death. A dramatic increase in cell killing can be observed from green, via yellow to red color emission. This was tested by colony forming ability. The generation of ROS and the bacterial protection mechanisms are discussed. We outline some possibilities for use the protein’s properties for treatment of antibiotic multi-resistant and difficult to treat bacteria like the methicillin-resistant Staphylococcus aureus (MRSA).     

LETHALITY AND THE INDUCTION AND REPAIR OF DNA DAMAGE IN FAR, MID OR NEAR UV-IRRADIATED HUMAN FIBROBLASTS: COMPARISON OF EFFECTS IN NORMAL, XERODERMA PIGMENTOSUM AND BLOOM'S SYNDROME CELLS

Abstract Using normal human fibroblasts we have determined the ability of far (254 nm), mid (310 nm) or near (365 nm) UV radiation to: (i) induce pyrimidine dimers (detected as UV endonuclease sensitive sites) and DNA single-strand breaks (detected in alkali); (ii) elicit excision repair, monitored as unscheduled DNA synthesis (UDS); and (iii) reduce colony-forming ability. Unscheduled DNA synthesis studies were also performed on dimer excision-defective xeroderma pigmentosum (XP) cells, and the survival studies were extended to include XP and Bloom's syndrome (BS) strains. UV-induced cell killing in normal, BS and XP cells was found to relate to an equivalent dimer load per genome after 254 or 310 nm exposure, whereas at 365 nm the lethal effects of non-dimer damage appeared to predominate. Lethality could not be correlated with DNA strand breakage at any wavelength. The two XP strains examined showed the same relative UDS repair deficiency at the two shorter wavelengths in keeping with a predominant role for pyrimidine dimer repair in the expression of UDS. However, UDS was not detected in 365 nm UV-irradiated normal and XP cells despite dimer induction; this effect was due to the inhibition of DNA repair functions since 365 nm UV-irradiated normal cells showed reduced capacity to perform UDS subsequent to challenge with 254 nm UV radiation.
In short, the near UV component of sunlight apparently induces biologically important non-dimer damage in human cells and inhibits DNA repair processes, two actions which should be considered when assessing the deleterious actions of solar UV.     

ACTION OF HYDROGEN PEROXIDE ON HUMAN FIBROBLAST IN CULTURE

Abstract— Human fibroblasts in culture lose the capacity of proliferating when exposed to hydrogen peroxide in the concentration range of 1 to 10 μ M . The toxicity of H₂O₂ to xeroderma pigmentosum cells (XP12RO). defective in excision repair of lesions produced by UV-irradiation, was about twice as high as to cells proficient in excision repair (VA13). This compound produces single-strand breaks in intracellular DNA but not in purified DNA. These breaks are in situ physical discontinuities rather than alkali-labile bonds, and their generation occurs at the same extent at 4°C and 37° indicating that they are not produced by an endonuclease. The results favor the hypothesis that H₂O₂ reacts in the cell producing a radical species which brings about the formation of DNA single-strand breaks. These breaks are effectively repaired by both XP12RO and VA13 fibroblasts. The possible reason for the lethality of H₂O₂ is discussed.     

Differential response of excision proficient and deficient L5178Y cells to UVC irradiation and benzamide

L5178Y-R and L5178Y-S cells differ in sensitivity to UVC radiation (D0 values: 2.8 and 9.0 J m-2 respectively, exposure in Fischer's medium). The UVC sensitivity is related to the excision repair ability. Benzamide (Bz), an inhibitor of adenosine diphosphoribosyl transferase (ADPRT), does not modify the lethal effect of UVC radiation in L5178Y-R cells, whereas it sensitizes L5178Y-S cells. The content of NAD+ after irradiation decreases only in the latter cells and this decrease can be prevented by 2 mM Bz treatment. In agreement with the survival data, in L5178Y-R cells neither the proportion of abnormal cells nor the frequency of chromatid aberration are affected by 2 mM Bz treatment, in contrast with L5178Y-S cells. Bz slightly reverses inhibition of 3H-thymidine incorporation only in L5178Y-S cells, but it does not affect the proportions of cells in the different phases of the cell cycle in either cell strain after UVC exposure. These data could be taken as an indirect indication of the involvement of ADPRT in DNA repair in UVC-irradiated L5178Y-S cells. However, the increase in the number of DNA strand breaks in UVC-exposed, Bz-treated cells compared with UVC-exposed untreated cells is the same in both L5178Y strains.     

THE REPAIR OF DNA STRAND BREAKS IN HUMAN LYMPHOCYTES EXPOSED TO NEAR UV-RADIATION (UVA) AND FAR UV-RADIATION (UVC)

Abstract— UVA irradiation of human lymphocytes induces DNA strand breaks and a portion of these breaks are closed at a slower rate than X-ray induced DNA strand breaks and the strand breaks generated during repair of UVC induced DNA lesions. In addition, the yield of DNA strand breaks in lymphocytes pretreated with UVA radiation and given a subsequent exposure with UVC radiation is higher and shows a slower decrease with increasing repair time in comparison with the expected yield based on additivity between UVA and UVC induced DNA strand breaks. This indicates that UVA delays the closure of the intermediate strand breaks formed in the repair process of UVC induced DNA lesions.     

SIRT1 promotes DNA repair activity and deacetylation of Ku70

Human SIRT1 controls various physiological responses including cell fate, stress, and aging, through deacetylation of its specific substrate protein. In processing DNA damage signaling, SIRT1 attenuates a cellular apoptotic response by deacetylation of p53 tumor suppressor. The present study shows that, upon exposure to radiation, SIRT1 could enhance DNA repair capacity and deacetylation of repair protein Ku70. Ectopically over-expressed SIRT1 resulted in the increase of repair of DNA strand breakages produced by radiation. On the other hand, repression of endogenous SIRT1 expression by SIRT1 siRNA led to the decrease of this repair activity, indicating that SIRT1 can regulate DNA repair capacity of cells with DNA strand breaks. In addition, we found that SIRT1 physically complexed with repair protein Ku70, leading to subsequent deacetylation. The dominant-negative SIRT1, a catalytically inactive form, did not induce deacetylation of Ku70 protein as well as increase of DNA repair capacity. These observations suggest that SIRT1 modulates DNA repair activity, which could be regulated by the acetylation status of repair protein Ku70 following DNA damage.     

HOECHST 33258 DYE GENERATES DNA-PROTEIN CROSS-LINKS DURING ULTRAVIOLET LIGHT-INDUCED PHOTOLYSIS OF BROMODEOXYURIDINE IN REPLICATED AND REPAIRED DNA

Abstract— Substitution of bromodeoxyuridine for thymidine in the DNA of mammalian cells sensitizes them to a range of wavelengths of ultraviolet light. Cells are also sensitized to photochemical reactions involving dyes such as Hoechst 33258, which is used to produce differential staining of chromatids according to their bromodeoxyuridine content. Irradiation with 313 nm light of human and hamster cells containing bromodeoxyuridine in their DNA produced single-strand breaks, detectable by alkaline elution, but no DNA-protein cross-links. Irradiation with 360 nm light in the presence of Hoechst 33258 produced extensive DNA-protein cross-linkage as well as single-strand breaks. These cross-links were observed in DNA containing bromodeoxyuridine incorporated by either semiconservative or repair replication, and may provide a method for identification of proteins in close proximity to replication forks or repair sites. When the protein was removed with proteinase K, bromodeoxyuridine in repair patches after irradiation by doses of ultraviolet (254 nm) light as low as 0.26 J/m² could readily be detected. Hoechst 33258-mediated photolysis, therefore, provides a sensitive new technique for measuring repair replication after ultraviolet light irradiation.     

IMPAIRED REPAIR OF UVC-INDUCED DNA DAMAGE IN L5178Y-R CELLS: DNA UNWINDING STUDIES WITH THE USE OF 1-β-D-ARABINOFURANOSYL CYTOSINE

Abstract— L5178Y-R (LY-R) and L5178Y-S (LY-S) cells differ in sensitivity to UVC radiation (D₀: 2.8 and 9.0 J/m²respectively, for cells exposed in Fischer's medium). We used these cells and a DNA unwinding technique in conjunction with 1-β-D-arabinosyl cytosine to determine DNA strand breaks accumulating as a result of enzymatic incision during DNA repair. Following UVC exposure DNA strand break accumulation was observed in LY-S cells but not in LY-R cells. The repair defect in LY-R cells is accompanied by a delayed recovery of [³H]thymidine incorporation.     

The use of silver-stained "comets" to visualize DNA damage and repair in normal and Xeroderma pigmentosum fibroblasts after exposure to simulated solar radiation

The alkaline and neutral comet assays have been widely used to assess DNA damage and repair in individual cells after in vivo or in vitro exposure to chemical or physical genotoxins. Cells processed under neutral conditions generate comets primarily from DNA double strand breaks, whereas under alkaline conditions, comets arise from DNA single and double strand breaks and alkali-labile lesions. A modified version of the alkaline comet assay, as described here, used silver stain to visualize the comets and a Gelbond base to facilitate the manipulation and processing of samples. To demonstrate how these modifications improve the assay, fibroblasts derived from both normal and Xeroderma pigmentosum (Xp) individuals were exposed to simulated solar radiation and the resulting DNA damage and repair evaluated and compared with results from the relevant literature. Comets from normal fibroblasts reached their maximum length at about an hour after irradiation. Dose-dependent increases in comet length were observed up to at least 360 mJ/cm2. In contrast, comet lengths from repair deficient Xp fibroblasts were shorter than normal cells reflecting their reduced capacity to generate single strand breaks by the excision of DNA dimers. For incubation times of more than 1 h, comet lengths from normal fibroblasts underwent a time-dependent decrease, supporting the contention that this change was related to the ligation step in the DNA repair process. These changes were compatible with the model of DNA damage and repair established by others for ultraviolet radiation.