Physiological and genetic effects of puromycin aminonucleoside on ultraviolet-irradiated Escherichia coli strains.

Puromycin aminonucleoside (PAN) increased significantly the mutation rate of Escherichia coli B/r strains when used in conjunction with certain ultraviolet dosages. PAN (2.5 mM) when added to the post-irradiation medium of hcr+ cells slowed down RNA synthesis to 65%, protein to 76% and DNA to 48% of the control rate. Purine ribosides such as adenosine decreased the inhibitory action of PAN on DNA, RNA and protein synthesis. Quantitatively quite different results were obtained with the hcr- strains. PAN did not increase killing of UV, but decreased the frequency of UV-induced mutations. Antimutagenic purine ribosides decreased the synergistic mutagenic activity of PAN. Increases in DNA synthesis in the presence of antimutagens correspond to reductions in the rate of mutation to streptomycin resistance. The excision of UV-induced pyrimidine dimers was investigated in the presence and absence of PAN. The pattern of repair-inhibition reversion of pre-mutagenic lesions by adenosine suggests that PAN behaves as a feedback inhibitor of purine biosynthesis in UV-irradiated cells. It is probable that this inhibition results in an impairment of repair which produces the increase in mutant numbers.     

Interactions between yeast photolyase and nucleotide excision repair proteins in Saccharomyces cerevisiae and Escherichia coli.

The PHR1 gene of Saccharomyces cerevisiae encodes a DNA photolyase that catalyzes the light-dependent repair of pyrimidine dimers. In the absence of photoreactivating light, this enzyme binds to pyrimidine dimers but is unable to repair them. We have assessed the effect of bound photolyase on the dark survival of yeast cells carrying mutations in genes that eliminate either nucleotide excision repair (RAD2) or mutagenic repair (RAD18). We found that a functional PHR1 gene enhanced dark survival in a rad18 background but failed to do so in a rad2 or rad2 rad18 background and therefore conclude that photolyase stimulates specifically nucleotide excision repair of dimers in S. cerevisiae. This effect is similar to the effect of Escherichia coli photolyase on excision repair in the bacterium. However, despite the functional and structural similarities between yeast photolyase and the E. coli enzyme and complementation of the photoreactivation deficiency of E. coli phr mutants by PHR1, yeast photolyase failed to enhance excision repair in the bacterium. Instead, Phr1 was found to be a potent inhibitor of dark repair in recA strains but had no effect in uvrA strains. The results of in vitro experiments indicate that inhibition of nucleotide excision repair results from competition between yeast photolyase and ABC excision nuclease for binding at pyrimidine dimers. In addition, the A and B subunits of the excision nuclease, when allowed to bind to dimers before photolyase, suppressed photoreactivation by Phr1. We propose that enhancement of nucleotide excision repair by photolyases is a general phenomenon and that photolyase should be considered an accessory protein in this pathway.     

Role of pyrimidine dimer excision in loss of potential streptomycin resistance mutations of ultraviolet-irradiated Escherichia coli on phosphate-buffered agar.

The frequency of ultraviolet (UV)-induced mutations to streptomycin resistance dropped rapidly when starved Escherichia coli strains WP-2 B/r and B/r T- were incubated on phosphate-buffered agar (PBA), but was reduced only slightly in a WP-2 hcr- mutant. During postirradiation, incubation viability remained approximately constant. Cells given an optimal recovery treatment with photo-reactivating light showed no further recovery if subsequently incubated on PBA. At least 70% of the mutations induced to streptomycin resistance by UV could be repaired. The loss of potential streptomycin-resistant mutants was markedly reduced in strain B/r T- when 5 mug of acriflavin or 700 mug of caffeine per ml was added to PBA. The excision of UV-induced thymine-containing dimers from E. coli tb/r T- was investigated. Dimer excision progressed more slowly when the cells were incubated on PBA containing acriflavin or caffeine. There was no congruity between the kinetics of dimer excision and the kinetics of mutant loss. Our results indicate that removal of potential streptomycin-resistant mutants is considerably faster than the excision of pyrimidine dimers.     

UV mutagenesis in E. coli with excision repair initiated by uvrABC or denV gene products

Mutation frequency responses produced by ultraviolet light are compared in 4 closely related strains of E. coli B/r having the same tyr(Oc) allele and different excision-repair capabilities: uvr+ (excision repair initiated by wild-type UvrABC activity), uvrA (excision repair defective), uvrA/pdenV-7 (excision repair initiated by endonuclease V of bacteriophage T4, DenV activity), and uvr+/pdenV-7 (excision repair initiated by UvrABC and DenV activities). The production of Tyr+ prototrophic mutants is classified into back-mutations and de novo or converted glutamine tRNA suppressor mutations to indicate different mutation events. Cells transformed with the plasmid pdenV-7 require larger exposures than the parent strains to produce comparable mutation frequency responses, indicating that DenV activity can repair mutagenic photoproducts. When damage reduction by UvrABC or DenV is compared for each of the specific categories of mutation, the results are consistent with the idea that pyrimidine dimers infrequently or never target back-mutations of this allele, frequently target the de novo suppressor mutations, and extensively or exclusively target the converted suppressor mutations. This analysis is based on the distinction that UvrABC-initiated excision repair recognizes dimer and non-dimer (pyrimidine (6-4) pyrimidone) photoproducts but that DenV-initiated repair recognizes only pyrimidine dimers.     

Thermal resistance to photoreactivation of specific mutations potentiated in E. coli B/r ung by ultraviolet light

Summary Mutagenesis by ultraviolet light was studied in a strain of E. coli ung, which lacks uracil-DNA glycosylase activity. Mutation potentiated by UV in cells already induced by nalidixic acid treatment was still photoreversible suggesting that pyrimidine dimers act directly as premutational photoproducts. Secondly, irradiated cells were held in buffer at 48°C for 0 to 135 min to allow for deamination of cytosines in pyrimidine dimers. The mutation frequencies for class 2 de novo suppressor mutation, for class 2 converted suppressor mutation and for backmutation were individually determined, before and after photoreactivation, as a function of this thermal treatment. Backmutation remained sensitive to photoreactivation throughout the treatment but de novo and converted suppressor mutations rapidly developed resistance to photoreactivation. This resistance was not seen in an ung ⁺ control. A model is proposed to account for the selective resistance based on the hypothesis that class 2 de novo and converted suppressor mutations normally result from UV by GC to AT transitions at T=C dimers. The model describes deamination of the cytosine residues in these dimers to become uracil residues. In consequence, monomerization by photoreactivation in cells that can not repair uracils in DNA no longer reverses mutation and GC to AT transitions are established at the sites of uracils.     

Mapping of the genes controlling excision repair of Pyrimidine photoproducts in Bacillus subtilis

Summary Twenty nine uvr mutations of Bacillus subtilis Marburg have been classified into two groups by mutual transformation crosses. One, uvrA, contained 20 mutations and involved uvrAl which had been isolated by Reiter and Strauss (1965) and linked to the hisA gene by Hoch and Anagnostopoulos (1970). Another group, uvrB, contained 9 mutations and the linkage relationship has been established as argA-polA-uvrB-leu-pheA. Both of them seemed to govern the incision reaction upon DNA containing pyrimidine dimers or spore photoproducts. It thus seems that in B. subtilis, as in Escherichia coli, the process of excision repair is controlled by the products of two or more genes not closely linked in the chromosome.     

Exposure to low dose of gamma radiation enhances the excision repair in Saccharomyces cerevisiae

The effect of low doses of ionizing and nonionizing radiation on the radiation response of yeast Saccharomyces cerevisiae toward ionizing and nonionizing radiation was studied. The wild-type strain D273-10B on exposure to 54 Gy gamma radiation (resulting in about 10% cell killing) showed enhanced resistance to subsequent exposure to UV radiation. This induced UV resistance increased with the incubation time between the initial gamma radiation stress and the UV irradiation. Exposure to low doses of UV light on the other hand showed no change in gamma or UV radiation response of this strain. The strains carrying a mutation at rad52 behaved in a way similar to the wild type, but with slightly reduced induced response. In contrast to this, the rad3 mutants, defective in excision repair, showed no induced UV resistance. Removal of UV-induced pyrimidine dimers in wild-type yeast DNA after UV irradiation was examined by analyzing the sites recognized by UV endonuclease from Micrococcus luteus. The samples that were exposed to low doses of gamma radiation before UV irradiation were able to repair the pyrimidine dimers more efficiently than the samples in which low gamma irradiation was omitted. The nature of enhanced repair was studied by scoring the frequency of induced gene conversion and reverse mutation at trp and ilv loci respectively in strain D7, which showed similar enhanced UV resistance induced by low-dose gamma irradiation. The induced repair was found to be essentially error-free. These results suggest that irradiation of strain D273-10B with low doses of gamma radiation enhances its capability for excision repair of UV-induced pyrimidine dimers.     

Repair inhibition of potential mutations of ultraviolet-irradiatedEscherichia coli by chloroquine and quinacrine

The frequency of ultraviolet(UV)-induced mutations drops rapidly whenEscherichia coli Hcr⁺ cells (strains WP-2 Hcr⁺; B/r) are incubated on phosphate-buffered agar (PBA), but is reduced only slightly if chloroquine or quinacrine are incorporated into the medium. The excision-deficient WP-2 Hcr^– strain shows little reduction in the number of mutants when incubated on PBA. During postirradiation incubation on PBA, cell viability was relatively unaffected by the presence of the chemicals in the PBA (25 g/ml quinacrine; 50 g/ml chloroquine). When cells were given optimal doses of photoreactivating light, no further decline in mutations was obtained during subsequent incubation on PBA. Approximately 64% of the mutants seen when cells are treated with UV-PBA-chloroquine and 90% seen with UV-PBA-quinacrine can be repaired if cells are incubated on PBA. When these chemicals were added to the PBA, both excision-proficient strains (WP-2 Hcr⁺; B/r) demonstrated a marked reduction in the repair of UV-induced mutations to streptomycin resistance. Our results indicate that these chemicals interfere with the excision of UV-induced pyrimidine dimers, a process that normally occurs during postirradiation incubation on PBA.     

Determination of pyrimidine dimers in Escherichia coli and Cryptosporidium parvum during UV light inactivation, photoreactivation, and dark repair

UV inactivation, photoreactivation, and dark repair of Escherichia coli and Cryptosporidium parvum were investigated with the endonuclease sensitive site (ESS) assay, which can determine UV-induced pyrimidine dimers in the genomic DNA of microorganisms. In a 99.9% inactivation of E. coli, high correlation was observed between the dose of UV irradiation and the number of pyrimidine dimers induced in the DNA of E. coli. The colony-forming ability of E. coli also correlated highly with the number of pyrimidine dimers in the DNA, indicating that the ESS assay is comparable to the method conventionally used to measure colony-forming ability. When E. coli were exposed to fluorescent light after a 99.9% inactivation by UV irradiation, UV-induced pyrimidine dimers in the DNA were continuously repaired and the colony-forming ability recovered gradually. When kept in darkness after the UV inactivation, however, E. coli showed neither repair of pyrimidine dimers nor recovery of colony-forming ability. When C. parvum were exposed to fluorescent light after UV inactivation, UV-induced pyrimidine dimers in the DNA were continuously repaired, while no recovery of animal infectivity was observed. When kept in darkness after UV inactivation, C. parvum also showed no recovery of infectivity in spite of the repair of pyrimidine dimers. It was suggested, therefore, that the infectivity of C. parvum would not recover either by photoreactivation or by dark repair even after the repair of pyrimidine dimers in the genomic DNA.     

Deamination of 5-methylcytosines within cyclobutane pyrimidine dimers is an important component of UVB mutagenesis

UVB mutagenesis is characterized by an abundance of C --> T and 5-methylcytosine --> T transitions at dipyrimidine sequences. It is not known how these mutations might arise. One hypothesis is that UV-induced mutations occur only after deamination of the cytosine or 5-methylcytosine within the pyrimidine dimer. It is not clear how methylation of cytosines at the 5-position influences deamination and how this affects mutagenesis. We have now conducted experiments with a CpG-methylated supF shuttle vector that was irradiated with UVB and then incubated at 37 degrees C to allow time for deamination before passage through a human cell line to establish mutations. This led to a significantly increased frequency of CC --> TT mutations and of transition mutations at 5'-PymCG-3' sequences. A spectrum of deaminated cis-syn cyclobutane pyrimidine dimers in the supF gene was determined using the mismatch glycosylase activities of MBD4 protein in combination with ligation-mediated PCR. Methylation at the C-5 position promoted the deamination of cytosines within cis-syn cyclobutane pyrimidine dimers, and these two events combined led to a significantly increased frequency of UVB-induced transition mutations at 5'-PymCG-3' sequences. Under these conditions, the majority of all supF mutations were transition mutations at 5'-PymCG-3', and they clustered at several mutational hot spots. Exactly these types of mutations are frequently observed in the p53 gene of nonmelanoma skin tumors. This particular mutagenic pathway may become prevalent under conditions of inefficient DNA repair and slow proliferation of cells in the human epidermis.     

Photoreactivation of UV induced cell killing, chromosome aberrations, sister chromatid exchanges, mutations and pyrimidine dimers in Xenopus laevis fibroblasts

Summary Fibroblasts from Xenopus laevis, which possess photoreactivating enzyme were used to study the influence of photoreactivating light on the frequency of pyrimidine dimers in DNA, chromosomal aberrations, sister chromatid exchanges, cell killing and the induction of gene mutations (ouabain-resistance) induced by 254 nm ultraviolet irradiation. The frequency of all biological endpoints studied were reduced following exposure to photoreactivating light parallel to the reduction in the frequencies of pyrimidine dimers (determined as endonuclease sensitive sites). However there was not always an absolute quantitative relationship between the reduction in the frequency of pyrimidine dimers and the reduction in the biological effects. This probably reflects a fast fixation process for the biological effects prior to removal of the dimers by photoreactivation.Abbreviations UV ultraviolet - PR photoreactivating - ESS endonuclease sensitive site - SCE sister chromatid exchanges - BrdUrd 5-brothodeoxyuridine     

DNA Repair in Potorous tridactylus

The DNA synthesized shortly after ultraviolet (UV) irradiation of Potorous tridactylis (PtK) cells sediments more slowly in alkali than that made by nonirradiated cells. The size of the single-strand segments is approximately equal to the average distance between 1 or 2 cyclobutyl pyrimidine dimers in the parental DNA. These data support the notion that dimers are the photoproducts which interrupt normal DNA replication. Upon incubation of irradiated cells the small segments are enlarged to form high molecular weight DNA as in nonirradiated cells. DNA synthesized at long times (~ 24 h) after irradiation is made in segments approximately equal to those synthesized by nonirradiated cells, although only 10-15% of the dimers have been removed by excision repair. These data imply that dimers are not the lesions which initially interrupt normal DNA replication in irradiated cells. In an attempt to resolve these conflicting interpretations, PtK cells were exposed to photoreactivating light after irradiation and before pulse-labeling, since photoreactivation repair is specific for only one type of UV lesion. After 1 h of exposure ~ 35% of the pyrimidine dimers have been monomerized, and the reduction in the percentage of dimers correlates with an increased size for the DNA synthesized by irradiated cells. Therefore, we conclude that the dimers are the lesions which initially interrupt DNA replication in irradiated PtK cells. The monomerization of pyrimidine dimers correlates with a disappearance of repair endonuclease-sensitive sites, as measured in vivo immediately after 1 h of photoreactivation, indicating that some of the sites sensitive to the repair endonuclease (from Micrococcus luteus) are pyrimidine dimers. However, at 24 h after irradiation and 1 h of photoreactivation there are no endonuclease-sensitive sites, even though ~ 50% of the pyrimidine dimers remain in the DNA. These data indicate that not all pyrimidine dimers are accessible to the repair endonuclease. The observation that at long times after irradiation DNA is made in segments equal to those synthesized by nonirradiated cells although only a small percentage of the dimers have been removed suggests that an additional repair system alters dimers so that they no longer interrupt DNA replication.     

Proximity of repair patches to persistent pyrimidine dimers in DNA of normal human and xeroderma pigmentosum cells

The proximity of repair patches to persistent pyrimidine dimers in normal human cells and xeroderma pigmentosum group C and D cells was analyzed by sequential digestion of repaired DNA with Micrococcus luteus UV-endonuclease and Escherichia coli DNA polymerase I. Although this enzymatic digestion removed one-third of the pyrimidine dimers, less than 3% of the label associated with repair patches and a similar amount of uniformly labeled DNA were removed. The repair patches therefore appear to be similarly distant from persistent dimers in all cell types, and, in particular, are not adjacent to unexcised dimers in xeroderma pigmentosum group D cells. A previous model that suggested that patches are inserted adjacent to dimers in xeroderma pigmentosum group D cells receives no support from these results.     

Thermal resistance to photoreactivation of ultraviolet light induced mutations in the lacI gene of E. coli ung

Ultraviolet light (UV) induced mutations in the lacI gene of Escherichia coli are thought to be targeted by DNA photoproducts. A number of reports suggest that both cyclobutyl pyrimidine dimers and pyrimidine (6−4) pyrimidone photoproducts may be involved. To investigate the potential contribution of each of these DNA photoproducts to mutagenesis in the lacI gene, we held UV-irradiated cells at a temperature of 44°C for 75 min and then exposed them to photoreactivating light (PR). This protocol is expected to preferentially deaminate specifically those cytosines that are contained in cyclobutyl dimers and subsequently monomerize the dimers to yield uracils in the DNA. In a strain deficient for uracil-DNA glycosylase (Ung⁻), these uracils would not be removed and a G : C → A : T transition would result at the site of the dimer. This protocol resulted in the enhancement of amber nonsense mutations that result from transitions at potential cytosine-containing dimer sites. The enhanced mutation frequencies resulting from this procedure were used to estimate the probability of dimer formation at the individual sites. A comparison of the dimer distribution with the mutation frequencies following UV alone suggests that both cyclobutyl dimers and (6−4) photoproducts contribute to UV-mutagenesis in the lacI gene. In addition, we conclude that the frequency of mutation at any particular site not only reflects the occurrence of DNA damage, but also the action of metabolic processes that are responsible for DNA repair and mutagenesis.     

Determination of Pyrimidine Dimers in Escherichia coli and Cryptosporidium parvum during UV Light Inactivation, Photoreactivation, and Dark Repair

UV inactivation, photoreactivation, and dark repair of Escherichia coli and Cryptosporidium parvum were investigated with the endonuclease sensitive site (ESS) assay, which can determine UV-induced pyrimidine dimers in the genomic DNA of microorganisms. In a 99.9% inactivation of E. coli, high correlation was observed between the dose of UV irradiation and the number of pyrimidine dimers induced in the DNA of E. coli. The colony-forming ability of E. coli also correlated highly with the number of pyrimidine dimers in the DNA, indicating that the ESS assay is comparable to the method conventionally used to measure colony-forming ability. When E. coli were exposed to fluorescent light after a 99.9% inactivation by UV irradiation, UV-induced pyrimidine dimers in the DNA were continuously repaired and the colony-forming ability recovered gradually. When kept in darkness after the UV inactivation, however, E. coli showed neither repair of pyrimidine dimers nor recovery of colony-forming ability. When C. parvum were exposed to fluorescent light after UV inactivation, UV-induced pyrimidine dimers in the DNA were continuously repaired, while no recovery of animal infectivity was observed. When kept in darkness after UV inactivation, C. parvum also showed no recovery of infectivity in spite of the repair of pyrimidine dimers. It was suggested, therefore, that the infectivity of C. parvum would not recover either by photoreactivation or by dark repair even after the repair of pyrimidine dimers in the genomic DNA.     

Mutagenic DNA repair in Escherichia coli

Summary The photoreversibility of UV-induced mutations to Trp⁺ in strain Escherichia coli WP2 uvrA trp (unable to excise pyrimidine dimers) was lost at different rates during incubation in different media. In Casamino acids medium after a short initial lag, photoreversibility was lost over about one generation time; in minimal medium with tryptophan, photoreversibility persisted for more than two generations; in Casamino acids medium with pantoyl lactone photoreversibility was lost extremely slowly. The rate of loss of photoreversibility was unaffected by UV dose in either Casamino acids medium or in minimal medium. The same eventual number of induced mutants was obtained when cells were incubated for two generations in any of the three media before being transferred to selective plates supplemented with Casamino acids. Thus in each the proportion of cells capable of giving rise to a mutant was the same and only the rate at which these cells did so during post-irradiation growth varied, suggesting that there might be a specific fraction of pyrimidine dimers at a given site capable of initiating a mutagenic repair event, and that the size of this fraction is dose dependent. Segregation experiments have shown that error-prone repair appears to occur once only and is not repeated in subsequent replication cycles, in contrast to (presumed error-free) recombination repair.The results are discussed in the light of current models of UV mutagenesis.     

Cooler temperatures slow the repair of DNA damage in tadpoles exposed to ultraviolet radiation: Implications for amphibian declines at high altitude

Ultraviolet B radiation (UVBR) damages the DNA of exposed cells, causing dimers to form between adjacent pyrimidine nucleotides. These dimers block DNA replication, causing mutations and apoptosis. Most organisms utilize biochemical or biophysical DNA repair strategies to restore DNA structure; however, as with most biological reactions, these processes are likely to be thermally sensitive. Tadpoles exposed to elevated UVBR at low environmental temperatures have significantly higher rates of mortality and developmental deformities compared with tadpoles exposed to the same levels of UVBR at higher environmental temperatures. We hypothesized that low environmental temperatures impair the primary enzymatic (photolyase) DNA repair pathway in amphibians, leading to the accumulation of DNA damage. To test this hypothesis, we compared DNA repair rates and photolyase gene expression patterns in Limnodynastes peronii. Tadpoles were acutely exposed to UVBR for 1 hr at either 20 or 30°C, and we measured DNA damage and photolyase expression levels at intervals following this exposure. Temperature had a significant effect on the rate of DNA repair, with repair at 30°C occurring twice as fast as repair at 20°C. Photolyase gene expression (6‐4 PP and CPD) was significantly upregulated by UVBR exposure, with expression levels increasing within 6 hr of UVBR exposure. CPD expression levels were not significantly affected by temperature, but 6‐4 PP expression was significantly higher in tadpoles in the 30°C treatment within 12 hr of UVBR exposure. These data support the hypothesis that DNA repair rates are thermally sensitive in tadpoles and may explain why enigmatic amphibian declines are higher in montane regions where UVBR levels are naturally elevated and environmental temperatures are lower.     

Repair-defective mutants of Alteromonas espejiana, the host for bacteriophage PM2

The in vivo repair processes of Alteromonas espejiana, the host for bacteriophage PM2, were characterized, and UV- and methyl methanesulfonate (MMS)-sensitive mutants were isolated. Wild-type A. espejiana cells were capable of photoreactivation, excision, recombination, and inducible repair. There was no detectable pyrimidine dimer-DNA N-glycosylase activity, and pyrimidine dimer removal appeared to occur by a pathway analogous to the Escherichia coli Uvr pathway. The UV- and MMS-sensitive mutants of A. espejiana included three groups, each containing at least one mutation involved with excision, recombination, or inducible repair. One group that was UV sensitive but not sensitive to MMS or X rays showed a decreased ability to excise pyrimidine dimers. Mutants in this group were also sensitive to psoralen plus near-UV light and were phenotypically analogous to the E. coli uvr mutants. A second group was UV and MMS sensitive but not sensitive to X rays and appeared to contain mutations in a gene(s) involved in recombination repair. These recombination-deficient mutants differed from the E. coli rec mutants, which are MMS and X-ray sensitive. The third group of A. espejiana mutants was sensitive to UV, MMS, and X rays. These mutants were recombination deficient, lacked inducible repair, and were phenotypically similar to E. coli recA mutants.     

Comparison of DNA damage responses following equimutagenic doses of UVA and UVB: a less effective cell cycle arrest with UVA may render UVA-induced pyrimidine dimers more mutagenic than UVB-induced ones

Mechanisms of UVA-mutagenesis remain a matter of debate. Earlier described higher rates of mutation formation per pyrimidine dimer with UVA than with UVB and other evidence suggested that a non-pyrimidine dimer-type of DNA damage contributes more to UVA- than to UVB-mutagenesis. However, more recently published data on the spectra of UVA-induced mutations in primary human skin cells and in mice suggest that pyrimidine dimers are the most common type of DNA damage-inducing mutations not only with UVB, but also with UVA. As this rebuts a prominent role of non-dimer type of DNA damage in UVA-mutagenesis, we hypothesized that the higher mutation rate at UVA-induced pyrimidine dimers, as compared to UVB-induced ones, is caused by differences in the way UVA- and UVB-exposed cells process DNA damage. Therefore, we here compared cell cycle regulation, DNA repair, and apoptosis in primary human fibroblasts following UVB- and UVA-irradiation, using the same physiologic and roughly equimutagenic doses (100-300 J m(-2) UVB, 100-300 kJ m(-2) UVA) we have used previously for mutagenesis experiments with the same type of cells. ELISAs for the detection of pyrimidine dimers confirmed that much fewer dimers were formed with these doses of UVA, as compared to UVB. We found that cell cycle arrests (intra-S, G1/S, G2/M), mediated at least in part by activation of p53 and p95, are much more prominent and long-lasting with UVB than with UVA. In contrast, no prominent differences were found between UVA and UVB for other anti-mutagenic cellular responses (DNA repair, apoptosis). Our data suggest that less effective anti-mutagenic cellular responses, in particular different and shorter-lived cell cycle arrests, render pyrimidine dimers induced by UVA more mutagenic than pyrimidine dimers induced by UVB.