Phage T4 endonuclease V stimulates DNA repair replication in isolated nuclei from ultraviolet-irradiated human cells, including xeroderma pigmentosum fibroblasts

Identification of ATIC as a Novel Target for Chemoradiosensitization

PURPOSE

Mutations in the gene encoding 5-aminoimidazole-4-carboxamide ribonucleotide formyltransferase/IMP cyclohydrolase (ATIC), a bifunctional enzyme that catalyzes the final 2 steps of the purine de novo biosynthetic pathway, were identified in a subject referred for radiation sensitivity testing. Functional studies were performed to determine whether ATIC inhibition was radiosensitizing and, if so, to elucidate the mechanism of this effect and determine whether small molecule inhibitors of ATIC could act as effective radiosensitizing agents.

METHODS AND MATERIALS

Both small interfering RNA knockdown and small molecule inhibitors were used to inactivate ATIC in cell culture. Clonogenic survival assays, the neutral comet assay, and γH2AX staining were used to assess the effects of ATIC inhibition or depletion on cellular DNA damage responses.

RESULTS

Depletion of ATIC or inhibition of its transformylase activity significantly reduced the surviving fraction of cells in clonogenic survival assays in multiple cancer cell lines. In the absence of ionizing radiation exposure, ATIC knockdown or chemical inhibition activated cell cycle checkpoints, shifting cells to the more radiosensitive G2/M phase of the cell cycle, and depleted cellular adenosine triphosphate but did not result in detectable DNA damage. Cells in which ATIC was knocked down or inhibited and then treated with ionizing radiation displayed increased numbers of DNA double-strand breaks and a delay in the repair of those breaks relative to irradiated, but otherwise untreated, controls. Supplementation of culture media with exogenous adenosine triphosphate ameliorated the DNA repair phenotypes.

CONCLUSIONS

These findings implicate ATIC as an effective, and previously unrecognized, target for chemoradiosensitization and, more broadly, suggest that purine levels in cells might have an underappreciated role in modulating the efficiency of DNA damage responses that could be exploited in radiosensitizing strategies.

NER factors are recruited to active promoters and facilitate chromatin modification for transcription in the absence of exogenous genotoxic attack

Upon gene activation, we found that RNA polymerase II transcription machinery assembles sequentially with the nucleotide excision repair (NER) factors at the promoter. This recruitment occurs in absence of exogenous genotoxic attack, is sensitive to transcription inhibitors, and depends on the XPC protein. The presence of these repair proteins at the promoter of activated genes is necessary in order to achieve optimal DNA demethylation and histone posttranslational modifications (H3K4/H3K9 methylation, H3K9/14 acetylation) and thus efficient RNA synthesis. Deficiencies in some NER factors impede the recruitment of others and affect nuclear receptor transactivation. Our data suggest that there is a functional difference between the presence of the NER factors at the promoters (which requires XPC) and the NER factors at the distal regions of the gene (which requires CSB). While the latter may be a repair function, the former is a function with respect to transcription unveiled in the current study.

Excision-repair patch lengths are similar for transcription-coupled repair and global genome repair in UV-irradiated human cells

We have used the buoyant density shift method to measure excision-repair patch lengths in UV-irradiated repair-proficient human cells and in primary fibroblasts belonging to xeroderma pigmentosum complementation group C (XP-C), in which excision repair of UV-induced photoproducts is dependent upon transcription. The patch size was found to be about 30 nucleotides for both cell types. This agrees with the size of the DNA fragments excised in vitro by the dual incisions of the structure-specific nucleases XPG and ERCC1-XPF. We conclude that the XPC protein is not required to target the excision nucleases to sites of DNA cleavage in transcribed strands of expressed genes or to protect the newly incised DNA from further processing by exonucleases.

Incomplete complementation of the DNA repair defect in cockayne syndrome cells by the denV gene from bacteriophage T4 suggests a deficiency in base excision repair

Endonuclease V (denV) from bacteriophage T4 has been examined for its ability to complement the repair defect in Cockayne syndrome (CS) cells of complementation groups A and B. CS is an autosomal recessive disorder characterized by hypersensitivity to UV light and a defect in the preferential repair of UV-induced lesions in transcriptionally active DNA by the nucleotide excision repair (NER) pathway. The denV gene was introduced into non-transformed normal and CS fibroblasts transiently via a recombinant adenovirus (Ad) vector and into SV40-transformed normal and CS cells via a retroviral vector. Expression of denV in CS-A cells resulted in partial correction of the UV-sensitive phenotype in assays of gene-specific repair and cell viability, while correction of CS-B cells by expression of denV in the same assays was minimal or non-existent. In contrast, denV expression led to enhanced host cell reactivation (HCR) of viral DNA synthesis in both CS complementation groups to near normal levels. DenV is a glycosylase which is specific for cyclobutane-pyrimidine dimers (CPDs) but does not recognize other UV-induced lesions. Previous work has indicated that CS cells can efficiently repair all non-CPD UV-induced transcription blocking lesions (S.F. Barrett et al.. Mutation Res. 255 (1991) 281-291 [1]) and that denV incised lesions are believed to be processed via the base excision repair (BER) pathway. The inability of denV to complement the NER defect in CS cells to normal levels implies an impaired ability to process denV incised lesions by the BER pathway, and suggests a role for the CS genes, particularly the CS-B gene, in BER.

Enzymatic activities involved in the DNA resynthesis step of nucleotide excision repair are firmly attached to chromatin

In this study the role of nuclear architecture in nucleotide excision repair (NER) was investigated by gentle dismantling of the cell and probing the capability of chromatin to carry out repair in vitro. The rationale behind this approach is that compartmentalization of NER at nuclear structures would make the enzymatic activities refractory to extraction by buffers that solubilize cellular membranes. In order to obtain intact chromatin primary human fibroblasts were encapsulated in agarose microbeads and lysed in isotonic buffers containing the non-ionic detergent Triton X-100. Under these conditions the majority of cellular proteins diffuse out of the beads, but the remaining chromatin is able to replicate and to transcribe DNA in the presence of triphosphates and Mg2+. UV irradiation of confluent repair-proficient human fibroblasts prior to lysis stimulated the incorporation of deoxynucleotide triphosphates in Triton X-100-isolated chromatin, even under stringent lysis conditions. In addition, experiments with UV-sensitive xeroderma pigmentosum (complementation groups A and C) and Cockayne's syndrome fibroblasts (complementation group A) revealed that this repair synthesis was due to global genome repair activity. Transcription-coupled repair was only detectable in cells permeabilized by streptolysin O (SLO). Repair synthesis in Triton X-100-isolated chromatin amounted to 15% of the total repair synthesis as measured in SLO-permeabilized cells. To allow the detection of these activities in vitro, presynthesis complexes have to be formed in intact cells, indicating that chromatin from Triton X-100-lysed cells is unable to initiate NER in vitro. Our data indicate that the components involved in the resynthesis step of NER are tightly associated with chromatin. A substantial fraction of total proliferating cell nuclear antigen (PCNA), which is required for the resynthesis step in NER, has been reported to become Triton X-100 non-extractable and tightly associated with nuclear structures after UV irradiation of cells. We propose that Triton X-100-resistant repair synthesis might be mediated by this chromatin-bound fraction of total PCNA.

Sites in human nuclei where damage induced by ultraviolet light is repaired: localization relative to transcription sites and concentrations of proliferating cell nuclear antigen and the tumour suppressor protein, p53

The repair of damage induced in DNA by ultraviolet light involves excision of the damaged sequence and synthesis of new DNA to repair the gap. Sites of such repair synthesis were visualized by incubating permeabilized HeLa or MRC-5 cells with the DNA precursor, biotin-dUTP, in a physiological buffer; then incorporated biotin was immunolabeled with fluorescent antibodies. Repair did not take place at sites that reflected the DNA distribution; rather, sites were focally concentrated in a complex pattern. This pattern changed with time; initially intense repair took place at transcriptionally active sites but when transcription became inhibited it continued at sites with little transcription. Repair synthesis in vitro also occurred in the absence of transcription. Repair sites generally contained a high concentration of proliferating cell nuclear antigen but not the tumour-suppressor protein, p53.

Sites in human nuclei where DNA damaged by ultraviolet light is repaired: visualization and localization relative to the nucleoskeleton

The repair of damage induced in DNA by ultraviolet light involves excision of the damage and then repair synthesis to fill the gap. We investigated the sites of repair synthesis using MRC-5 fibroblasts and HeLa cells in G1 phase. Cells were encapsulated in agarose microbeads to protect them during manipulation, irradiated, incubated to allow repair to initiate, and permeabilized with streptolysin O to allow entry of labelled triphosphates; [32P]dTTP was incorporated into acid-insoluble material in a dose-dependent manner. Incubation with biotin-16-dUTP allowed sites of incorporation to be indirectly immunolabeled using a FITC-conjugated antibody; sites were not diffusely spread throughout nuclei but concentrated in discrete foci. This is similar to sites of S phase activity that are attached to an underlying nucleoskeleton. After treatment with an endonuclease, most repaired DNA electroeluted from beads with chromatin fragments; this was unlike nascent DNA made during S phase and suggests that repaired DNA is not as closely associated with the skeleton. However, the procedure destroyed repair activity, so repaired DNA might be attached in vivo through a polymerase that was removed electrophoretically. Therefore this approach cannot be used to determine decisively whether repair sites are associated with a skeleton in vivo.

Enhanced pyrimidine dimer repair in cultured murine epithelial cells transfected with the denV gene of bacteriophage T4

The patch size for excision repair of ultraviolet radiation (UV)-induced pyrimidine dimers was determined in cultured murine epithelial cells with normal and enhanced pyrimidine dimer repair capabilities. Cells with enhanced pyrimidine dimer repair were produced by transfecting 308 cells with the denV gene of bacteriophage T4; this gene encodes the enzyme endonuclease V. Pyrimidine dimer repair following exposure to UV from an FS-40 sunlamp was determined by micrococcal dimer-specific nuclease digestion and alkaline sucrose ultracentrifugation. Patch size ws estimated based on the photolytic lability of bromodeoxyuridine-substituted DNA. Excision repair of UV-induced pyrimidine dimers in denV-transfected 308 cells was enhanced two- to threefold. Production of mRNA from the denV gene in cell lines with enhanced repair was confirmed by RNA blotting. In control cells, the patch size for excision repair of DNA photoproducts was estimated to be 34 nucleotides per photoproduct removed; in denV-transfected cells, a smaller average patch size of 10-16 nucleotides per photoproduct removed was calculated. Thus, endonuclease V activity appears to alter not only the extent, but also the nature of excision repair in UV-exposed mammalian epithelial cells.

Cyclobutane dimers and (6-4) photoproducts in human cells are mended with the same patch sizes

The size of excision repair patches corresponding to excision of (6-4) pyrimidine-pyrimidone photoproducts and (5-5, 6-6) cyclobutane dimers have been independently determined by using bromodeoxyuridine substitution and density increases in isopycnic gradients of small DNA fragments. The two classes of photoproducts were distinguished by using (a) a xeroderma pigmentosum (XP) revertant cell line that excises (6-4) photoproducts normally, but does not excise cyclobutane dimers from bulk DNA or from an actively transcribed sequence; (b) an XP cell line containing the denV gene of bacteriophage T4, which repairs only cyclobutane dimers by a unique glycosylase mechanism, and (c) normal cells analyzed during time intervals in which cyclobutane dimer repair is the main repair process in action. The patch sizes for the two lesions were similar under all conditions and were estimated to be approximately 30-40 bases. These values are slightly large than corresponding estimates for Escherichia coli and Saccharomyces cerevisiae but close to estimates from in vitro experiments with human cell extracts. The size of 30 bases may consequently be very close to the actual distance between cleavage sites made on either side of a photoproduct during repair.

Encapsulation of the UV-DNA repair enzyme T4 endonuclease V in liposomes and delivery to human cells

T4 endonuclease V, a pyrimidine-dimer-specific DNA repair enzyme, was encapsulated in liposomes under mild conditions. The encapsulated enzyme was active, and when applied to ultraviolet (UV)-irradiated human cells in culture, the liposomes increased incision of UV-irradiated cellular DNA, enhanced DNA repair replication, and enhanced survival of UV-irradiated cells. This method is a first step in a new approach for topical application of DNA repair enzymes to human skin to prevent skin cancer.

Excision repair characteristics of denV-transformed xeroderma pigmentosum cells

Introduction of the denV gene of phage T4, encoding the pyrimidine dimer-specific endonuclease V, into xeroderma pigmentosum cells XP12RO(M1) was reported to result in partial restoration of colony-forming ability and excision repair synthesis. We have further characterized 3 denV-transformed XP clones in terms of rates of excision of pyrimidine dimers and size of the resulting resynthesized regions following exposure to 100 J/m2 from an FS-40 sunlamp. In the denV-transformed XP cells we observed 50% dimer removal within 3-6 h after UV exposure as compared to no measurable removal in the XP12RO(M1) line and 50% dimer excision after 18 h in the GM637A human, control cells. Dimer removal was assayed with Micrococcus luteus UV-endonuclease in conjunction with sedimentation of treated DNA in alkaline sucrose gradients. The size of the resulting repaired regions was determined by the bromouracil photolysis technique. Based on the photolytic sensitivity of DNA repaired in the presence of bromodeoxyuridine, we calculated that the excision of a dimer in the GM637A cells appears to be accompanied by the resynthesis of a region approximately 95 nucleotides in length. Conversely, the resynthesized regions in the denV-transformed clones were considerably smaller and were estimated to be between 13 and 18 nucleotides in length. These results may indicate that either the endonuclease that initiated dimer repair dictated the size of the resynthesized region or that the long-patch repair observed in the normal cells resulted from the repair of non-dimer DNA lesions.

The molecular genetics of the incision step in the DNA excision repair process

This review describes the evolution of research into the genetic basis of how different organisms use the process of excision repair to recognize and remove lesions from their cellular DNA. One particular aspect of excision repair, DNA incision, and how it is controlled at the genetic level in bacteriophage, bacteria, S. cerevisae, D. melanogaster, rodent cells and humans is examined. In phage T4, DNA is incised by a DNA glycosylase-AP endonuclease that is coded for by the denV gene. In E. coli, the products of three genes, uvrA, uvrB and uvrC, are required to form the UVRABC excinuclease that cleaves DNA and releases a fragment 12-13 nucleotides long containing the site of damage. In S. cerevisiae, genes complementing five mutants of the RAD3 epistasis group, rad1, rad2, rad3, rad4 and rad10 have been cloned and analyzed. Rodent cells sensitive to a variety of mutagenic agents and deficient in excision repair are being used in molecular studies to identify and clone human repair genes (e.g. ERCC1) capable of complementing mammalian repair defects. Most studies of the human system, however, have been done with cells isolated from patients suffering from the repair defective, cancer-prone disorder, xeroderma pigmentosum, and these cells are now beginning to be characterized at the molecular level. Studies such as these that provide a greater understanding of the genetic basis of DNA repair should also offer new insights into other cellular processes, including genetic recombination, differentiation, mutagenesis, carcinogenesis and aging.

Permeabilization of ultraviolet-irradiated Chinese hamster cells with polyethylene glycol and introduction of ultraviolet endonuclease from Micrococcus luteus

Chinese hamster V-79 cells were made permeable by treatment with polyethylene glycol and then incubated with a Micrococcus luteus extract containing ultraviolet-specific endonuclease activity. This treatment introduced nicks in irradiated, but not in unirradiated, deoxyribonucleic acid. The nicks remained open for at least 3 h; there was no loss of endonuclease-sensitive sites, and no excision of dimers as measured by chromatography was detected. In addition, there was no increase in ultraviolet resistance in treated cells. This suggests that the absence of a significant amount of excision repair in rodent cells is due to the lack of both incision and excision capacity.

The effect of preincubation of HeLa cell nuclei with ATP on the degradation of mononucleosomal DNA by micrococcal nuclease

HeLa cell nuclei with DNA labeled with [3H] thymidine have been preincubated under varying conditions and then incubated with micrococcal nuclease. Aliquots, removed at increasing times, were analyzed for mononucleosomal size DNA and for acid-soluble DNA, the ratios were plotted and a slope was determined. Preincubation with ATP and a regenerating system increased the slope 2 fold. Optimum ATP concentrations were above 0.25 mM. The ATP effect was reversed by novobiocin. No inhibition of the ATP effect was observed with nalidixic acid, coumermycin, oxolinic acid, VM-26, aphidicolin, or 3 amino-benzamide. NAD or cAMP or cGMP had no effect with or without ATP. Other nucleoside triphosphates could substitute to varying degrees for ATP as could ATP analogues. Nuclei from log phase cells showed no ATP effect, but log phase cells, partially depleted of ATP by incubation with deoxyglucose, showed the effect. The effect was lost in nuclei on long-term storage. No evidence was found for differential degradation of core histones, histone H-1 or DNA, and there was no evidence of nucleosome sliding.

Genetic complementation of UV-induced DNA repair in Chinese hamster ovary cells by the denV gene of phage T4

The denV gene of phage T4, encoding the pyrimidine dimer-specific DNA repair enzyme endonuclease V, has been introduced by DNA transfection into the UV-sensitive DNA repair-deficient Chinese hamster ovary (CHO) cell line UV5. Transformants were first selected for resistance to the antibiotic G418 conferred by the neo gene from Tn5 carried by the same plasmid. A majority of the isolated G418-resistant UV5 clones also showed an increased resistance to 254-nm UV light. One clone, designated I-A1, was found to have an intermediate level of colony-forming ability after UV irradiation when compared to UV5 and wild-type AA8 cells. A Southern blot showed that I-A1 carries a single integrated intact copy of the denV gene. Alkaline sucrose gradients revealed a dose-dependent appearance of breaks in the DNA of I-A1 cells following UV-irradiation, while unirradiated cells did not exhibit any significant breaks. Analysis of DNA repair by isopycnic sedimentation showed that DNA excision repair by I-A1 was at least equal to the level of repair in AA8 cells. These results show that the prokaryotic denV gene can restore UV repair capabilities in vivo to CHO UV5 cells defective in repair of UV-induced damage.

Identification, physical map location and sequence of the denV gene from bacteriophage T4

The denV gene from bacteriophage T4, which codes for endonuclease V, a small DNA repair enzyme, has been cloned and identified by an approach combining DNA sequencing and genetics, independent of the phenotypic effect of the cloned gene. Appropriate DenV+ and DenV- deletion mutants were mapped physically to define precisely a region encompassing the denV gene. This region was sequenced in order to identify a protein-coding sequence of the correct size for the denV gene (400-500 bp). Finally, identification was confirmed by sequencing the corresponding fragments cloned from four genetically and phenotypically well-characterized denV mutants. The denV gene is located at 64 kb on the T4 genome, adjacent to the ipII gene, and codes for a basic protein of 138 amino acids with a deduced molecular weight of 16,078.

Microinjection of human cell extracts corrects xeroderma pigmentosum defect

Cultured fibroblasts of patients with the DNA repair syndrome xeroderma pigmentosum (XP) were injected with crude cell extracts from various human cells. Injected fibroblasts were then assayed for unscheduled DNA synthesis (UDS) to see whether the injected extract could complement their deficiency in the removal of u.v.-induced thymidine dimers from their DNA. Microinjection of extracts from repair-proficient cells (such as HeLa, placenta) and from cells belonging to XP complementation group C resulted in a temporary correction of the DNA repair defect in XP-A cells but not in cells from complementation groups C, D or F. Extracts prepared from XP-A cells were unable to correct the XP-A repair defect. The UDS of phenotypically corrected XP-A cells is u.v.-specific and can reach the level of normal cells. The XP-A correcting factor was found to be sensitive to the action of proteinase K, suggesting that it is a protein. It is present in normal cells in high amounts, it is stable on storage and can still be detected in the injected cells 8 h after injection. The microinjection assay described in this paper provides a useful tool for the purification of the XP-A (and possibly other) factor(s) involved in DNA repair.

Demonstration of pyrimidine dimer-DNA glycosylase activity in vivo: bacteriophage T4-infected Escherichia coli as a model system

An approach to the detection of pyrimidine dimer-DNA glycosylase activity in living cells is presented. Mutants of Escherichia coli defective in uvr functions required for incision of UV-irradiated DNA were infected with phage T4 denV+ or denV- (defective in the T4 pyrimidine dimer-DNA glycosylase activity). In the former case the denV gene product catalyzed the incision of UV-irradiated host DNA, facilitating the subsequent excision of thymine-containing pyrimidine dimers. Isolation of these dimers from the acid-soluble fraction of infected cells was achieved by a multistep thin-layer chromatographic system. Exposure of the dimers to irradiation that monomerizes pyrimidine dimers (direct photoreversal) resulted in the stoichiometric formation of free thymine. Thus, in vivo incision of UV-irradiated DNA dependent on a pyrimidine dimer-DNA glycosylase can be demonstrated.

Alterations in chromatin structure during DNA excision repair

Work from a number of laboratories recently has demonstrated that alterations in chromatin structure occur during excision repair in mammalian cells. It is now clear that when cells are damaged with a wide variety of chemical agents or ultraviolet radiation, almost all of the repair synthesis is initially sensitive to staphylococcal nuclease. With time, there is a redistribution of the counts incorporated during excision repair synthesis so that many of them become nuclease resistant and associated with nucleosome core length DNA. In our laboratory, we have demonstrated this phenomenon in human cells damaged with N-acetoxy-2-acetylaminofluorene, 7-bromomethylbenz[a]anthracene, and ultraviolet radiation. It is clear from the work of others that the phenomenon is not unique to human cells since African green monkey cells damaged with either ultraviolet radiation or angelicin also show an initial nuclease sensitivity of repair-incorporated nucleotides follow by rearrangement. Two models to explain these observations have been proposed; one suggests that there is an unfolding of nucleosomes during excision repair followed by a refolding, while the other suggests that sliding of core proteins with respect to DNA occurs during excision repair. These models, as well as recent data bearing on them, will be discussed.

Expression of the denV gene of bacteriophage T4 cloned in Escherichia coli

The denV gene of bacteriophage T4 has been cloned into Escherichia coli K-12 by inserting appropriate fragments of cytosine-containing T4 DNA into the Sal I site of the plasmid pBR322. The denV gene codes for an enzyme that initiates the excision repair of pyrimidine dimers produced in DNA by UV. In uvrA recA mutants, deficient in an early step in excision repair, the cloned DNA results in enhanced UV resistance that is more pronounced in stationary- than in exponential-phase cultures. The expression of the cloned DNA also results in the enhanced survival of UV-irradiated phage lambda or of a denV mutant of phage T4 and in removal of dimers from the DNA of UV-irradiated cells.

Permeabilization of ultraviolet-irradiated Chinese hamster cells with polyethylene glycol and introduction of ultraviolet endonuclease from Micrococcus luteus

Chinese hamster V-79 cells were made permeable by treatment with polyethylene glycol and then incubated with a Micrococcus luteus extract containing ultraviolet-specific endonuclease activity. This treatment introduced nicks in irradiated, but not in unirradiated, deoxyribonucleic acid. The nicks remained open for at least 3 h; there was no loss of endonuclease-sensitive sites, and no excision of dimers as measured by chromatography was detected. In addition, there was no increase in ultraviolet resistance in treated cells. This suggests that the absence of a significant amount of excision repair in rodent cells is due to the lack of both incision and excision capacity.

Insertion of E. coli photoreactivating enzyme into V79 hamster cells

Photoreactivating enzyme mediates the specific repair of UV light [220-300 nm, (UV)]-induced cyclobutyl pyrimidine dimers in DNA. It binds to dimer-containing DNA, and on absorption of light in the wavelength range 300-500 nm monomerizes the dimer, restoring biological activity to DNA. The specificity of the enzyme for pyrimidine dimers in DNA allows its use as an analytical tool. If UV-induced biological damage is photoreactivable, dimers are probably a major cause of that damage. Thus dimers have been implicated in producing death and mutation in prokaryotes and in simple eukaryotes. Although this photoreactivation test has great potential value in assessing the role of dimers in UV-induced damage in mammalian cells, its use in cultured mammalian cells has been limited by the dependence of photoreactivating enzyme levels on the cell species, genotype and culture medium. We have developed a method for insertion of Escherichia coli photoreactivating enzyme into mammalian cells, and show here that the inserted bacterial enzyme can mediate photoreactivation of pyrimidine dimers in V79 rodent cells.

Evidence implying DNA polymerase beta function in excision repair

Comparison was made of the ability of calf thymus DNA polymerases alpha and beta to replicate the following templates: native E. coli CR-34 DNA (T-DNA), calf thymus DNA activated by DNase I (act.DNA), BU-DNA (from E. coli CR-34 cells cultured on BUdR-containing medium) with damages resulting from incomplete excision repair, as well as thermally denatured act.DNA and BU-DNA (s.s.act.DNA and s.s.BU-DNA). 3H-TTP incorporation during extensive replication of act.DNA was similar for both enzymes, being, as expected, 40 times higher than for T-DNA. Likewise, the differences in the yield of the s.s.act.DNA or s.s.BU-DNA replication between both enzymes were negligible. In contrast, damaged native DNA was 6 - 30 times more extensively replicated by DNA polymerase beta than alpha. We propose that this is due to the greater ability of DNA polymerase beta compared with alpha to replicate single-stranded gaps, the presence of which is more likely in damaged BU-DNA than in T-DNA and act.DNA.

Association of poly(adenosine diphosphoribose) synthesis with DNA damage and repair in normal human lymphocytes

A permeable cell technique was used to measure the alterations in synthesis of DNA and poly-(adenosine diphosphoribose) in normal human lymphocytes after treatment of the cells with different types of DNA-damaging agents. The lymphocytes showed an abrupt increase in the unscheduled synthesis of DNA and poly(adenosine diphosphoribose) in response to ultraviolet (UV) irradiation. The increases were apparent within 1 h and reached a maximum between 2 and 4 h after irradiation. The magnitude of the increases in DNA and poly(adenosine diphosphoribose) synthesis was dependent upon the UV dose. Alkaline CsCl gradient studies, with bromodeoxyuridine triphosphate density labeling of DNA, demonstrated that the unscheduled DNA synthesis, which occurred in response to UV irradiation, was actually a result of the repair mode of DNA synthesis. Similar increases in DNA synthesis, and poly(adenosine diphosphoribose) synthesis occurred when lymphocytes were treated with several other DNA-damaging agents, including bleomycin, N-methyl-N'-nitro-N-nitrosoguanidine or N-acetoxyacetyl aminofluorene. Treatment of lymphocytes with DNase, under conditions which allowed degradation of cellular DNA, also resulted in increased synthesis of poly(adenosine diphosphoribose). Cycloheximide did not inhibit the increase in synthesis of DNA or poly(adenosine diphosphoribose) that occurred in response to treatment with the DNA-damaging agents.