Partial purification and characterization of a uracil-DNA glycosylase from wheat germ

Base Excision DNA Repair in Plants: Arabidopsis and Beyond

Base excision DNA repair (BER) is a key pathway safeguarding the genome of all living organisms from damage caused by both intrinsic and environmental factors. Most present knowledge about BER comes from studies of human cells, E. coli, and yeast. Plants may be under an even heavier DNA damage threat from abiotic stress, reactive oxygen species leaking from the photosynthetic system, and reactive secondary metabolites. In general, BER in plant species is similar to that in humans and model organisms, but several important details are specific to plants. Here, we review the current state of knowledge about BER in plants, with special attention paid to its unique features, such as the existence of active epigenetic demethylation based on the BER machinery, the unexplained diversity of alkylation damage repair enzymes, and the differences in the processing of abasic sites that appear either spontaneously or are generated as BER intermediates. Understanding the biochemistry of plant DNA repair, especially in species other than the Arabidopsis model, is important for future efforts to develop new crop varieties.

Identification of a new and diverse set of Mycobacterium tuberculosstais uracil-DNA glycosylase (MtUng) inhibitors using structure-based virtual screening: experimental validation and molecular dynamics studies

Mycobacterium tuberculosis uracil-DNA glycosylase (MtUng), a key DNA repair enzyme, represents an attractive target for the design of new antimycobacterial agents. However, only a limited number of weak MtUng inhibitors are reported, primarily based on the uracil ring, and hence, lack diversity. We report the first structure-based virtual screening (SBVS) using three separate libraries consisting of uracil and non-uracil small molecules, together with the FDA-approved drugs. Twenty diverse virtual hits with the highest predicted binding were procured and screened using a fluorescence-based assay to evaluate their potential to inhibit MtUng. Several of these molecules were found to inhibit MtUng activity at low mM and µM levels, comparable to or better than several other reported Ung inhibitors. Thus, these molecules represent a diverse set of scaffolds for developing next-generation MtUng inhibitors. The most active uracil-based compound 5 (IC₅₀ = 0.14 mM) was found to be ∼15-fold more potent than the positive control, uracil. The binding stability and conformation of compound 5 in complex with the enzyme were further confirmed using molecular dynamics simulation.

Use of a molecular beacon based fluorescent method for assaying uracil DNA glycosylase (Ung) activity and inhibitor screening

Uracil DNA glycosylases are an important class of enzymes that hydrolyze the N-glycosidic bond between the uracil base and the deoxyribose sugar to initiate uracil excision repair. Uracil may arise in DNA either because of its direct incorporation (against A in the template) or because of cytosine deamination. Mycobacteria with G, C rich genomes are inherently at high risk of cytosine deamination. Uracil DNA glycosylase activity is thus important for the survival of mycobacteria. A limitation in evaluating the druggability of this enzyme, however, is the absence of a rapid assay to evaluate catalytic activity that can be scaled for medium to high-throughput screening of inhibitors. Here we report a fluorescence-based method to assay uracil DNA glycosylase activity. A hairpin DNA oligomer with a fluorophore at its 5′ end and a quencher at its 3′ ends was designed incorporating five consecutive U:A base pairs immediately after the first base pair (5′ C:G 3’) at the top of the hairpin stem. Enzyme assays performed using this fluorescent substrate were seen to be highly sensitive thus enabling investigation of the real time kinetics of uracil excision. Here we present data that demonstrate the feasibility of using this assay to screen for inhibitors of Mycobacterium tuberculosis uracil DNA glycosylase. We note that this assay is suitable for high-throughput screening of compound libraries for uracil DNA glycosylase inhibitors.

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A novel molecular beacon based fluorescent method to assay uracil DNA glycosylase (UDG) activity has been developed.

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The single step assay is useful to determine real-time kinetics of uracil release.

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The assay is useful for high throughput screening of uracil DNA glycosylase inhibitors.

DNA Base Excision Repair in Plants: An Unfolding Story With Familiar and Novel Characters

Base excision repair (BER) is a critical genome defense pathway that deals with a broad range of non-voluminous DNA lesions induced by endogenous or exogenous genotoxic agents. BER is a complex process initiated by the excision of the damaged base, proceeds through a sequence of reactions that generate various DNA intermediates, and culminates with restoration of the original DNA structure. BER has been extensively studied in microbial and animal systems, but knowledge in plants has lagged behind until recently. Results obtained so far indicate that plants share many BER factors with other organisms, but also possess some unique features and combinations. Plant BER plays an important role in preserving genome integrity through removal of damaged bases. However, it performs additional important functions, such as the replacement of the naturally modified base 5-methylcytosine with cytosine in a plant-specific pathway for active DNA demethylation.

The major Arabidopsis thaliana apurinic/apyrimidinic endonuclease, ARP is involved in the plant nucleotide incision repair pathway

Apurinic/apyrimidinic (AP) endonucleases are important DNA repair enzymes involved in two overlapping pathways: DNA glycosylase-initiated base excision (BER) and AP endonuclease-initiated nucleotide incision repair (NIR). In the BER pathway, AP endonucleases cleave DNA at AP sites and 3'-blocking moieties generated by DNA glycosylases, whereas in NIR, the same AP endonucleases incise DNA 5' to a wide variety of oxidized bases. The flowering plant Arabidopsis thaliana contains three genes encoding homologues of major human AP endonuclease 1 (APE1): Arp, Ape1L and Ape2. It has been shown that all three proteins contain AP site cleavage and 3'-repair phosphodiesterase activities; however, it was not known whether the plant AP endonucleases contain the NIR activity. Here, we report that ARP proteins from Arabidopsis and common wheat (Triticum aestivum) contain NIR and 3'→5' exonuclease activities in addition to their AP endonuclease and 3'-repair phosphodiesterase functions. The steady-state kinetic parameters of reactions indicate that Arabidopsis ARP cleaves oligonucleotide duplexes containing α-anomeric 2'-deoxyadenosine (αdA) and 5,6-dihydrouridine (DHU) with efficiencies (k_cat/K_M=134 and 7.3 μM^-1·min^-1, respectively) comparable to those of the human counterpart. However, the ARP-catalyzed 3'-repair phosphodiesterase and 3'→5' exonuclease activities (k_cat/K_M=314 and 34 μM^-1·min^-1, respectively) were about 10-fold less efficient as compared to those of APE1. Interestingly, homozygous A. thaliana arp^-/- mutant exhibited high sensitivity to methyl methanesulfonate and tert-butyl hydroperoxide, but not to H₂O₂, suggesting that ARP is a major plant AP endonuclease that removes abasic sites and specific types of oxidative DNA base damage. Taken together, these data establish the presence of the NIR pathway in plants and suggest its possible role in the repair of DNA damage generated by oxidative stress.

Structural plasticity inMycobacterium tuberculosisuracil-DNA glycosylase (MtUng) and its functional implications

17 independent crystal structures of family I uracil-DNA glycosylase fromMycobacterium tuberculosis(MtUng) and its complexes with uracil and its derivatives, distributed among five distinct crystal forms, have been determined. Thermodynamic parameters of binding in the complexes have been measured using isothermal titration calorimetry. The two-domain protein exhibits open and closed conformations, suggesting that the closure of the domain on DNA binding involves conformational selection. Segmental mobility in the enzyme molecule is confined to a 32-residue stretch which plays a major role in DNA binding. Uracil and its derivatives can bind to the protein in two possible orientations. Only one of them is possible when there is a bulky substituent at the 5′ position. The crystal structures of the complexes provide a reasonable rationale for the observed thermodynamic parameters. In addition to providing fresh insights into the structure, plasticity and interactions of the protein molecule, the results of the present investigation provide a platform for structure-based inhibitor design.

Cloning and characterization of a wheat homologue of apurinic/apyrimidinic endonuclease Ape1L

BACKGROUND

Apurinic/apyrimidinic (AP) endonucleases are key DNA repair enzymes involved in the base excision repair (BER) pathway. In BER, an AP endonuclease cleaves DNA at AP sites and 3'-blocking moieties generated by DNA glycosylases and/or oxidative damage. A Triticum aestivum cDNA encoding for a putative homologue of ExoIII family AP endonucleases which includes E. coli Xth, human APE1 and Arabidopsis thaliana AtApe1L has been isolated and its protein product purified and characterized.

METHODOLOGY/PRINCIPAL FINDINGS

We report that the putative wheat AP endonuclease, referred here as TaApe1L, contains AP endonuclease, 3'-repair phosphodiesterase, 3'-phosphatase and 3' → 5' exonuclease activities. Surprisingly, in contrast to bacterial and human AP endonucleases, addition of Mg(2+) and Ca(2+) (5-10 mM) to the reaction mixture inhibited TaApe1L whereas the presence of Mn(2+), Co(2+) and Fe(2+) cations (0.1-1.0 mM) strongly stimulated all its DNA repair activities. Optimization of the reaction conditions revealed that the wheat enzyme requires low divalent cation concentration (0.1 mM), mildly acidic pH (6-7), low ionic strength (20 mM KCl) and has a temperature optimum at around 20 °C. The steady-state kinetic parameters of enzymatic reactions indicate that TaApe1L removes 3'-blocking sugar-phosphate and 3'-phosphate groups with good efficiency (kcat/KM = 630 and 485 μM(-1) · min(-1), respectively) but possesses a very weak AP endonuclease activity as compared to the human homologue, APE1.

CONCLUSIONS/SIGNIFICANCE

Taken together, these data establish the DNA substrate specificity of the wheat AP endonuclease and suggest its possible role in the repair of DNA damage generated by endogenous and environmental factors.

Base-excision repair activity of uracil-DNA glycosylase monitored using the latch zone of α-hemolysin

Nanopores have been investigated as a simple and label-free tool to characterize DNA nucleotides when a ssDNA strand translocates through the constriction of the pore. Here, a wild-type α-hemolysin protein nanopore was used to monitor DNA repair enzyme activity based on base-specific interactions of dsDNA with the vestibule constriction "latch", a previously unrecognized sensing zone in α-hemolysin specific for dsDNA structure. The presence of a single abasic site within dsDNA that is in proximity to the latch zone (±2 nucleotides) results in a large increase in ion channel current, allowing accurate quantitation of the kinetics of base repair reactions involving an abasic site product. Taking advantage of the high resolution for abasic site recognition, the rate of uracil-DNA glycosylase hydrolysis of the N-glycosidic bond, converting 2'-deoxyuridine in DNA to an abasic site, was continuously monitored by electrophoretically capturing reaction substrate or product dsDNA in the ion channel vestibule. Our work suggests use of the nanopore as an enzymology tool and provides a means to identify single base structural changes in dsDNA.

Presence of base excision repair enzymes in the wheat aleurone and their activation in cells undergoing programmed cell death

Cereal aleurone cells are specialized endosperm cells that produce enzymes to hydrolyze the starchy endosperm during germination. Aleurone cells can undergo programmed cell death (PCD) when incubated in the presence of gibberellic acid (GA) in contrast to abscisic acid (ABA) which inhibits the process. The progression of PCD in aleurone layer cells of wheat grain is accompanied by an increase in deoxyribonuclease (DNase) activities and the internucleosomal degradation of nuclear DNA. Reactive oxygen species (ROS) are increased during PCD in the aleurone cells owing to the β-oxidation of triglycerides and inhibition of the antioxidant enzymes possibly leading to extensive oxidative damage to DNA. ROS generate mainly non-bulky DNA base lesions which are removed in the base excision repair (BER) pathway, initiated by the DNA glycosylases. At present, very little is known about oxidative DNA damage repair in cereals. Here, we study DNA repair in the cell-free extracts of wheat aleurone layer incubated or not with phytohormones. We show, for the first time, the presence of 8-oxoguanine-DNA and ethenoadenine-DNA glycosylase activities in wheat aleurone cells. Interestingly, the DNA glycosylase and AP endonuclease activities are strongly induced in the presence of GA. Based on these data we propose that GA in addition to activation of nuclear DNases also induces the DNA repair activities which remove oxidized DNA bases in the BER pathway. Potential roles of the wheat DNA glycosylases in GA-induced oligonucleosomal fragmentation of DNA and metabolic activation of aleurone layer cells via repair of transcribed regions are discussed.

Structure of uracil-DNA glycosylase from Mycobacterium tuberculosis: insights into interactions with ligands

Uracil N-glycosylase (Ung) is the most thoroughly studied of the group of uracil DNA-glycosylase (UDG) enzymes that catalyse the first step in the uracil excision-repair pathway. The overall structure of the enzyme from Mycobacterium tuberculosis is essentially the same as that of the enzyme from other sources. However, differences exist in the N- and C-terminal stretches and some catalytic loops. Comparison with appropriate structures indicate that the two-domain enzyme closes slightly when binding to DNA, while it opens slightly when binding to the proteinaceous inhibitor Ugi. The structural changes in the catalytic loops on complexation reflect the special features of their structure in the mycobacterial protein. A comparative analysis of available sequences of the enzyme from different sources indicates high conservation of amino-acid residues in the catalytic loops. The uracil-binding pocket in the structure is occupied by a citrate ion. The interactions of the citrate ion with the protein mimic those of uracil, in addition to providing insights into other possible interactions that inhibitors could be involved in.

Arabidopsis uracil DNA glycosylase (UNG) is required for base excision repair of uracil and increases plant sensitivity to 5-fluorouracil

Uracil in DNA arises by misincorporation of dUMP during replication and by hydrolytic deamination of cytosine. This common lesion is actively removed through a base excision repair (BER) pathway initiated by a uracil DNA glycosylase (UDG) activity that excises the damage as a free base. UDGs are classified into different families differentially distributed across eubacteria, archaea, yeast, and animals, but remain to be unambiguously identified in plants. We report here the molecular characterization of AtUNG (Arabidopsis thaliana uracil DNA glycosylase), a plant member of the Family-1 of UDGs typified by Escherichia coli Ung. AtUNG exhibits the narrow substrate specificity and single-stranded DNA preference that are characteristic of Ung homologues. Cell extracts from atung(-/-) mutants are devoid of UDG activity, and lack the capacity to initiate BER on uracil residues. AtUNG-deficient plants do not display any apparent phenotype, but show increased resistance to 5-fluorouracil (5-FU), a cytostatic drug that favors dUMP misincorporation into DNA. The resistance of atung(-/-) mutants to 5-FU is accompanied by the accumulation of uracil residues in DNA. These results suggest that AtUNG excises uracil in vivo but generates toxic AP sites when processing abundant U:A pairs in dTTP-depleted cells. Altogether, our findings point to AtUNG as the major UDG activity in Arabidopsis.

Analysis of the impact of a uracil DNA glycosylase attenuated in AP-DNA binding in maintenance of the genomic integrity in Escherichia coli

Uracil DNA glycosylase (Ung) initiates the uracil excision repair pathway. We have earlier characterized the Y66W and Y66H mutants of Ung and shown that they are compromised by ∼7- and ∼170-fold, respectively in their uracil excision activities. In this study, fluorescence anisotropy measurements show that compared with the wild-type, the Y66W protein is moderately compromised and attenuated in binding to AP-DNA. Allelic exchange of ung in Escherichia coli with ung::kan, ungY66H:amp or ungY66W:amp alleles showed ∼5-, ∼3.0- and ∼2.0-fold, respectively increase in mutation frequencies. Analysis of mutations in the rifampicin resistance determining region of rpoB revealed that the Y66W allele resulted in an increase in A to G (or T to C) mutations. However, the increase in A to G mutations was mitigated upon expression of wild-type Ung from a plasmid borne gene. Biochemical and computational analyses showed that the Y66W mutant maintains strict specificity for uracil excision from DNA. Interestingly, a strain deficient in AP-endonucleases also showed an increase in A to G mutations. We discuss these findings in the context of a proposal that the residency of DNA glycosylase(s) onto the AP-sites they generate shields them until recruitment of AP-endonucleases for further repair.

Base excision repair and its role in maintaining genome stability

For all living organisms, genome stability is important, but is also under constant threat because various environmental and endogenous damaging agents can modify the structural properties of DNA bases. As a defense, organisms have developed different DNA repair pathways. Base excision repair (BER) is the predominant pathway for coping with a broad range of small lesions resulting from oxidation, alkylation, and deamination, which modify individual bases without large effect on the double helix structure. As, in mammalian cells, this damage is estimated to account daily for 10(4) events per cell, the need for BER pathways is unquestionable. The damage-specific removal is carried out by a considerable group of enzymes, designated as DNA glycosylases. Each DNA glycosylase has its unique specificity and many of them are ubiquitous in microorganisms, mammals, and plants. Here, we review the importance of the BER pathway and we focus on the different roles of DNA glycosylases in various organisms.

Selective inhibition of herpes simplex virus type-1 uracil-DNA glycosylase by designed substrate analogs

Cytosine deamination and the misincorporation of 2'-dUrd into DNA during replication result in the presence of uracil in DNA. Uracil-DNA glycosylases (UDGs) initiate the excision repair of this aberrant base by catalyzing the hydrolysis of the N-glycosidic bond. UDGs are expressed by nearly all known organisms, including some viruses, in which the functional role of the UDG protein remains unresolved. This issue could in principle be addressed by the availability of designed synthetic inhibitors that target the viral UDG without affecting the endogenous human UDG. Here, we report that double-stranded and single-stranded oligonucleotides incorporating either of two dUrd analogs tightly bind and inhibit the activity of herpes simplex virus type-1 (HSV-1) UDG. Both inhibitors are exquisitely specific for the HSV-1 UDG over the human UDG. These inhibitors should prove useful in structural studies aimed at understanding substrate recognition and catalysis by UDGs, as well as in elucidating the biologic role of UDGs in the life cycle of herpesviruses.

Purification and characterization of a cold-adapted uracil-DNA glycosylase from Atlantic cod (Gadus morhua)

Uracil-DNA glycosylase (UDG; UNG) has been purified 17000-fold from Atlantic cod liver (Gadus morhua). The enzyme has an apparent molecular mass of 25 kDa, as determined by gel filtration, and an isoelectric point above 9.0. Atlantic cUNG is inhibited by the specific UNG inhibitor (Ugi) from the Bacillus subtilis bacteriophage (PBS2), and has a 2-fold higher activity for single-stranded DNA than for double-stranded DNA. cUNG has an optimum activity between pH 7.0-9.0 and 25-50 mM NaCl, and a temperature optimum of 41 degrees C. Cod UNG was compared with the recombinant human UNG (rhUNG), and was found to have slightly higher relative activity at low temperatures compared with their respective optimum temperatures. Cod UNG is also more pH- and temperature labile than rhUNG. At pH 10.0, the recombinant human UNG had 66% residual activity compared with only 0.4% for the Atlantic cUNG. At 50 degrees C, cUNG had a half-life of 0.5 min compared with 8 min for the rhUNG. These activity and stability experiments reveal cold-adapted features in cUNG.

Pea chloroplast glyceraldehyde-3-phosphate dehydrogenase has uracil glycosylase activity

Pea (Pisum sativum) chloroplastic glyceraldehyde-3-P dehydrogenase (EC 1.2.1.13) was tested for uracil DNA glycosylase activity. It was found that both the chloroplast and the recombinant subunit B dehydrogenases remove uracil from poly(dA[3H]dU). The glycosylase activity of the recombinant subunit B enzyme and that of a truncated form corresponding in length to subunit A were associated with the dehydrogenase activity in gel-filtration experiments. Both activities of the chloroplast enzyme were inhibited by antisera raised against recombinant subunit B, and both activities of the recombinant subunit B enzyme were inhibited by antisera raised against pea chloroplast glyceraldehyde-3-P dehydrogenase. Antisera raised against Escherichia coli uracil glycosylase did not affect the glycosylase activity of the recombinant subunit B enzyme. The glycosylase pH activity profile of the chloroplast dehydrogenase was unique. It is distinct from the dehydrogenase pH activity profile and from the pH activity profiles of other plant glycosylases. The glycosylase activity, but not the dehydrogenase activity, of the recombinant subunit B enzyme was inhibited by uracil. Pyridine nucleotides stimulated the glycosylase activity. To our knowledge this is the first example of a nonhuman glyceraldehyde-3-P dehydrogenase, and of an NADP-dependent glyceraldehyde-3-P dehydrogenase, that exhibits uracil glycosylase activity.

Induction of fragile sites by fluorodeoxyuridine and caffeine accompanies with misincorpolation of endogenous uridine nucleotide into DNA of feline fibroblasts

Fluorodeoxyuridine, an inhibitor of thymidylate synthetase, is known to induce chromosomal fragile sites. The drug treatment may cause deprivation of intracellular thymidine nucleotide pool followed by a serious imbalance of deoxynucleotide pool. Though the stress is probably related to the induction of folate-sensitive fragile sites, the exact mechanism is still to be investigated. The present study has been carried out to test the possibility that the fragile sites are originated, at least in part, from incorpolated uracil residues. The incorpolated uracil residue can be detected by a novel assay for abasic sites after treatment with uracil-DNA N-glycosylase (UDG). About 2.7 abasic sites per 10(4) nucleotides were detected in the DNA extracted from feline fibroblasts after the treatment with FUdR and caffeine. By digesting the DNA with UDG prior to the assay, significant increase in the number of abasic sites were observed. These results indicate that the large amount of uracil residues are present in the feline fibroblast DNA under the condition which induces chromosomal fragile sites.

Formation, detection and repair of AP sites

The paper is an outline review of the main aspects concerning the formation and repair of AP (apurinic/apyrimidinic) sites in DNA as well as some of the chemical properties allowing their quantitative determination. A new method for the measurement of AP sites based on their reaction with [14C]methoxyamine is described. It has been applied to the measurement of AP sites produced in DNA either by physical (gamma-rays) or chemical (methyl methanesulphonate, osmium tetroxide) agents. The method has also been used to quantify the excision of abnormal bases from DNA under the action of specific DNA glycosylases and to prevent the chemical or enzymatic degradation of DNA containing AP sites. The paper contains data about the purification and characterization of uracil-DNA glycosylase and AP endodeoxyribonuclease from carrot cells, two enzymes involved in the first steps of base excision repair through AP site intermediates. The biological effects of unrepaired AP sites are also discussed.

The incorporation of deoxyuridine monophosphate into DNA increases the sister-chromatid exchange yield

The effect of a treatment with 5-fluoro-2'-deoxyuridine (FdUrd) in combination with 2'-deoxyuridine (dUrd) on cell proliferation, incorporation of DNA precursors into DNA and sister-chromatid exchanges (SCEs) has been analyzed in Allium cepa meristem cells. FdUrd in the range 10(-9)-5 X 10(-7) M produced a dose- and time-dependent decrease in the amount of cells in mitosis. This inhibitory effect could be reversed by 70-80% in short-term (6 h) experiments, by exogenously supplied dUrd at a concentration of 10(-4) M. However, at the highest FdUrd dose tested (10(-7) M), 10(-4) M dUrd could not reverse the FdUrd effect in long-term experiments (20 h, about one cell cycle interval), as shown by analyzing the kinetics of synchronous cell populations. DNA extracted from cells pulsed with [6-3H]dUrd in the presence of FdUrd and 6-amino-uracil (6-AU), an inhibitor of uracil-DNA glycosylase, contained a small amount of label (at least 3% of the total radioactivity incorporated into DNA) in the form of [6-3H]dUMP. Thus, we conclude that, under our experimental conditions, exogenously supplied dUrd may be metabolized intracellularly to 2'-deoxyuridine triphosphate (dUTP) and that this deoxynucleotide may eventually be mis-incorporated into DNA. As far as the formation of SCEs is concerned, analysis of second division chromosomes showed that 2'-deoxyuridine monophosphate (dUMP) residues present in newly-synthesized DNA strands are probably not relevant to SCE formation. However, by analyzing SCE levels in third division chromosomes of cells treated with FdUrd and dUrd during their second cycle, we have scored a 6-fold increase in the reciprocal SCE level which demonstrates that the replication of a dUMP-containing DNA template leads to a higher SCE yield.

Uracil-DNA glycosylase activity from Dictyostelium discoideum

We have isolated and partially characterized a uracil-DNA glycosylase activity from the cellular slime mold, Dictyostelium discoideum. This glycosylase has a broad pH optimum (6.5-8.5) and is fully active in 10 mM EDTA or in 5 mM Mg2+. Its molecular weight by gel filtration is about 55 000. This enzyme activity may work in concert with previously described apurinic/apyrimidinic (AP) endonuclease activities in the excision repair of uracil from the DNA of this lower eukaryote.

Inhibition of uracil-DNA glycosylase increases SCEs in BrdU-treated and visible light-irradiated cells

We have approached the study of the ability of different types of lesions produced by DNA-damaging agents to develop sister-chromatid exchanges (SCEs) by analyzing SCE levels observed in Allium cepa L cells with BrdU-substituted DNA and exposed to visible light (VL), an irradiation which produces uracil residues in DNA after debromination of bromouracil and enhances SCE levels but only above a certain dose. We have partially purified an uracil-DNA glycosylase activity from A. cepa L root meristem cells, which removes uracil from DNA, the first step in the excision repair of this lesion. This enzyme was inhibited in vitro by 6-amino-uracil and uracil but not by thymine. When cells exposed to VL, at a dose that did not produce per se an SCE increase, were immediately post-treated with these inhibitors of uracil-DNA glycosylase, a significant increase in SCE levels was obtained. Moreover, SCE levels in irradiated cells dropped to control level when a short holding time (less than 15 min) elapsed between exposure to VL and the beginning of post-treatment with the inhibitor. Thus, our results (1) showed that inhibitors of uracil-DNA glycosylase enhanced SCE levels in cells with unifilarly BrdU-substituted DNA exposed to visible light; (2) pointed to uracils and/or to some products of their repair as lesions responsible for SCE formation under our experimental conditions; and (3) indicated the existence of a very rapid repair of SCE-inducing lesions produced by visible light irradiation of cells with unifilarly BrdU-containing DNA.

The cloning and overproduction of Escherichia coli uracil-DNA glycosylase

Escherichia coli cells containing elevated levels of the DNA repair enzyme uracil-DNA glycosylase (the ung gene product) have been constructed by in vitro recombination methods. First, lambdanadB transducing phages were isolated from two E. coli DNA libraries by selection of nicotinate-independent lysogens. lambdanadB phage from one of the libraries were also ung+ and carried the ung-nadB genes on an 8.3-kb HindIII restriction fragment. The ung and nadB genes were subcloned into plasmids and a restriction map of the ung region of the E. coli chromosome was constructed. The uracil glycosylase gene was localized to a 1.4-kb restriction fragment by subcloning the gene into pBR322. Uracil glycosylase was overproduced (relative to the specific activity of wild type cells) by about two-fold in lambdaung lysogens and by 15- to 20-fold in cells containing pBR322ung derivatives. When the ung gene and its promoter were placed downstream from the bacteriophage lambdaPL promoter in the plasmid pKC30, uracil glycosylase production was heat-induced to more than 100-fold above the levels of a wild-type cell. By relating the insertion orientation of the lambdaung gene in the plasmid pKC30 to its orientation in lambdaung-nadB transducing phages, the transcription direction of the ung gene on the E. coli linkage map was found to be clockwise.

The occurrence and consequences of deoxyuridine in DNA

Deoxyuridine can become resident in the DNA of prokaryotic and eukaryotic cells via two general mechanisms - deamination of cytosine to uracil, and nucleotide pool changes that lead to misincorporation of deoxyuridine in place of thymidine. In this paper we have examined the chemical basis of deamination reactions in DNA and discussed a possible mechanism for an increased rate of deamination by means of cross-strand protonation of cytosine by alkylated guanine. In addition, we have examined the genetic and drug-induced conditions that lead to dUMP misincorporation into DNA in place of thymidine and have presented experimental evidence indicating that the antifolate-induced lesion is a general drug-dose dependent lesion of human blood cells. Finally, the toxic and genetic impact of this lesion has been evaluated within the context of a review of the repair mechanisms elicited by dUMP in DNA.