Effects of O6-benzylguanine on growth and differentiation of P19 embryonic carcinoma cells treated with alkylating agents

The purpose of this study was to evaluate the impact of modulating the repair of O6-alkylguanine adducts on the developmental toxicity of alkylating agents. Alkylating agents that have been shown to induce developmental toxicity following either in vitro or in vivo exposure were chosen for this investigation, and include methylnitrosourea (MNU), ethylnitrosourea (ENU), methyl methanesulfonate (MMS), and ethyl methanesulfonate (EMS). P19 cells are pluripotent murine embryonic carcinoma cells that can be induced by all trans retinoic acid (RA) to differentiate into cells that are biochemically and morphologically very similar to cells of the central nervous system. These cells are useful for studying the ability of chemicals to affect neuronal viability and differentiation. Neuronally differentiating P19 cells were pretreated with O6-benzylguanine (O6-Bg), a potent and specific inhibitor of the O6-alkylguanine-DNA-alkyltransferase (AT) protein that repairs lesions at the O6-position of guanine. In previous studies using micromass rat embryo midbrain cells, O6-Bg greatly potentiated the ability of MNU but not ENU to inhibit differentiation, and did not significantly alter the effects of either MNU or ENU on viability. In the P19 cells, we found that AT inhibition potentiated the effects of MMS, MNU, and EMS to inhibit both viability and differentiation. Additionally, AT inhibition had a much greater effect on toxicity of the methylating agents, as compared to the ethylating agents. These results suggest that O6-alkylguanine adducts can inhibit both viability and differentiation in P19 cells treated with alkylating agents.

The lyase activity of the DNA repair protein beta-polymerase protects from DNA-damage-induced cytotoxicity

Small DNA lesions such as oxidized or alkylated bases are repaired by the base excision repair (BER) pathway. BER includes removal of the damaged base by a lesion-specific DNA glycosylase, strand scission by apurinic/apyrimidinic endonuclease, DNA resynthesis and ligation. BER may be further subdivided into DNA beta-polymerase (beta-pol)-dependent single-nucleotide repair and beta-pol-dependent or -independent long patch repair subpathways. Two important enzymatic steps in mammalian single-nucleotide BER are contributed by beta-pol: DNA resynthesis of the repair patch and lyase removal of 5'-deoxyribose phosphate (dRP). Fibroblasts from beta-pol null mice are hypersensitive to mono-functional DNA-methylating agents, resulting in increases in chromosomal damage, apoptosis and necrotic cell death. Here we show that only the dRP lyase activity of beta-pol is required to reverse methylating agent hypersensitivity in beta-pol null cells. These results indicate that removal of the dRP group is a pivotal step in BER in vivo. Persistence of the dRP moiety in DNA results in the hypersensitivity phenotype of beta-pol null cells and may signal downstream events such as apoptosis and necrotic cell death.

Chlorophyllin [CHLN] and the mutagenicity of monofunctional alkylating agents in Drosophila: the action of CHLN need not include an influence on metabolic activation

The effect of chlorophyllin (CHLN) on the mutagenicity of four monofunctional alkylating agents (MFAAs) was evaluated in the wing spot test in Drosophila. Three of the compounds are direct-acting (ethylnitrosamine (ENU), methylnitrosourea (MNU), and methylmethanesulfonate (MMS)) and one indirect-acting (diethylnitrosamine, DEN). Results indicate that the mutagenicity of all four compounds is strongly inhibited by CHLN. The findings are not in agreement with the conclusion of Romert et al. (1992) that CHLN has no effect on the mutagenicity of direct acting MFFAs inferred from their work with MNU and ethylmethanesulfonate (EMS) in the V79 and Salmonella in vitro test systems. The results suggest the possibility that the action of CHLN need not include an inhibiting effect on metabolic activation.

Ras-related GTPase RhoB forces alkylation-induced apoptotic cell death

rhoB encoding a Ras-related GTPase is immediate-early inducible by genotoxic treatments. To address the question of the physiological role of RhoB in cellular defense, cells stably overexpressing wild-type RhoB protein were generated. Overexpression of RhoB renders cells hypersensitive to the killing effect of alkylating agents including antineoplastic drugs but not to UV-light and doxorubicin. As compared to control cells, RhoB overexpressing cells revealed an increase in the frequency of alkylation-induced apoptotic cell death. This indicates that RhoB is involved in modulating apoptotic signaling. Furthermore, overexpression of RhoB resulted in a prolonged transient block to DNA replication upon MMS treatment. UV-induced replication blockage was not affected by RhoB. Based on the data we suggest RhoB to be a novel regulatory factor which takes influence on the level of cytotoxicity of DNA damaging drugs and forces cells to alkylation-induced apoptosis. The data indicate that this might be due to RhoB mediated delay in cell cycle progression upon alkylation treatment.

Application of yeast cells transformed with GFP expression constructs containing the RAD54 or RNR2 promoter as a test for the genotoxic potential of chemical substances

Yeast strains transformed with high copy number plasmids carrying the gene encoding a green fluorescent protein optimised for yeast (yEGFP3) under the control of the RAD54 or RNR2 promoter were used to investigate the activity of potentially DNA-damaging substances. The assays were performed on 96-well microtitre plates in the presence of different concentrations of the test substances. The synthesis of GFP protein was measured through the fluorescence signal and cell growth was monitored by absorption. Here, we demonstrate that this system can be used as a biosensor to assess the genotoxic potential of drugs and other chemical substances. The use of microtitre plates will enable full automation of the system and allows the inclusion of internal reference standards in each assay.

Interference of tannic acid on the genotoxicity of mitomycin C, methylmethanesulfonate, and nitrogen mustard in somatic cells of Drosophila melanogaster

The modulating effects of tannic acid (TA) on somatic mutation and mitotic recombination induced by methylmethanesulfonate (MMS), nitrogen mustard (HN2), and mitomycin C (MMC) were evaluated in the standard (ST) cross of the wing spot test in Drosophila melanogaster using co- and posttreatment protocols. It was shown that TA alone did not modify the spontaneous frequencies of single and twin spots, which means that this polyphenol neither acts as a genotoxin nor exerts any antigenotoxic effect over spontaneous DNA lesions. However, the simultaneous administration of genotoxins with TA can lead to considerable alterations of the frequencies of induced wing spots in comparison to those with administration of the genotoxins alone. In fact, TA produced a significant increase in HN2-induced wing spots with enhancements between 90 and 160%. For MMS, the enhancement was 38% in the highest TA concentration tested. In contrast, a significant protective action of this polyphenol was observed in combined treatments with MMC (64 to 99% inhibition). Moreover, the data from TA posttreatments demonstrated that this agent is not effective in exerting protective or enhancing effects on the genotoxicity of MMS, HN2, or MMC. One feasible mechanism of TA action is its interaction with the enzyme systems catalyzing the metabolic detoxification of MMS and HN2, which may also be involved in the bioactivation of MMC.

A novel method for the parallel monitoring of mitotic recombination and clastogenicity in somatic cells in vivo

Both homologous mitotic recombination (HMR), causing loss of heterozygosity (LOH) of the wild-type allele, and structural chromosome aberrations (CA) involve the formation of double-strand breaks in DNA. Whether the induction of CAs is always accompanied by HMR, or whether there exist DNA lesions specifically forming only one of the two end-points is unknown. Answering this fundamental question requires a system for the parallel detection of CAs and HMR, because only then is their analysis under strictly identical condition (dose, repair, genetic background) possible. We describe here a novel system for the parallel detection of HMR and loss of a whole chromosome as a measure of CA, utilizing somatic cells of Drosophila. In haploid germ cells of Drosophila, loss of a ring-shaped X-chromosome (rX) constitutes a frequent event providing an efficient method for measuring clastogenicity. For somatic cells, however, it was unclear whether the development of such a system would be feasible. The generally accepted notion has been that in XX female genotypes, loss of an entire X-chromosome acts as a cell lethal when generated at or shortly after blastoderm stage. However, here we show that rX-loss, if induced in pre-ommatidia cells of 3rd instar larvae, generates viable clones visible as small white patches in the red compound eye. To set up optimal conditions for the detection and quantification of rX-loss compared to HMR, several protocols were developed and tested against model carcinogens (methyl methanesulfonate, cisplatin and 7,12-dimethylbenz[a]anthracene). Generally, we find striking differences in the efficiency of these carcinogens for recombination when compared with clastogenicity. The cross-linking agent cisplatin is 4- to 6-fold more clastogenic than recombinagenic. 7,12-Dimethylbenz[a]-anthracene, on the contrary, produced less than a doubling effect for rX-loss but was highly active (20-times the background) for HMR. It appears therefore that both processes can be separated from each other. To the best of our knowledge, this is the first report suggesting, in terms of DNA adducts involved, qualitative differences between homologous recombination and clastogenic effects. Application of our system for studies on DNA repair may therefore provide new insight into the linkage of repair pathways in either of the two mechanisms.

Spontaneous and mutagen-induced transformation of primary cultures of Msh2-/- p53-/- colonocytes

Loss of function of mismatch repair (MMR) genes underlies hereditary nonpolyposis colorectal cancer (HNPCC). However, the inability to maintain primary colon epithelial cells in culture has limited the analysis of the contribution of MMR gene defects to colorectal tumorigenesis. We have now established primary cultures of epithelial cells from the colon crypts of Msh2-/- p53-/- double-knockout mice. These cells undergo spontaneous transformation (soft agar colonies and s.c. tumor formation), with a progressively shorter latency as a function of increasing passages in culture. Treatment of early passage cells with the mutagen methylmethane thiosulfonate (MMS) further decreases the transformation latency of Msh2-/- p53-/- cells. Spontaneous transformation of p53-/- colonocytes is only observed using late passage cells, and methylmethane thiosulfonate-treated early passage p53-/- colonocytes do not form tumors when injected into immunodeficient mice. Together, these findings support the pathogenic role of MMR gene inactivation in colorectal tumorigenesis and provide an experimental model for the serial assessment of the molecular phenotype associated with Msh2 deficiency.

The influence of temperature during alkaline treatment and electrophoresis on results obtained with the comet assay

The alkaline comet assay (single-cell gel electrophoresis) is becoming established as a genotoxicity test with many fold applications in vitro and in vivo. While the underlying principles are identical, various modifications of the method are in use which clearly affect the sensitivity and resolving power of the assay. One variable of potential importance that has been disregarded until now is temperature during alkaline treatment and electrophoresis. We therefore performed comet assay experiments with human blood and V79 Chinese hamster cells using two different temperatures (4 and 20 degrees C, i.e. room temperature) during alkaline treatment and electrophoresis. DNA damage was induced by the two standard mutagens gamma irradiation and methyl methanesulfonate (MMS). The results clearly indicate significant differences in the detection of background and mutagen-induced DNA damage at these two temperatures. Under otherwise identical test conditions (including the duration of alkaline treatment and electrophoresis), increased temperature during alkaline treatment and electrophoresis strongly enhances DNA migration. Our findings suggest that the comet assay should be performed under strictly controlled and reproducible temperature conditions. In any case the temperature during alkaline treatment and electrophoresis should be stated in a publication to allow for a critical evaluation of results obtained with the comet assay.

Modulation of chemical carcinogen-induced unscheduled DNA synthesis by dehydroepiandrosterone (DHEA) in the primary rat hepatocytes

Modulation of unscheduled DNA synthesis by dehydroepiandrosterone (DHEA) after exposure to various chemical carcinogens was investigated in the primary rat hepatocytes. Unscheduled DNA synthesis was induced by treatment of such direct acting carcinogens as methyl methanesulfonate (MMS) and ethyl methanesulfonate (EMS) or procarcinogens including benzo(a)pyrene (BaP) and 7,12-dimethylbenz(a)anthracene (DMBA). Unscheduled DNA synthesis was determined by measuring [methyl-3H]thymidine radioactivity incorporated into nuclear DNA of hepatocytes treated with carcinogens in the presence or absence of DHEA. Hydroxyurea (5x10(-3) M) was added to growth medium to selectively suppress normal replication. DHEA at concentrations ranging from 1x10(-6) M to 5x10(-4) M did not significantly inhibit unscheduled DNA synthesis induced by either MMS (1x10(-4) M) or EMS (1x10(-2) M). In contrast, DHEA significantly inhibited unscheduled DNA synthesis induced by BaP (6.5x10(-5) M) and DMBA (2x10(-5) M). DHEA-induced hepatotoxicity in rats was examined using lactate dehydrogenase (LDH) release as an indicator of cytotoxicity. DHEA exhibit no significant increase in LDH release compared with the solvent control at 18 h. These data suggest that nontoxic concentration of DHEA does not affect the DNA excision repair process, but it probably influence the enzymatic system responsible for the metabolic activation of procarcinogens and thereby decreases the amount of the effective DNA adducts formed by the ultimate reactive carcinogenic species.

The Drosophila melanogaster DmRAD54 gene plays a crucial role in double-strand break repair after P-element excision and acts synergistically with Ku70 in the repair of X-ray damage

The RAD54 gene has an essential role in the repair of double-strand breaks (DSBs) via homologous recombination in yeast as well as in higher eukaryotes. A Drosophila melanogaster strain deficient in the RAD54 homolog DmRAD54 is characterized by increased X-ray and methyl methanesulfonate (MMS) sensitivity. In addition, DmRAD54 is involved in the repair of DNA interstrand cross-links, as is shown here. However, whereas X-ray-induced loss-of-heterozygosity (LOH) events were completely absent in DmRAD54(-/-) flies, treatment with cross-linking agents or MMS resulted in only a slight reduction in LOH events in comparison with those in wild-type flies. To investigate the relative contributions of recombinational repair and nonhomologous end joining in DSB repair, a DmRad54(-/-)/DmKu70(-/-) double mutant was generated. Compared with both single mutants, a strong synergistic increase in X-ray sensitivity was observed in the double mutant. No similar increase in sensitivity was seen after treatment with MMS. Apparently, the two DSB repair pathways overlap much less in the repair of MMS-induced lesions than in that of X-ray-induced lesions. Excision of P transposable elements in Drosophila involves the formation of site-specific DSBs. In the absence of the DmRAD54 gene product, no male flies could be recovered after the excision of a single P element and the survival of females was reduced to 10% compared to that of wild-type flies. P-element excision involves the formation of two DSBs which have identical 3' overhangs of 17 nucleotides. The crucial role of homologous recombination in the repair of these DSBs may be related to the very specific nature of the breaks.

Processing of DNA damage induced by hydrogen peroxide and methyl methanesulfonate in human lymphocytes: analysis by alkaline single cell gel electrophoresis and cytogenetic methods

The persistence of induced DNA damage in human lymphocytes after mitogen stimulation and its relationship to subsequent cytogenetic alterations were investigated. The analysis of single-strand breaks and alkali-labile sites by single cell gel electrophoresis (SCGE) showed the almost complete repair of damage induced in resting lymphocytes by methyl methanesulfonate (MMS, 140-210 microM) and hydrogen peroxide (H(2)O(2), 25-100 microM) during the first 16 h of culture. On the other hand, DNA damage was shown to persist to a large extent when cells were cultured in the presence of the repair inhibitor cytosine beta-D-arabinofuranoside (Ara-C) (1 microg/ml). Although highly effective in the induction of DNA lesions detectable by SCGE, both agents failed to significantly increase the rate of micronucleus formation in cytokinesis-blocked cells harvested 66 h after treatment. However, when Ara-C was present during the first 16 h of culture, micronuclei were significantly increased at all doses. Conversely, sister chromatid exchange (SCE) rates were increased by chemical treatments to a higher extent in cultures without Ara-C. Delayed treatments, 16 h after mitogen stimulation, led to a significant induction of micronuclei in the case of MMS but not with H2O(2). These results suggest that only a minor fraction of DNA damage induced in resting lymphocytes is available for fixation through misreplication, because of its effective repair prior to S phase. However, the processing of damage through recombination pathways can lead to increased SCE rates in treated cells. These features of the processing of DNA damage in human lymphocytes should be taken into account when structural cytogenetic alterations in cultured lymphocytes are used in monitoring human exposure to genotoxic agents.

Study of a rat skin in vivo micronucleus test: data generated by mitomycin C and methyl methanesulfonate

We have developed an in vivo micronucleus (MN) test that uses rat skin as the target organ. Sample preparation involves cold-treating the epidermis with trypsin, peeling it off with a fine forceps, treating it in hypotonic solution, and staining it with acridine orange (A.O.). We evaluated the assay using mitomycin C (MMC) and methyl methanesulfonate (MMS) as model clastogens, applying them as single and repeat treatments. Both chemicals induced a significant, dose-dependent increase in MN frequency in basal cells. One treatment per day for 3 days was optimal for MN induction.

Development of a new bioluminescent mutagenicity assay based on the Ames test

A newly developed rapid mutagenicity assay based on the adenosine triphosphate (ATP)-bioluminescence technique and the Ames test is described. Salmonella typhimurium strains TA98 and TA100 were exposed in an appropriate liquid medium to the direct mutagens 4-nitroquinoline-N-oxide and methyl methanesulphonate, respectively, and to the indirect mutagen 2-aminoanthracene. Both auxotrophic and prototrophic growth were monitored throughout the incubation period as variations in the intracellular ATP levels by means of the luciferin-luciferase assay. After 9-12 h of incubation a dose-response increase in the levels of ATP was readily detected. In order to demonstrate that this increase was due to the growth of revertant bacteria, aliquots from each culture were plated on minimal agar plates. A very good correlation between the changes in ATP levels and the appearance of revertant colonies on the plates was found. Given the rapidity of this method as compared with conventional mutagenicity assays, it has potential for industrial and environmental applications. Other potential applications are also discussed.

Evaluation of transcriptional fusions with green fluorescent protein versus luciferase as reporters in bacterial mutagenicity tests

A bacterial plasmid was constructed on which the regulatory region of the umuC gene of Escherichia coli was fused to the coding sequence of the green fluorescent protein gene (gfp) from the jellyfish Aequorea victoria. Escherichia coli AB1157 strains carrying the plasmid emitted fluorescence in the presence of mutagens that induce the SOS DNA repair system. Data on tests with nitrosoguanidine, methylmethane sulphonate and UV radiation (254 nm) are presented. Although fluorescent detection using this system was not as rapid or sensitive as a similar luminescent equivalent (umuC-luxAB), the gfp reporter system was more robust. Escherichia coli umu gene induction was also analysed in Salmonella typhimurium TA1537 cells following plasmid transfer and exposure to the same range of mutagens. There was no significant difference in sensitivity between the two species. These preliminary results will provide the basis for development of mutagenicity test systems useful in the testing of complex mixtures, such as environmental samples, and the investigation of physiological parameters influencing spontaneous mutagenesis in bacteria.

Methyl methanesulfonate and hydrogen peroxide differentially regulate p53 accumulation in hepatoblastoma cells

Genotoxic chemicals not only damage cellular DNA, but may also induce cell apoptosis if they are lethal to the cell. p53, Bcl-2 and Bax play important roles in the regulation of genotoxic chemical induced cell apoptosis. Since the mechanisms by which cellular DNA damaged by different DNA-damaging chemicals may not be the same, we studied the involvement of p53, Bcl-2 and Bax in apoptosis induced by methyl methanesulfonate (MMS) and hydrogen peroxide (H2O2). H2O2 damages DNA by free radical generation and MMS damages DNA by DNA methylation. At non-lethal doses, both H2O2 and MMS induced high level of p53 protein accumulation. Nevertheless, while the amount of p53 protein increased with the dose of MMS and the occurrence of apoptotic cell death events, H2O2 doses that induce cell apoptosis attenuated the p53 protein accumulation level. Lethal MMS treatment also increased Bax, but not Bcl-2 expression, whereas in H2O2 induced apoptosis, the level of both Bcl-2 and Bax declined. These results indicate that toxic chemicals differentially regulate the accumulation of p53 protein. Thus, the pathways of toxic chemicals induced cell apoptosis are different and independent.

Correcting for toxic inhibition in quantification of genotoxic response in the umuC test

An improved procedure for quantification of results from the umuC tests for genotoxicity is presented. The calculation method better separates toxic growth inhibition (cytotoxicity) from genotoxic effects than currently used methods and therefore, greatly extends the applicability of genotoxicity tests on environmental samples. The basic principle is to normalize the genotoxic response compensating for both decreasing biomass and growth rate reduction that results from cytotoxicity. The improved method and the currently used method was compared for umuC tests on the pure compounds: methylmethanesulfonate (MMS), N-methyl-N'-nitro-N-nitroguanidine (MNNG), sodium azide (NaN3), and 4-nitroquinoline-1-oxide (4-NQO). For compounds with no or low cytotoxicity, the two calculation methods gave practically identical results, while for highly cytotoxic compounds, the traditional method overestimated genotoxicity. umuC tests were also carried out on leachate polluted groundwater sampled downgradient of a landfill (Grindsted, Denmark). All polluted samples showed high cytotoxicity concomitant with high genotoxicity when the results were quantified in the traditional way. The new method showed that these results were in fact false positive, as the apparent genotoxicity was a result of cytotoxicity. Based on the mathematical analysis leading to the improved procedure for correction for cytotoxicity, it is suggested to alter the present test design of the umuC test in order to obtain well-defined exposure concentrations as well as mathematical consistency in the quantification of genotoxicity.

Inhibitory effect of nordihydroguaiaretic acid on the frequency of micronuclei induced by methyl methanesulfonate in vivo

Nordihydroguaiaretic acid (NDGA) is an antioxidant originally obtained from plants of the genus Larrea. This chemical has shown antigenotoxic activity measuring gene mutations and sister-chromatid exchanges. The aim of this investigation was to determine if NDGA is also an antigenotoxic agent and can inhibit the induction of micronucleus (MN) formation by methyl methanesulfonate (MMS) in mouse. The frequency of micronucleated polychromatic erythrocytes (MPE) was scored for 4 days, and a MN induction curve by a single injection of MMS (40 mg/kg) was obtained. The results of this experiment showed that the highest MN incidence was reached at the second day of exposure with a mean of 13.2%+/-1.0. This value is more than 4 times the control mean. Thus, the modulatory study by NDGA was established at a 2-day exposure time using three doses (6.0, 11.0, and 17.0 mg/kg) against the damage induced by 40 mg/kg of MMS. The results of this study showed a significant reduction of the clastogenic damage at the two highest doses, where the inhibitory values corresponded to 62.2% and 66.7%, respectively. With respect to the ratio polychromatic erythrocytes/normochromatic erythrocytes, a marked toxicity was detected with 2 days of MMS exposure; however, the combination of the two high doses of NDGA plus MMS significantly reduced the cytotoxic damage produced by MMS alone.

Modulation of the toxic and mutagenic effects induced by methyl methanesulfonate in Chinese hamster ovary cells by overexpression of the rat N-alkylpurine-DNA glycosylase

Exposure of mammalian cells to alkylating agents causes transfer of alkyl groups to N- as well as O-atoms of DNA bases. Especially the O-alkylated G and T bases have strong mutagenic properties, since they are capable of mispairing during replication. The mutagenic potential of N-alkylbases is less clear although specific base excision repair (BER) pathways exist which remove those lesions from the DNA. We investigated the relative contribution of N-alkylations to mutation induction at the Hprt gene in cultured Chinese hamster ovary cells (CHO). To this end BER activity in CHO cells was modulated by introduction of an expression vector carrying the rat N-alkylpurine-DNA glycosylase (APDG) gene, which codes for a glycosylase that is able to remove 3-methyladenine and 7-methylguanine from DNA thereby generating apurinic sites. Upon selection of a CHO clone which 10 times overproduced APDG compared to control CHO cells, mutation induction, the mutational spectrum, and cell survival were determined in both cell lines following treatment with methyl methanesulfonate (MMS). The results show that over-expression of APDG renders CHO cells more sensitive for mutation induction as well as cytotoxicity induced by MMS. The involvement of apurinic sites in induction of base pair changes at positions where 3-methyladenine was induced is inferred from the observation that the mutational spectrum of MMS-induced mutations in APDG-CHO cells showed twice as much base pair changes at AT base pairs (33.3%) compared to the spectrum of MMS-induced mutations in CHO-control cells (15.8%).

Cells deficient in DNA polymerase beta are hypersensitive to alkylating agent-induced apoptosis and chromosomal breakage

DNA polymerase beta (beta-pol), which is involved in base excision repair, was investigated for its role in protection of cells against various genotoxic agents and cytostatic drugs using beta-pol knockout mouse fibroblasts. We show that cells lacking beta-pol are highly sensitive to induction of apoptosis and chromosomal breakage by methylating agents, such as N-methyl-N'-nitro-N-nitrosoguanidine and methyl methanesulfonate and the cross-linking antineoplastic drugs mitomycin C and mafosfamide. The cross-sensitivity between the agents observed suggests that beta-pol is involved in repair not only of DNA methylation lesions but also of other kinds of DNA damage induced by various cytostatic drugs. Cells deficient in beta-pol were not hypersensitive to cisplatin, melphalan, benzo(a)pyrene diol epoxide, chloroethylnitrosourea, or UV light. Because both established and primary beta-pol knockout fibroblasts displayed the hypersensitive phenotype, which, moreover, was complemented by transfection with a beta-pol expression vector, the alkylating agent hypersensitivity can clearly be attributed to the beta-pol deficiency. The results demonstrate that beta-pol-driven base excision repair is highly important for protection of cells against cell killing due to apoptosis and induced chromosomal breakage and suggest that incompletely repaired DNA damage causes chromosomal changes and may act as a trigger of DNA damage-induced apoptosis.

Analysis of the Bacillus subtilis recO gene: RecO forms part of the RecFLOR function

The deduced protein product of the Bacillus subtilis gene yqfI, which is 255 residues long, shares homology (25% identity) with the Escherichia coli RecO protein. A null allele of yqfI, when present in an otherwise Rec+ B. subtilis strain, causes cells to become highly sensitive to DNA-damaging agents, and plasmid transformation (intramolecular recombination) is reduced by 25-fold while chromosomal transformation (intermolecular recombination) is only moderately affected (2.5-fold reduction). Therefore, the yqfI gene was renamed recO and its null allele is referred to as recO1. The recO1 mutation was introduced into recombination-deficient strains representative of the epistatic groups alpha (recF, recR and recL strains), beta (addA5 addB72), gamma (recH342) and epsilon (recU40). The recO mutation did not affect the sensitivity of recF, recR or recL cells to DNA-damaging agents, increased the sensitivity of recU and addAB cells and abolished the DNA repair capacity of recH cells. The recO mutation did not affect intermolecular recombination in recF, recL, recH or recU cells, but reduced (by about 9-fold) the incidence of intermolecular recombination in addAB cells. The recO mutation did not affect intramolecular recombination in the addAB, recU, recF or recL cells, but reduced it by about 75-fold in recH cells. The defects caused by the recO1 mutation can be partially suppressed by a common suppressor of the recF, recL and recR phenotypes. We therefore assigned recO to epistatic group alpha and predict that the RecO protein acts at the same stage of recombination as the RecF, RecL and RecR proteins, in a RecFLOR complex.

Involvement of Arabidopsis thaliana ribosomal protein S27 in mRNA degradation triggered by genotoxic stress

A recessive Arabidopsis mutant with elevated sensitivity to DNA damaging treatments was identified in one out of 800 families generated by T-DNA insertion mutagenesis. The T-DNA generated a chromosomal deletion of 1287 bp in the promoter of one of three S27 ribosomal protein genes (ARS27A) preventing its expression. Seedlings of ars27A developed normally under standard growth conditions, suggesting wild-type proficiency of translation. However, growth was strongly inhibited in media supplemented with methyl methane sulfate (MMS) at a concentration not affecting the wild type. This inhibition was accompanied by the formation of tumor-like structures instead of auxiliary roots. Wild-type seedlings treated with increasing concentrations of MMS up to a lethal dose never displayed such a trait, neither was this phenotype observed in ars27A plants in the absence of MMS or under other stress conditions. Thus, the hypersensitivity and tumorous growth are mutant-specific responses to the genotoxic MMS treatment. Another important feature of the mutant is its inability to perform rapid degradation of transcripts after UV treatment, as seen in wild-type plants. Therefore, we propose that the ARS27A protein is dispensable for protein synthesis under standard conditions but is required for the elimination of possibly damaged mRNA after UV irradiation.

Comparison of DNA damage in plants as measured by single cell gel electrophoresis and somatic leaf mutations induced by monofunctional alkylating agents

The use of single cell gel electrophoresis (SCGE) has recently been applied to plant systems. We optimized the experimental conditions for SCGE analysis using nuclei isolated from different tissues of intact plants. Concentration-response curves of genomic DNA migration were analyzed in intact plants treated with the monofunctional alkylating agents ethyl methanesulfonate (EMS), methyl methanesulfonate (MMS), N-ethyl-N-nitrosourea (ENU), and N-methyl-N-nitrosourea (MNU). These data were used to calibrate SCGE tail moment values to induced somatic mutation in plant leaves. We used a genotoxicity index to compare genomic DNA damage and the induction of somatic mutation in the leaf tissues. The rank order of the genotoxic potency of these alkylating agents assayed by SCGE was MNU >> MMS > ENU > EMS. The rank order for the mutagenic potency of these agents was MNU >> ENU congruent with MMS > EMS. The data demonstrate the utility of SCGE analysis in plant systems. The use of SCGE will permit a larger range of plants for use as in situ environmental monitors. Also, this approach may be used to search for crop plant germplasm accessions with enhanced genomic stability. We investigated whether the intragenomic distributions of DNA damage induced by these alkylating agents were uniform and random. When a plot of the ratio of the %tail DNA and tail length versus the concentration of the test mutagen was generated, the induced SCGE data deviated from a random distribution of genomic DNA damage.