Classifying mutagens as to their specificity in causing the six possible transitions and transversions: a simple analysis using the Salmonella mutagenicity assay

A deep dive into historical Ames study data for N-nitrosamine compounds

Mutagenicity data is a core component of the safety assessment data required by regulatory agencies for acceptance of new drug compounds, with the OECD-471 bacterial reverse mutation (Ames) assay most widely used as a primary screen to assess drug impurities for potential mutagenic risk. N-Nitrosamines are highly potent mutagenic carcinogens in rodent bioassays and their recent detection as impurities in pharmaceutical products has sparked increased interest in their safety assessment. Previous literature reports indicated that the Ames test might not be sensitive enough to detect the mutagenic potential of N-nitrosamines in order to accurately predict a risk of carcinogenicity. To explore this hypothesis, public Ames and rodent carcinogenicity data pertaining to the N-nitrosamine class of compounds was collated for analysis. Here we present how variations to the OECD 471-compliant Ames test, including strain, metabolic activation, solvent type and pre-incubation/plate incorporation methods, may impact the predictive performance for carcinogenicity. An understanding of optimal conditions for testing of N-nitrosamines may improve both the accuracy and confidence in the ability of the Ames test to identify potential carcinogens.

Mutations induced by Bleomycin, 4-nitroquinoline-1-oxide, and hydrogen peroxide in the rpoB gene of Escherichia coli: Perspective on Mutational Hotspots

We report the mutational spectra in a segment of the E. coli rpoB gene of bleomycin (BLEO), 4-nitroquinoline-1-oxide (NQO), and hydrogen peroxide (H₂O₂). We compare these spectra with those of other mutagens and repair deficient strains in the same rpoB system, and review the key elements determining mutational hotspots and outline the questions that remain unanswered. We consider three tiers of hotspots that derive from 1) the nature of the sequence change at a specific base, 2) the direct nearest neighbors and 3) some aspect of the larger sequence context or the local 3D-structure of segments of DNA. This latter tier can have a profound effect on mutation frequencies, even among sites with identical nearest neighbor sequences. BLEO is dependent on the SOS-induced translesion Pol V for mutagenesis, and has a dramatic hotspot at a single mutational site in rpoB. NQO is not dependent on any of the translesion polymerases, in contrast to findings with plasmids treated in vitro and transformed into E. coli. The rpoB system allows one to monitor both G:C -> A:T transitions and G:C -> T:A transversions at the same site in 11 cases, each site having the identical sequence context for each of the two mutations. The combined preference for G:C -> A:T transitions at these sites is 20-fold. Several of the favored sites for hydrogen peroxide mutagenesis are not seen in the spectra of BLEO and NQO mutations, indicating that mutagenesis from reactive oxygen species is not a major cause of BLEO or NQO mutagenesis, but rather specific adducts. The variance in mutation rates at sites with identical nearest neighbors suggests that the local structure of different DNA segments is an important factor in mutational hotspots.

Mutagen Synergy: Hypermutability Generated by Specific Pairs of Base Analogs

We tested pairwise combinations of classical base analog mutagens in Escherichia coli to study possible mutagen synergies. We examined the cytidine analogs zebularine (ZEB) and 5-azacytidine (5AZ), the adenine analog 2-aminopurine (2AP), and the uridine/thymidine analog 5-bromodeoxyuridine (5BrdU). We detected a striking synergy with the 2AP plus ZEB combination, resulting in hypermutability, a 35-fold increase in mutation frequency (to 53,000 × 10(-8)) in the rpoB gene over that with either mutagen alone. A weak synergy was also detected with 2AP plus 5AZ and with 5BrdU plus ZEB. The pairing of 2AP and 5BrdU resulted in suppression, lowering the mutation frequency of 5BrdU alone by 6.5-fold. Sequencing the mutations from the 2AP plus ZEB combination showed the predominance of two new hot spots for A·T→G·C transitions that are not well represented in either single mutagen spectrum, and one of which is not found even in the spectrum of a mismatch repair-deficient strain. The strong synergy between 2AP and ZEB could be explained by changes in the dinucleoside triphosphate (dNTP) pools.

IMPORTANCE

Although mutagens have been widely studied, the mutagenic effects of combinations of mutagens have not been fully researched. Here, we show that certain pairwise combinations of base analog mutagens display synergy or suppression. In particular, the combination of 2-aminopurine and zebularine, analogs of adenine and cytidine, respectively, shows a 35-fold increased mutation frequency compared with that of either mutagen alone. Understanding the mechanism of synergy can lead to increased understanding of mutagenic processes. As combinations of base analogs are used in certain chemotherapy regimens, including those involving ZEB and 5AZ, these results indicate that testing the mutagenicity of all drug combinations is prudent.

A comparative study of the sensitivities of Salmonella typhimurium strains TA 98, TA 100 and TA 102 to hospital waste waters

Hospitals are a release source of many chemical compounds in their wastewaters. In the present study Salmonella typhimurium strains TA 98, TA 100 and TA 102 were analyzed for their sensitivity to hospital waste waters. The results of the study showed that hospital waste waters consists of mutagens causing frame shift mutations and base pair substitutions and amongst the three strains used in this study, TA 102 was most effective which along with TA 98 can be used for quick assessment of genotoxicity of hospital waste waters prior to its discharge. Genotoxic potential of hospital waste waters from five major hospitals located in Jaipur and Delhi was studied. Such waste waters should be treated prior to their discharge. The results of this study call for further detailed study in this area of research.

A system for the rapid determination of the mutation spectrum in Escherichia coli

The first step toward elucidating the mutagenic effects of chemicals and pathways is to determine the specificity of the mutations generated spontaneously or in response to treatment with mutagens. We constructed a set of plasmid-encoded probes for the specific detection of each type of base substitution mutation. Using these probes, we were able to quickly determine both the mutation rate and the specificity of the mutations caused by different types of mutagens and mutagenic conditions. We also developed a PCR-based method to rapidly and robustly determine the mutation spectrum in response to various mutagenic samples in parallel. This system allows one to not only analyze the mutation specificity of various chemicals, but also to search for novel genetic elements that promote the specific mutation events.

Determination of hypersensitivity to genotoxic agents among Escherichia coli single gene knockout mutants

We have tested the KEIO collection of 3985 different viable single gene knockouts in Escherichia coli to identify genes whose loss increases sensitivity to one or more of six different chemotherapeutic agents and mutagens: Bleomycin (BLM), Cisplatin (CPT), ICR-191 (ICR), 5-azacytidine (5AZ), Zebularine (ZEB), and 5-bromo-2'-deoxyuridine (5BdU). We discovered a set of 156 strains that display a significant increase in sensitivity to at least one of the agents tested. Each genotoxic agent generates a distinct "sensitivity profile" that is characteristic of the agent. Comparison with an independent study of sensitivity profiles for an extensive set of antibiotics pinpoints those effects that are relatively specific for each agent. In some cases engineered double mutants have greatly increased effects. These results provide insight into the mechanism of action of each agent, and define targets for the design of co-drugs that can potentiate these agents. An example is the finding that mutants lacking one of several genes in the folate biosynthetic pathway are hypersensitive to ZEB, leading to a demonstration of synergy between trimethoprim and ZEB.

Antimutagenic activity of a secoisopimarane diterpenoid from Salvia cinnabarina M. Martens et Galeotti in the bacterial reverse mutation assay

The effects of 3,4-secoisopimar-4(18),7,15-trien-3-oic acid, a diterpenoid isolated from Salvia cinnabarina, were evaluated in the Ames test on Salmonella typhimurium TA98 and TA100 and on Escherichia coli WP2uvrA, in presence and in absence of the metabolic activation system. The secoisopimarane diterpenoid not only showed to be devoid of mutagenic activity, but significantly inhibited the effect of some known mutagens, in all strains tested. The reduction of the number of chemically-induced revertant colonies reached the value of 92.2% against 2-aminoanthracene, 59.6% against 2-nitrofluorene, 50.9% against sodium azide and 39.9% against methyl methane sulfonate. It is hypothesized that the secoisopimarane diterpenoid acts by aspecific mechanisms, by alterating the cell permeability thus blocking the mutagen adsorption across the bacterial membrane, or by chemical or enzymatic inhibition of the mutagens.

Transgenic Plants as Sensors of Environmental Pollution Genotoxicity

Rapid technological development is inevitably associated with manyenvironmental problems which primarily include pollution of soil, water and air. In manycases, the presence of contamination is difficult to assess. It is even more difficult toevaluate its potential danger to the environment and humans. Despite the existence ofseveral whole organism-based and cell-based models of sensing pollution and evaluationof toxicity and mutagenicity, there is no ideal system that allows one to make a quick andcheap assessment. In this respect, transgenic organisms that can be intentionally altered tobe more sensitive to particular pollutants are especially promising. Transgenic plantsrepresent an ideal system, since they can be grown at the site of pollution or potentiallydangerous sites. Plants are ethically more acceptable and esthetically more appealing thananimals as sensors of environmental pollution. In this review, we will discuss varioustransgenic plant-based models that have been successfully used for biomonitoringgenotoxic pollutants. We will also discuss the benefits and potential drawbacks of thesesystems and describe some novel ideas for the future generation of efficient transgenicphytosensors.

Tandem repeat mutation, global DNA methylation, and regulation of DNA methyltransferases in cultured mouse embryonic fibroblast cells chronically exposed to chemicals with different modes of action

Mutations at expanded simple tandem repeat (ESTR) DNA sequences provide a useful tool for screening germline mutation. However, the mechanisms resulting in induced mutations are unknown and provide an impediment to the utility of the method. Induced ESTR mutations arise through a nontargeted mechanism resulting in destabilization of the repeat locus. We hypothesized that alterations in DNA methylation, or in DNA methyltransferase expression, may be associated with this indirect mechanism of mutation. DNA mutation frequency was measured in C3H/10T1/2 mouse embryonic fibroblast cells following chronic exposure to six chemicals exhibiting different modes of genotoxic action: N-nitroso-N-ethylurea (ENU); benzo(a)pyrene (BaP); etoposide (ETOP); okadaic acid (OA); cisplatin (CisPt); and 5-azacytidine (5azadC). Induced mutation ranged from 2-fold (ENU, BaP, ETOP), to 1.3-1.4 fold (OA, 5azadC), to nonresponsive (CisPt). Global DNA methylation, measured using the cytosine extension assay, revealed hypomethylation following exposure to ENU and 5azadC, hypermethylation following BaP and OA exposure, and no change following treatment with ETOP or CisPt. DNA methyltransferase transcription (Dnmt1, Dnmt3a, Dnmt3b) was significantly affected by all treatments except ETOP, with the vast majority of changes being downregulation. There was no direct correlation between ESTR mutation, global methylation, or DNA methyltransferase transcription. However, 4/5 ESTR mutagens caused changes in global methylation, while the noninducer (CisPt) did not cause changes in methylation. We hypothesize that chemicals that modify chromatin conformation through changes in methylation may compromise the ability of mismatch repair enzymes (or other enzymes) to access and repair secondary structures that may form across ESTR loci resulting in mutation.

DNA-damaging activity and mutagenicity of 16 newly synthesized thiazolo[5,4-a]acridine derivatives with high photo-inducible cytotoxicity

The discovery of the potent anticancer properties of natural alkaloids in the pyrido-thiazolo-acridine series has suggested that thiazolo-acridine derivatives could be of great interest. In a continuous attempt to develop DNA-binding molecules and DNA photo-cleavers, 16 new thiazolo[5,4-a]acridines were synthesized and studied for their photo-inducible DNA-intercalative, cytotoxic and mutagenic activities, by use of the DNA methyl-green bioassay, the Alamar Blue viability assay and the Salmonella mutagenicity test using strains TA97a and TA98 with and without metabolic activation and photo-activation. Without photo-activation, one compound showed a DNA-intercalative activity in the DNA major groove while three compounds displayed intercalating properties after photo-activation. In the dark, four molecules possessed cytotoxic activities against a THP1 acute monocytic leukemia cell line while 15 derivatives displayed photo-inducible cytotoxic activity against this cell line. All compounds were mutagenic in strain TA97a with metabolic activation (+S9mix) and 15 molecules were mutagenic in strain TA98 without activation (-S9mix). Study of the quantitative structure-activity relationships (QSAR) from the Salmonella mutagenicity data revealed that several descriptors could describe cytotoxic and mutagenic activities after photo-activation. From the results of the mutagenicity test, four compounds with elevated mutagenic activities were selected for additional experiments. Their capacities to induce single-strand breaks (SSB) and chromosome-damaging effects were monitored by the comet and the micronucleus assays in normal human keratinocytes. Comparison of the minimal genotoxic concentrations showed that two compounds possessed higher capacities to induce SSB after photo-activation. In the micronucleus assay, three molecules were able to induce high numbers of micronuclei following photo-activation. Overall, the results of this study confirm that acridines are predominantly genotoxic via a DNA-intercalating mechanism in the dark, while DNA-adducts were probably induced following photo-activation.

A novel genotoxic aspect of thiabendazole as a photomutagen in bacteria and cultured human cells

Thiabendazole (TBZ) is a post-harvest fungicide commonly used on imported citrus fruits. We recently found that TBZ showed photomutagenicity with UVA-irradiation in the Ames test using plate incorporation method. In the present study, potential of DNA-damaging activity, mutagenicity, and clastogenicity were investigated by short pulse treatment for 10 min with TBZ (50-400 microg/ml) and UVA-irradiation (320-400 nm, 250 microW/cm2) in bacterial and human cells. UVA-irradiated TBZ caused DNA damage in Escherichia coli and human lymphoblastoid WTK1 cells assayed, respectively, by the umu-test and the single cell gel electrophoresis (comet) assay. In a modified Ames test using Salmonella typhimurium and E. coli, strong induction of -1 frameshift mutations as well as base-substitution mutations were detected. TBZ at 50-100 microg/ml with UVA-irradiation significantly induced micronuclei in WTK1 cells in the in vitro cytochalasin-B micronucleus assay. Pulse treatment for 10 min with TBZ alone did not show any genotoxicity. Although TBZ is a spindle poison that induces aneuploidy, we hypothesize that the photogenotoxicity of TBZ in the present study was produced by a different mechanism, probably by DNA adduct formation. We concluded that UVA-activated TBZ is genotoxic in bacterial and human cells in vitro.

Suppression of chemically induced and spontaneously occurring oxidative mutagenesis by three alleles of human OGG1 gene encoding 8-hydroxyguanine DNA glycosylase

8-Hydroxyguanine (8-OH-G) is an oxidatively damaged guanine base that causes G:C to T:A transversion mutations. To counteract the mutagenicity of 8-OH-G in DNA, humans possess the hOGG1 gene, which encodes 8-OH-G DNA glycosylase. Interestingly, genetic polymorphisms at codon 326 (hOGG1-Ser326 versus hOGG1-Cys326) and at codon 46 (hOGG1-Arg46 versus hOGG1-Gln46) exist in human populations. hOGG1-Ser326 and -Cys326 have Arg at codon 46, and hOGG1-Gln46 has Ser at codon 326. In this study, we examined the abilities of three forms of GST-hOGG1 (hOGG1-Ser326, -Cys326 and -Gln46) to suppress chemically induced oxidative mutagenesis using Salmonella typhimurium strains YG3001 and YG3002. These strains are the mutMST derivatives of Ames tester strains TA1535 (uvrB-) and TA1975 (uvrB+), respectively. The mutMST gene encodes a functional counterpart of the OGG1 gene. Mutations induced by 4-nitroquinoline 1-oxide were by more than 95% suppressed by the expression of any of three forms of GST-hOGG1 in strain YG3002. Expression of GST-hOGG1 also reduced by 40 and 60%, respectively, the numbers of His+ revertants induced by methylene blue plus visible light and benzo[a]pyrene plus visible light in strain YG3001. hOGG1-Gln46 displayed a slightly weaker ability to suppress the mutations induced by methylene blue plus visible light than did other two forms although the differences were not statistically significant. About 85 and 95% of spontaneous mutagenesis in strain YG3021 and YG3022, the mutMST mutYST double mutants of strain TA1535 and TA1975, respectively, were suppressed by the expression of any of hOGG1 alleles. hOGG1-Gln46 displayed a weaker suppression than did other two forms in strain YG3022 and the difference was statistically significant. These results suggest that three alleles of the hOGG1 gene efficiently suppress chemically induced and spontaneously occurring oxidative mutagenesis, and that hOGG1-Gln46 may have a weaker ability to suppress the mutations.

Mutant spectra analysis at hisG46 in Salmonella typhimurium strain YG1029 induced by mammalian S9- and plant-activated aromatic amines

Mutant spectra analysis was conducted with spontaneous hisG46 revertants of Salmonella typhimurium strain YG1029 and revertants induced by the plant- and mammalian S9-activation of benzidine and 4-aminobiphenyl (4-ABP). Under preincubation conditions, YG1029 cells were exposed to benizidine or 4-ABP with mammalian S9 activation or to a high molecular weight fraction that contained the plant-activated products. The induced revertants were isolated at mutagen concentrations that caused an increased mutant frequency of approximately 4- to 10-fold above background. Genomic DNA from each revertant was isolated and the hisG region was amplified using polymerase chain reaction (PCR). Using a series of specific probes and a modified version of the ECL3's-oligolabelling and detection system, each of the six possible base-pair substitution mutations at hisG46 that leads to a reversion event was determined. Of the YG1029 spontaneous revertants, transition mutations were 31.8% and transversion mutations were 68.2%. The YG1029 spontaneous mutant spectrum differed significantly from the spontaneous spectrum of TA1535 but did not significantly differ from the spontaneous TA100 mutant spectrum. The differences of the spontaneous mutant spectra among these highly related strains illustrate that the introduction of the plasmid pKM101 into S. typhimurium increased the frequency of transversions (CCC-->ACC; CCC-->CAC) and reduced site 2 (CCC-->CTC) transitions. With plant-activated benzidine, 21.1% of recovered revertants resulted from transitions and 78.9% from transversions while S9 activated-benzidine induced revertants were recovered as 14.2% from transition and 85.8% from transversion mutations. Plant-activated 4-ABP recovered 20.0% transitions and 80.0% transversions. S9-activated 4-ABP-induced 21.4% transitions and 78.6% transversions. Chi-square analysis of mutant spectra indicated that the DNA lesions that resulted in reversion at the hisG46 allele induced by plant-activated benzidine or 4-ABP were different from those generated after mammalian S9 activation of these promutagens. The plant-activated benzidine and 4-ABP induced statistically identical mutant spectra. Also, the mammalian-activated benzidine and 4-ABP induced statistically similar mutant spectra. These data show that the plant-activated and mammalian-activated aromatic amine products inflicted different types or distributions of DNA lesions that were reflected in the resulting induced mutant spectra.

Photomutagenicity of thiabendazole, a postharvest fungicide, in bacterial assays

We investigated the photomutagenicity of thiabendazole (TBZ), a postharvest fungicide commonly used on imported citrus fruits. Using UVA light (320-400 nm), we irradiated bacterial cultures with or without TBZ in a 24-well multiplate. UVA-irradiation without TBZ was not mutagenic to the tester strains, nor was unirradiated TBZ. TBZ was strongly photomutagenic in Escherichia coli WP2uvrA and WP2uvrA/pKM101 strains, weakly photomutagenic in Salmonella typhimurium TA100 and TA98, and not photomutagenic in S. typhimurium TA1535 and TA1538. The photomutagenicity of TBZ was more evident in WP2uvrA/pKM101, which carries the trpE65 ochre mutation (TAA), than in TA100, which carries the hisG46 missense mutation (CCC). In E. coli WP3101-WP3106 and the corresponding pKM101-containing strains, photoactivated TBZ induced predominantly G:C-->A:T transitions and A:T-->T:A transversions. In the plasmid-containing strains only, TBZ induced a moderate number of A:T-->G:C transitions and a few A:T-->C:G and G:C-->T:A transversions. The observation that UVA-irradiated TBZ mutated both G:C and A:T basepairs may explain why WP2uvrA/pKM101 was more sensitive to its mutagenicity than TA100. TBZ that was irradiated before it was added to the WP2uvrA/pKM101 cells was not photomutagenic, which suggests that the photomutagenic products of TBZ were unstable or rapidly reacted with other molecules before being incorporated into cells.

Genotoxicity of 2-nitro-7-methoxy-naphtho[2,1-b]furan (R7000): a case study with some considerations on nitrofurantoin and nifuroxazide

Two nitrofurans present broad-spectrum antimicrobial properties and some of them are used in human and veterinary medicine. Most of these molecules are mutagens and some of them were reported as carcinogens. Due to its extreme mutagenic potency in bacteria, the nitronaphtho derivative 2-nitro-7-methoxy-naphtho[2,1-b]furan (R7000) was used as a tool to analyze the mechanism of the genotoxic action of this family of chemicals. In the present paper, we review essential data on the genotoxicity of R7000 and briefly discuss the case of nitrofurantoin and nifuroxazide, two nitrofurans, still in use as urinary and gastrointestinal disinfectants.

Genotoxicity of Brosimum gaudichaudii measured by the Salmonella/microsome assay and chromosomal aberrations in CHO cells

The root bark of Brosimum gaudichaudii Trécul (Moraceae) is popularly used for treatment of vitiligo. In the present study the mutagenic activity of the aqueous and methanolic extract as well as of the n-butanolic fraction of this medicinal plant were evaluated using Salmonella typhimurium assays, TA100, TA98, TA102 and TA97a strains, while the clastogenic effect in Chinese hamster ovary (CHO) cells in the G(1)/S, S and G(2)/S phases of the cell cycle. The results showed mutagenic activity of the aqueous extract against TA102 in the presence of S9, and of methanolic extract, with and without metabolic activation. TA100 mutagenicity was only observed for the methanolic extract in the absence of S9. The n-butanolic fraction did not present mutagenic activity. In CHO cells only the methanolic extract induced a significant increase of chromosomal aberrations in the G(1)/S and S phases, whereas a decrease in the mitotic index was observed in the G(1)/S and G(2)/S phases. No clastogenicity was observed for the aqueous extract. The furocoumarins (psoralen and bergapten) presented in the extracts might contribute to the mutagenicity. The lower activity of the aqueous extract was probably due to the presence of smaller amount of furocoumarins compared to the methanolic extract.

Genetic toxicity of a standardized mixture of citrus polymethoxylated flavones

Flavonoids are a ubiquitous family of phytochemicals that display a variety of biological effects, both beneficial and adverse depending on the individual compound. Certain flavonoids are genotoxic while others inhibit the genotoxicity of other mutagens. In the present studies, the mutagenicity of a mixture of polymethoxylated flavones (PMFs) purified from citrus peel oil was evaluated. The mixture consisted of nobiletin (32.5%), 3,3',4',5,6,7,8-heptamethoxyflavone (25.0%), tangeretin (14.0%), trimethylscutellarein (9.1%), sinensetin (3.9%), 5-demethyl-nobiletin (2.8%), hexa-O-methylquercetagetin (3.3%), 5-demethyl-tetramethylscutellarein (0.7%), 5-hydroxy-3,3',4',6,7,8-hexamethoxyflavone (0.7%), and a small quantity of unidentified flavonoid compounds (3.9%). In vitro addition of the PMF mixture over a concentration range that spanned four log doses (0.0005-5.0 mg/plate) did not reveal any evidence of mutagenicity in five bacterial tester strains (Salmonella typhimurium TA98, TA100, TA102, TA1535 and TA1537) either in the absence or presence of S9 activation. The PMF mixture exhibited a statistically significant increase in mutagenicity of L5178Y tk(+/-) mouse lymphoma cells at 0.05 (38.5 x 10(-6); P<0.05) and 0.1 mg/ml (61 x 10(-6); P<0.01) compared with vehicle-treated controls (mutation frequency=19.7 x 10(-6)). However, these responses were within historical values observed in negative control cultures and extremely small compared to the positive control (EMS 0.5 microl/ml; 1685.3 x 10(-6)). Furthermore, in the presence of S9 there was no indication of genetic toxicity in L5178Y tk(+/-) cells. These results demonstrate that the PMF mixture is not genotoxic in in vitro assay systems.

A sensitive transgenic plant system to detect toxic inorganic compounds in the environment

We describe a transgenic plant-based assay to study the genetic effects of heavy metals. Arabidopsis thaliana plants carrying a beta-glucuronidase (GUS) marker gene either with a point mutation or as a recombination substrate were used to analyze the frequency of somatic point mutations and homologous recombination in whole plants. Transgenic test plants sown on media contaminated by the salts of the heavy metals Cd2+, Pb2+, Ni2+, Zn2+, Cu2+, and As2O3 exhibited a pronounced uptake-dependent increase in the frequencies of both somatic intrachromosomal recombination and point mutation. The test was applied to monitor the genotoxicity of soils sampled in sites contaminated with several heavy metals. Our results indicate that this is a highly sensitive system for monitoring metal contamination in soils and water.

Large accumulation of mRNA and DNA point modifications in a plant senescent tissue

Although nucleic acids are the paradigm of genetic information conservation, they are inherently unstable molecules that suffer intrinsic and environmental damage. Oxidative stress has been related to senescence and aging and, recently, it has been shown that mutations accumulate at high frequency in mitochondrial DNA with age. We investigated RNA and DNA modifications in cork, a senescent plant tissue under high endogenous oxidative stress conditions. When compared to normally growing young tissue, cork revealed an unexpected high frequency of point modifications in both cDNA (Pn = 1/1784) and nuclear DNA (Pn = 1/1520). Cork should be viewed as a mosaic of genetically heterogeneous cells. This has biological implications: it supports somatic mutation models for aging and challenges 'single cDNA clone' as descriptor for the molecular genetics of senescent tissues.

Effect of interaction between 5-azacytidine and DNA (cytosine-5) methyltransferase on C-to-G and C-to-T mutations in Escherichia coli

The purpose of this study was to determine the effect of the Dcm cytosine methyltransferase on 5-azacytidine (5-azaC) mutagenesis in Escherichia coli. We used a Lac reversion assay to measure C-to-G and C-to-T mutations at a single, methylatable cytosine in the lacZ gene, in the presence and absence of Dcm. C-to-G mutations are stimulated by 5-azaC but are largely independent of Dcm. In contrast, C-to-T mutations are not stimulated by 5-azaC in either wild type or dcm cells. However, in cells which contain Dcm but are defective in very short patch repair, the normally high frequency of spontaneous C-to-T mutations is decreased by the analog in a dose-dependent manner.

Detection of mutator subpopulations in Salmonella typhimurium LT2 by reversion of his alleles

Defects in the methyl-directed mismatch repair lead to both the hypermutability phenotype and removal of a barrier to genetic exchange between species. Mutator bacteria carrying such defects occur frequently among bacterial pathogens, suggesting that subpopulations of mutators are contained within pathogen clones and give rise to the genetic variants that are acted upon by selective forces to allow survival or successful infection. We report here on the detection of the mutator subpopulation in Salmonella typhimurium and determination of its frequency in laboratory cultures. The analysis involved screening for mutators among revertants of S. typhimurium histidine auxotrophs selected for the His+ phenotype, since the frequency of mutators is expected to be increased in the selected mutant population they helped to spawn. The increases in spontaneous reversion of histidine mutations were first measured in isogenic strains carrying mismatch repair-defective mutH, mutL, mutS, or uvrD alleles, relative to their mismatch repair-proficient counterparts. Screening for the mutator phenotype in nearly 12,000 revertants of repair-proficient strains carrying his mutations highly stimulated for reversion in mutator backgrounds, the base substitution in hisG428 and frameshift in hisC3076, yielded five mutator strains (0.04%). The his+ reversion mutations contained within the newly-arisen mutator strains were characteristic of the predominant nucleotide changes expected in such mutators, as assessed by comparison with the spectra for reversion events in wild-type and mismatch correction-defective backgrounds. The results show that subpopulations of mutators, residing in normal populations at a finite frequency, can be culled from the culture by strong selection for a required phenotype. We calculate that the frequency of mutators in the unselected population of S. typhimurium is 1-4x10-6, an incidence 10-fold lower than that expected based on studies of laboratory cultures of Escherichia coli.

New Escherichia coli WP2 tester strains highly sensitive to reversion by oxidative mutagens

New Escherichia coli strains have been added to the WP2 mutagenicity test for the specific detection of oxidative mutagens. Strain IC203 derives from WP2 uvrA/pKM101 and is highly sensitive to oxidative stress due to a deficiency in the OxyR function. Following exposure to t-butyl hydroperoxide (BuOOH) or menadione (MD), but not to 4-nitroquinoline 1-oxide (4NQO), strain IC203 (oxyR) shows increased mutability with respect to the oxyR+ parent. The advantage that the OxyR deficiency confers on IC203 strain in detecting oxidative mutagens is not obtained with strains deficient in either katG or ahpCF, two OxyR-regulated genes. Strain IC206, a derivative of WP2 uvrA carrying a deletion of the umuDC genes and deficient in the MutY glycosylase, has also been added to the WP2 test for the detection of SOS-independent mutations promoted by 8-oxoguanine lesions. Induction of these mutations was observed after treatment with BuOOH, but not after MD or 4NQO exposure. The two new strains, IC203 and IC206, can be useful for the screening of mutations resulting from oxidative stress as well as in studies on antioxidants preventing mutagenesis.

Comparison of responses of base-specific Salmonella tester strains with the traditional strains for identifying mutagens: the results of a validation study

The ability of a TA7000 series of Salmonella his- mutant tester strains to detect mutagens as classified by the traditional tester strains (TA100, TA98, TA1535, TA1537, TA97, TA102 and TA104) was evaluated using 30 coded chemicals, 5 of which were duplicates with different code numbers. The TA7000 series of tester strains were TA7001, TA7002, TA7003, TA7004, TA7005 and TA7006, each of which reverts by a specific base substitution. In addition, each chemical was tested in a mixture of the base-specific strains (the Mix), plus the traditional strains, TA98 and TA1537. A liquid version of the Salmonella mutagenicity assay was performed in microtiter plates to allow partial automation for increased throughput. The results were compared to those in the National Toxicology Program (NTP) database, which were obtained from the traditional strains in the preincubation assay. In the two strains common to both protocols, TA98 and TA1537, the agreement was 80% and 85%, respectively. When compared to the NTP results for TA100, the Mix gave a 72% concordance, while the addition of the frameshift tester strain, TA98, increased the agreement to 76%. The overall agreement on positive or negative classifications of mutagenicity was 88% for the 25 chemicals tested. There were three notable exceptions to the overall agreement. Benzaldehyde was detected as a mutagen in TA7005 in contrast to its classification as a non-mutagen in the NTP database. This does not necessarily contradict the NTP results because the base-specific strains may respond to different mutagens. Two weak mutagens in the NTP database, 1-chloro-2-propanol and isobutyl nitrite, were not detected as mutagens in the base-specific new strains in the liquid protocol. While there are a number of major differences in the two assays, it was concluded that the results from each procedure are comparable.

Base-pair mutation caused by four nitro-group containing aromatic amines in Salmonella typhimurium TA100, TA104, TA4001 and TA4006

Among p-phenylenediamine, benzidine and the analogues we previously tested, only the nitro-group containing 2-nitro-p-phenylenediamine, 3-nitro-o-phenylenediamine, 4-nitro-o-phenylenediamine and 4,4'-dinitro-2-biphenylamine caused base-pair reversion in the histidine locus of Salmonella typhimurium TA100. In order to determine the types of mutations involved, such as transversion or transition, these four nitro-group containing compounds were tested with S. typhimurium strains TA100, TA104, TA4001 and TA4006. Dose-mutagenicity relationships occurred with TA100 and TA104. However, the majority of revertants from TA100 and TA104 were insensitive to inhibition by histidine analogue, DL-1,2,4-triazole-3-alanine. These results suggested that the occurrence of histidine revertants was predominantly induced by base-pair (point) mutations and not by suppressor gene mutations. The CG-TA transition and CG-AT transversion are the major types of mutation induced by all these compounds in TA100. The TA-AT transversion also contributed to the mutagenicity of 4-nitro-o-phenylenediamine and 4,4'-dinitro-2-biphenylamine in TA104. These nitro-group containing compounds showed no mutagenicity in TA4001, but induced CG-GC transversion in TA4006.

Construction of mutants of Salmonella typhimurium deficient in 8-hydroxyguanine DNA glycosylase and their sensitivities to oxidative mutagens and nitro compounds

8-Hydroxyguanine (8-OH-G) DNA glycosylase is an enzyme involved in repair of oxidative DNA damage, e.g., 8-OH-G in DNA. In order to assess the roles of 8-OH-G in spontaneous and chemically-induced mutagenesis, the mutMST gene encoding 8-OH-G DNA glycosylase of Salmonella typhimurium was disrupted in several Ames tester strains, i.e., S. typhimurium TA1535 (hisG46, uvrB-, rfa), TA1975 (hisG46, uvr+, rfa) and TA102 (hisG428, uvr+, rfa). The spontaneous mutation frequencies were increased 2.4 and 1.6 times, respectively, by the mutMST deletions in strains TA1535 and TA1975, which are spontaneously reverted to His+ by mutations mainly at G:C base pairs. The resulting strains YG3001 (TA1535 delta mutMST) and YG3002 (TA1975 delta mutMST) were 2 to 8 times more sensitive to the mutagenicities of methylene blue plus visible light, neutral red plus visible light and 2-nitrofluorene than the parent strains. The strain YG3002 but not YG3001 was about 30 times more sensitive to the mutagenicity of 4-nitroquinoline N-oxide than the parent strain TA1975. Neither hydrogen peroxide nor phenazine methosulfate was mutagenic in the mutMST-deletion strains as well as in the parent strains. In contrast, the mutMST deletion did not affect the spontaneous mutation frequency of strain TA102, which has an A:T base pair at the critical site for reversion. The sensitivities of strain TA102 to the chemicals were not enhanced by the mutMST deletion except for hydrogen peroxide. These results suggest that 8-OH-G in DNA plays important roles in spontaneous mutagenesis occurring at G:C base pairs in S. typhimurium, and some nitro aromatics such as 4-nitroquinoline N-oxide or 2-nitrofluorene as well as the photosensitizers plus visible light can produce 8-OH-G in DNA, thereby inducing mutations. In the case of 4-nitroquinoline N-oxide, 8-OH-G rather than DNA adducts seems to play major roles in mutagenesis in uvr+ background. The new strains could be useful for the evaluation of the roles of 8-OH-G in mutagenesis in S. typhimurium and permit the efficient detection of some oxidative mutagens in the environment.

Protection of DNA during oxidative stress by the nonspecific DNA-binding protein Dps

Reactive oxygen species can damage most cellular components, but DNA appears to be the most sensitive target of these agents. Here we present the first evidence of DNA protection against the toxic and mutagenic effects of oxidative damage in metabolically active cells: direct protection of DNA by Dps, an inducible nonspecific DNA-binding protein from Escherichia coli. We demonstrate that in a recA-deficient strain, expression of Dps from an inducible promoter prior to hydrogen peroxide challenge increases survival and reduces the number of chromosomal single-strand breaks. dps mutants exhibit increased levels of the G x C-->T x A mutations characteristic of oxidative damage after treatment with hydrogen peroxide. In addition, expression of Dps from the inducible plasmid reduces the frequency of spontaneous G x C-->T x A and A x T-->T x A mutations and can partially suppress the mutator phenotype of mutM (fpg) and mutY alleles. In a purified in vitro system, Dps reduces the number of DNA single-strand breaks and Fpg-sensitive sites introduced by hydrogen peroxide treatment, indicating that the protection observed in vivo is a direct effect of DNA binding by Dps. The widespread conservation of Dps homologs among prokaryotes suggests that this may be a general strategy for coping with oxidative stress.

Mutagenicity of 5-aza-2'-deoxycytidine is mediated by the mammalian DNA methyltransferase

The cytosine analog 5-aza-2'-deoxycytidine has been used clinically to reactivate genes silenced by DNA methylation. In particular, patients with beta-thalassemia show fetal globin expression after administration of this hypomethylating drug. In addition, silencing of tumor suppressor gene expression by aberrant DNA methylation in tumor cells may potentially be reversed by a similar regimen. Consistent with its function in maintaining tumor suppressor gene expression, 5-aza-2'-deoxycytidine significantly reduces intestinal tumor multiplicity in the predisposed Min mouse strain. Despite its utility as an anti-cancer agent, the drug is highly mutagenic by an unknown mechanism. To gain insight into how 5-aza-2'-deoxycytidine induces mutations in vivo, we examined the mutational spectrum in an Escherichia coli lac I transgene in colonic DNA from 5-aza-2'-deoxycytidine-treated mice. Mutations induced by 5-aza-2'-deoxycytidine were predominantly at CpG dinucleotides, which implicates DNA methyltransferase in the mutagenic mechanism. C:G-->G:C transversions were the predominant class of mutations observed. We suggest a model for how the mammalian DNA methyltransferase may be involved in facilitating these mutations. The observation that 5-aza-2'-deoxycytidine-induced mutations are mediated by the enzyme suggests that novel inhibitors of DNA methyltransferase, which can inactivate the enzyme before its interaction with DNA, are needed for chemoprevention or long term therapy.

Effect of overexpression of E. coli 3-methyladenine-DNA glycosylase I (Tag) on survival and mutation induction in Salmonella typhimurium

Salmonella typhimurium, compared to Escherichia coli, is deficient in an inducible glycosylase activity harbouring only constitutive glycosylase functions. 3-Methyladenine-DNA glycosylase I encoded by the E. coli tag gene is a constitutively expressed repair enzyme that primarily removes N3-methyladenine but also N3-methylguanine from DNA by glycosylic cleavage in the first step of the base excision repair. In order to investigate in vivo effect of the overexpressed glycosylase I activity on survival capacity and mutation induction in S. typhimurium, and thereby elucidate the significance of both 3-methylpurines in cellular sensitivity to methylating agents (e.g., DMS), we transformed four his- S. typhimurium strains with the plasmid pCY5 carrying the E. coli tag gene under the control of the lac promoter. Although the 3-methyladenine-DNA glycosylase activity in cells carrying pCY5 was only 10-fold higher on exposure to IPTG compared to the TA1535 control strain carrying pUC8, the overexpression of the Tag protein completely suppressed deficiency in an inducible glycosylase activity, rendering cells resistance to toxic effects of DMS. The suppression was not influenced by the nucleotide excision repair pathway since there was no difference in recovered survival among NER-proficient and NER-deficient strains. The yield of mutation induction in the reversion assay was decreased to the level of spontaneous (his-->his+) revertant colonies showing that in the overall population in overexpressed conditions in vivo 3-methyl-guanine, in addition to 3-methyladenine, must have been removed from DNA by the E. coli Tag protein and therefore accounts for the second most important cytotoxic lesion.

The selection-induced His+ reversion in Salmonella typhimurium

It was previously shown that spontaneous reversion to His+ of the allele hisG46 Salmonella typhimurium occurs under the influence of histidine starvation. No pre-existing His+ revertants arisen in rich medium were observed. We have now shown that the pre-existing His+ revertants are seen under increased cell concentration (10(10) cells/ml). At the same time, it was established that the selection-induced His+ reversion events of hisG46 begin to occur after 2-3 h of incubation on histidine starvation plates, and this process continues for about 4 days. In parallel, considerable DNA synthesis was observed for the initial hours of starvation. Chloramphenicol and novobiocin inhibited this DNA synthesis, whereas the addition of trace of histidine as well as novobiocin produced the delay of adaptive His+ reversion. It was found that adaptive reversion of hisG46 is recA-independent, although it requires some activity of RecA on the mucAB genetic background. Based on these data, we suggest that the cause of adaptive His+ reversion is the DNA replication operating under histidine starvation. Using a number of mutation models, we showed that histidine starvation did not increase the general mutation rate. It was also demonstrated that intragenic revertants and extragenic ochre suppressors of the allele hisG428 arise under the influence of histidine deprivation.

Mutation induction and mutation spectra of S. typhimurium TA100 after exposure to isohistidines

L-Isohistidine and D,L-isohistidine, but not D-isohistidine, caused an increase of the number of mutant colonies in S. typhimurium strain TA100. Spontaneous and also sodium azide or 2-aminoanthracene induced mutant numbers were enhanced by L-isohistidine and by an isomeric mixture of D,L- and L-isohistidine. These effects could not be attributed to a growth-enhancing property. The colony probe hybridization procedure was used to investigate the effects of the histidines on the spontaneous and azide-induced spectra of the hisG46 allele in strain TA100. D,L-Isohistidine, but not the D-isomer, caused and increase of transitions (CCC-->CTC) and transversions (CCC-->CAC) in the spontaneous spectrum. Sodium azide alone induced a strong increase of CCC-->CTC transitions; combination with the D,L-isohistidine led to a further enhancement of this type of base substitutions, whereas with the L-isomer, no such effect was observed. This supports the hypothesis that the activity of D,L-isohistidine is probably not due to DNA-damaging properties, but rather to indirect mechanisms, such as enhancement of the infidelity of DNA replication and/or interference with DNA-repair or proofreading functions.

Specificity of spontaneous and t-butyl hydroperoxide-induced mutations in delta oxyR strains of Escherichia coli differing with respect to the SOS mutagenesis proficiency and to the MutY and MutM functions

Mutations induced by oxidative DNA damage appear to occur by two pathways, differing in their dependence on SOS mutagenesis. We have analysed the specificity of mutations produced by each pathway. Base substitutions generating extragenic suppressors were characterized in Trp+ revertants of Escherichia coli strains carrying the trpE65 ochre mutation, which were hypersensitive to oxidative mutagenesis due to a deletion of the oxyR gene. In strain IC3821, containing MucA/B proteins and therefore proficient for SOS mutagenesis, the more frequently scored base substitutions, either spontaneous or induced by t-butyl hydroperoxide (BuOOH), were T:A-A:T transversions, followed by G:C-A:T transitions, while the frequency of G:C-T:A transversions was lower. This SOS-dependent mutability could be promoted by abasic sites. In strains IC3894 (mutY) and IC3981 (mutY mutM), lacking mutagenesis proteins, SOS-independent revertants arose almost exclusively via G:C-T:A transversions probably derived from oxidatively damaged 8-oxoguanine/adenine mispairs. Formation of these mispairs in IC3894 and IC3981 would be enhanced by BuOOH treatment since it caused a significant increase in the revertant number. Strains IC3894 and IC3981 could have a complementary role to that of IC3821 to analyse the mutagenicity and the mutational specificity of oxidants.

Isolation of glutamate-inserting ochre suppressor mutants of Salmonella typhimurium and Escherichia coli

Glutamate-inserting ochre suppressors have been identified among late-arising, spontaneous revertants of a hisG428 mutant of Salmonella typhimurium and an argE3 mutant of Escherichia coli. The S. typhimurium suppressors mapped in the tRNA2(Glu) gene gltU at 82 min; those in E. coli were found to be in tRNA2(Glu) genes gltW at 56 min, gltU at 85 min, and gltT at 90 min.

Analysis of the ciprofloxacin-induced mutations in Salmonella typhimurium

The mutagenic events induced by ciprofloxacin, a potent antimicrobial agent, have been characterized. For this, a battery of His mutants of Salmonella typhimurium (hisG428, his G46, His C9070, and his G1775 targets) that detects the six possible transitions and transversions [Levin and Ames (1986): Environ Mutagen 8:9-28] and two additional His strains (hisC3076 and his D3052 targets) carrying frameshift mutations have been used. Our results indicate that GC-TA transversions are the major base-pair substitution induced by ciprofloxacin and that GC-At transitions are also produced, but to a lesser degree. However, we cannot discard the fact that At-Ta transversions are also induced. In addition, the data indicate that the mutational specificity of ciprofloxacin depends on the location of the target. Intragenic base-pair substitutions are the most frequent mutations at the hisG428 target when it is on the chromosome, whereas 3 or 6 base-pair deletions are the major mutagenic events when this target is on the plasmid pAQ1. We have shown that ciprofloxacin also induces deletions/insertions at the hisC3076 and hisD3052 frameshift targets. Therefore, this inhibitor of DNA gyrase promotes a wide pattern of mutations including different kinds of base-pair substitutions, 3 or 6 base-pair deletions, and insertions/deletions resulting in frameshifts. All of these mutagenic events require the MucAb proteins involved in the error-prone repair, with the exception of base-pair insertions/deletions at the hisD3052 target, which are independent of the presence of plasmid pKM101.

SOS chromotest results in a broader context: empirical relationships between genotoxic potency, mutagenic potency, and carcinogenic potency

Environmental monitoring requires that large numbers of samples be processed in a relatively short period of time. While microbioassays facilitate rapid testing, the results are often difficult to interpret in the broader context of human or animal health. Determining the consequences of exposure to genotoxic substances will ultimately require in situ monitoring of exposed organisms. However, it is immediately possible to construct a broad empirical framework within which available microbioassay results can be interpreted. To do this for SOS Chromotest results, we investigated the empirical relationships between SOS genotoxic potency and mutagenic potency (as measured with the Salmonella/microsome assay), as well as between genotoxic potency and carcinogenic potency (as measured using standard, chronic animal bioassays). Strong relationships were identified between; 1) genotoxic potency and mutagenic potency for 268 direct-acting substances (r2=0.76) and 2) genotoxic potency and mutagenic potency for 126 S9-activated substances (r2=0.65). Ordinary least squares regression analyses of the SOS genotoxicity-Salmonella mutagenicity relationship revealed a significant effect of SOS genotoxicity as well as differences in mutagenic potency that can be attributed to the Salmonella strain used to measure mutagenic potency. Analyses of S9-activated substances revealed a significant interaction between the SOS genotoxic potency (SOSIP) effect and the Salmonella strain effect. Two regression models relating SOS genotoxicity and Salmonella mutagenicity were used to predict the mutagenic potency of several industrial effluent extracts previously analyzed for SOS genotoxicity by White et al. [(1996): Environ Mol Mutagen 27:116-139]. Predictions are consistent with published mutagenic potency values for similar industrial waste materials. A consistent relationship was also identified between genotoxic potency and carcinogenic potency for 51 substances. Linear regression analyses revealed an effect of SOS genotoxic potency as well as differences in carcinogenic potency that may be attributable to experimental animal and route of exposure. The correlation between genotoxicity and carcinogenicity was fairly weak (maximum r value = 0.51). Previous studies revealed similar strength of association between Ames mutagenicity and carcinogenicity. Predicted carcinogenic potencies of previously examined genotoxic, industrial effluent extracts are generally low compared to the pure substances included in the data set.

Determination of mutational spectrum of the pesticide, captan, with an improved set of Escherichia coli LacZ mutants

The mutational spectrum of the fungicide, captan, was determined using a set of improved Escherichia coli lacZ mutants. Captan created mutations mostly at dA-dT sites (83%) with only 17% occurring at dG-dC sites. The hydrolysis products of captan do not appear to be mutagenic because samples of captan at different hydrolysis stages showed basically the same mutational spectra: 31% at AT --> CG transversions, 8% of GC --> AT transitions, 2% of GC --> CG transversions, 8% of GC --> TA transversions, 19% of AT --> TA transversions, and 32% of AT --> GC transitions. Prepared solutions of captan lost their mutational activity gradually over time, indicating that the rate of decrease in mutagenicity agreed with the kinetics of captan hydrolysis reported in other studies. Using the change in mutagenicity to predict degradation, the hydrolysis of captan in pH 7.0 buffer was about three times faster than the hydrolysis carried out in pH 4.5 buffer. To our knowledge, this is the first presentation of mutational spectrum of captan.

Involvement of umuDCST genes in nitropyrene-induced -CG frameshift mutagenesis at the repetitive CG sequence in the hisD3052 allele of Salmonella typhimurium

Expression of the umuDC operon is required for UV and most chemical mutagenesis in Escherichia coli. The closely related species Salmonella typhimurium has two sets of umuDC-like operons, umuDCST on the chromosome and samAB on a 60-MDa cryptic plasmid. The roles of the umuDC-like operons in chemically induced frameshift mutagenesis of the hisD3052 allele of S. typhimurium were investigated. Introduction of a pBR322-derived plasmid carrying umuDCST increased the rate of reversion of hisD3052, following treatment with 1-nitropyrene (1-NP) or 1,8-dinitropyrene (1,8-DNP) tenfold and fivefold, respectively, whereas it did not substantially increase the rate of reversion induced by other frameshift mutagens, i.e. 2-nitrofluorene (2-NF) and 2-amino-3-methyldipyrido[1,2-a:3',2'-d]imidazole (Glu-P-1). Introduction of a pBR322-derived plasmid carrying samAB did not increase the incidence of reversion of hisD3052 observed with any of the mutagens examined. Deletion of umuDCST substantially lowered the reversion rate induced by 1-NP or 1,8-DNP, but it did not affect reversion induced by 2-NF, Glu-P-1 or N-hydroxyacetylaminofluorene (N-OH-AAF). Deletion of samAB had little impact on reversion incidence induced by any of the five frameshift mutagens. DNA amplification using the polymerase chain reaction technique followed by restriction enzyme analysis using BssHII, suggested that the mutations induced by the five frameshift mutagens were all CG deletions at the CGCGCGCG sequence in hisD3052. These results suggest that umuDCST, but not samAB, is involved in the -2 frameshift mutagenesis induced by 1-NP and 1,8-DNP at the repetitive CG sequence, whereas neither operon participates in induction of the same type of mutations by 2-NF, Glu-P-1 or N-OH-AAF.

Salmonella typhimurium LT7 and LT2 strains carrying the imp operon on colIa

The imp operon is carried on a transmissible plasmid, ColIa, in original isolates of Salmonella typhimurium LT7. LT2 strain recipients of F' factors from LT7 strains harboring ColIa can acquire ColIa and imp under nonselective conditions. Thus, S. typhimurium LT2 strains that have received plasmids by conjugal transfer from LT7 strains might be inadvertently harboring ColI factors.

Effects of DNA repair deficiency on the mutational specificity in the lacZ gene of Escherichia coli

The mutational specificities of various chemical mutagens were compared in isogenic E. coli strains with different DNA repair capabilities (wild-type, uvrA, umuC, and uvrA umuC) in a reversion assay employing a set of mutant lacZ genes that can detect two types of transitions, four types of transversions, and five kinds of specific frameshift events. A uvrA derivative was more sensitive than the wild-type strain to 3-chloro-4-(dichloromethyl)-5-hydroxy-2(5H)-furanone for +1G, -1G, -2(C-G), +1A and -1A frameshifts, G.C-->A.T transitions, and G.C-->T.A transversions. In a uvrA background, G.C-->T.A transversions and +1G, +1A, and -1A frameshifts appeared to be umuC-dependent, while G.C-->A.T transitions were not. N-Ethyl-N'-nitro-N-nitrosoguanidine was more mutagenic in a uvrA background for five kinds of frameshifts and G.C-->A.T transitions, but not for G.C-->T.A, A.T-->C.G, and A.T-->G.C base substitutions. A.T-->C.G transversions were totally dependent on umuC gene function. For the investigation of mutational specificities induced by frameshift mutagens, an rfa mutation was additionally introduced. The rfa strain responded to 2-nitrofluorene, which induced primarily -2(C-G) frameshift mutations. In an rfa uvrA background, benzo[a]pyrene induced +1G, -1G, +1A, and -1A frameshifts. 2-Aminoanthracene induced +1G, -1G, and +1A, but not -1A, frameshifts, with -1G frameshifts predominating. Ethidium bromide induced only two types of frameshifts, -1G and +1A. Frameshifts induced by ICR-170 were independent of umuC gene function, while those by induced 1-nitropyrene were partly umuC-dependent.

Nucleoside and nucleobase analog mutagens

Compounds with structures close to those of normal nucleosides or nucleobases may be incorporated into cells and then become constituents of their DNA. Proliferation of such cells could yield mutants. In this article, the current status of studies on such nucleoside and nucleobase analogs is described. Base mispairing mechanisms for these analogs are discussed in light of recent biochemical and biophysical findings.

Detection and classification of mutagens: a set of base-specific Salmonella tester strains

A detection and classification system for mutagens has been developed that identifies the six possible base-pair substitution mutations. A set of six Salmonella typhimurium (TA7001 to TA7006) strains has been constructed, each of which carries a unique missense mutation in the histidine biosynthetic operon. In addition to the his mutation, these strains carry different auxiliary features that enhance the mutability of the target his mutation. These include the R factor pKM101, which has the SOS-inducible mucAB system; a deletion of the uvrB component of excision repair; and rfa mutations to increase the accessibility of bulky chemicals to the bacteria. Another set of strains (TA7041 to TA7046) contain a wild-type rfa gene. Reversion via the base substitution unique to each strain was verified by sequence analyses of > 800 revertants obtained from different types of mutagens. The strains have considerably lower spontaneous reversion frequencies and detect a variety of mutagens at a sensitivity comparable to the Salmonella tester strains TA100, TA102, and TA104. The low spontaneous frequency of reversion of a mixture of the six tester strains (approximately 10 revertants per plate) enables a single mutation assay with the mixture that is followed by classification of the type of mutation with the individual strains.

Genetic safety evaluation of pesticides in different short-term tests

Cyanazine, cyhexatin, dicamba and DNOC are pesticides commonly and broadly used in agriculture pest control. However, there is little information on their toxicity and mutagenicity in human cells and in whole animals. Therefore, UDS assay and SCE assay in human peripheral lymphocytes, and chromosome aberration analysis in bone marrow of rats have been used to assess the DNA-damaging activity of the above pesticides. Cyanazine proved non-genotoxic in all the test systems. Cyhexatin showed only weakly positive results for SCE induction in human lymphocytes, providing no concern for genotoxicological hazard. While dicamba did not show clastogenic effects in rodents, DNOC gave significant dose-related increases of structural chromosome aberrations in rat bone marrow cells. Female animals showed increased sensitivity to the toxic effects by DNOC at the highest dose. The results provide further information on the intrinsic genotoxic activity of the tested pesticides, which may contribute to the toxicological assessment of the risk associated with human exposure.

DNA-directed aniline mustards with high selectivity for adenine or guanine bases: mutagenesis in a variety of Salmonella typhimurium strains differing in DNA-repair capability

Two closely-related aniline monomustards (1 and 2), linked to a DNA-targeting acridine chromophore by a linker chain of different length, show high selectivity for alkylation of polymer DNA. The shorter-chain derivative (2) alkylates mainly at guanine N7 sites, while the longer-chain analogue (1) reacts almost exclusively at adenine N1. The biological effects of these compounds have been studied in standard Ames Salmonella typhimurium strains in order to determine the mutagenic consequences of such well-defined DNA lesions, and the effect of DNA-repair systems on them. Both compounds caused detectable mutations in strains TA1537, TA98 or TA100 and some related strains. Mutation rates were greatly enhanced in strains carrying either a uvrB deletion or the plasmid pKM101. Frameshift mutagenesis by both compounds was completely eliminated by recA deletion, in both the presence or absence of the plasmid. The adenine-selective compound (1) appeared more sensitive to the DNA-repair defects than the guanine-selective derivative (2). Additionally, only the adenine-selective compound (1) caused statistically significant levels of detectable mutation in the repair-proficient strains TA102, TA4001 or TA4006. The bacterial mutagenesis evidence suggests that a bulky, major groove-residing adenine lesion may be more readily recognised by DNA-repair systems, and more likely to lead to a wider range of mutagenic events, than a similar guanine lesion.

Effects of the umuDC, mucAB, and samAB operons on the mutational specificity of chemical mutagenesis in Escherichia coli: II. Base substitution mutagenesis

Mutational spectra induced by different classes of chemical mutagens including two ultraviolet-mimetic mutagens, an alkylating agent, intercalators, a crosslinking agent, and base analogs were characterized by means of a set of mutant lacZ genes in E. coli. These strains can be used to detect each of two types of transition and four types of transversion, simply by measuring the number of Lac+ revertant colonies. 4-Nitroquinoline 1-oxide induced G.C-->A.T, G.C-->C.G, or G.C-->T.A changes almost equally, whereas furylfuramide and mitomycin C induced only G.C-->A.T transitions and G.C-->T.A transversions, respectively. No base substitutional mutations were detected by the treatment with 9-aminoacridine. A weak stimulation of G.C-->A.T transitions by ICR-191 was observed. Both the G.C-->A.T and A.T-->G.C transitions were induced by N-methyl-N'-nitro-N-nitrosoguanidine and N4-aminocytidine. 5-Azacytidine was a specific inducer of G.C-->C.G transversions. In addition, a comparative study of mutational specificity was performed in the strains bearing either the umuDC, mucAB, or the samAB operon on a multicopy plasmid. Regardless of the kind of mutagen, G.C-->T.A transversions were greatly potentiated by the introduction of plasmids in the order of pGW1700 (mucAB) > pSE117 (umuDC) > or = pYG8011 (samAB). Besides G.C-->T.A transversions, the introduction of pGW1700, but not pSE117 and pYG8011, enhanced the mutations of A.T-->C.G and A.T-->T.A transversions. The mucAB plasmid also enhanced the G.C-->A.T transitions and G.C-->C.G transversions induced by some mutagens.