Specificity of replicative and SOS-inducible DNA polymerases in frameshift mutagenesis: mutability of Salmonella typhimurium strains overexpressing SOS-inducible DNA polymerases to 30 chemical mutagens

DNA replication is frequently hindered because of the presence of DNA lesions induced by endogenous and exogenous genotoxic agents. To circumvent the replication block, cells are endowed with multiple specialized DNA polymerases that can bypass a variety of DNA damage. To better understand the specificity of specialized DNA polymerases to bypass lesions, we have constructed a set of derivatives of Salmonella typhimurium TA1538 harboring plasmids carrying the polB, dinB or mucAB genes encoding Escherichia coli DNA polymerase II, DNA polymerase IV or DNA polymerase RI, respectively, and examined the mutability to 30 chemicals. The parent strain TA1538 possesses CGCGCGCG hotspot sequence for -2 frameshift. Interestingly, the chemicals could be classified into four groups based on the mutagenicity to the derivatives: group I whose mutagenicity was highest in strain YG5161 harboring plasmid carrying dinB; group II whose mutagenicity was almost equally high in strain YG5161 and strain TA98 harboring plasmid carrying mucAB; group III whose mutagenicity was highest in strain TA98; group IV whose mutagenicity was not affected by the introduction of any of the plasmids. Introduction of plasmid carrying polB did not enhance the mutagenicity except for benz[a]anthracene. We also introduced a plasmid carrying polA encoding E. coli DNA polymerase I to strain TA1538. Strikingly, the introduction of the plasmid reduced the mutagenicity of chemicals belonging to groups I, II and III, but not the chemicals of group IV, to the levels observed in the derivative whose SOS-inducible DNA polymerases were all deleted. These results suggest that (i) DNA polymerase IV and DNA polymerase RI possess distinct but partly overlapping specificity to bypass lesions leading to -2 frameshift, (ii) the replicative DNA polymerase, i.e., DNA polymerase III, participates in the mutagenesis and (iii) the enhanced expression of E. coli polA may suppress the access of Y-family DNA polymerases to the replication complex.     

A comparative study of mutagenic and SOS-inducing activity of biphenyls, phenanthrenequinones and fluorenones

A total of 23 chemicals--biphenyls, phenanthrenequinones and fluorenones--were tested for mutagenicity towards Salmonella typhimurium strains TA1538, TA1535 and TA98. SOS-inducing activity of the same chemicals was studied in terms of the SOS-inducing potency in Escherichia coli PQ37, using an automated instrument controlled by a dedicated computer program for the SOS Chromotest. Of the 23 chemicals studied 14 induced His+ revertants in S. typhimurium TA1538 hisD305 (-1 frameshift); none induced His+ reversions in TA1535 (base-pair substitution). The mutagenicity of the chemicals in S. typhimurium TA98 (pKM 101) was lower than in TA1538. There was a close correlation between mutagenicity and SOS-inducing activity of fluorenones and phenanthrenequinones. None of the biphenyls tested induced SOS response and this property does not depend upon the mutagenic activity of the chemicals. SOS Chromotest is particularly valid in detecting chemicals which give rise to base-pair substitutions through SOS induction. If positive results are obtained, the Salmonella assay may be omitted. However, this test cannot replace the Ames test especially for the primary screening of mutagenicity of chemicals with unknown structure.     

Epidermal enzyme-mediated mutagenicity of the skin carcinogen, 2-aminoanthracene

Using four Salmonella typhimurium tester strains (TA1537, TA1538, TA98 and TA100) and the promutagen 2-aminoanthracene, an epidermal S9-mediated mutagenicity assay was developed. Using an activation mixture derived from whole skin of the rat, mutagenicity was observed in tester strain TA98 whereas an activation mixture derived from the dermis resulted in mutagenicity in tester strains TA1538, TA98 and TA100. Activation mixtures from both the epidermis and the liver produced a positive response in all of the tester strains studied. Activation mixtures from liver were shown to have the highest specific activity followed in decreasing order of potency by epidermis, dermis and whole skin. These results indicate that the skin, a target tissue directly exposed to environmental chemicals, is capable of converting 2-aminoanthracene to mutagenic moieties. Since the skin of the rat is known to be susceptible to tumor induction by 2-aminoanthracene our findings re-emphasize that membrane-bound enzymes can influence toxic responses including mutagenicity to xenobiotics in cutaneous tissue.     

Mutagenic potency of some conjugated nitroaromatic compounds and its relationship to structure

The mutagenicities of 12 conjugated non-fused nitroaromatic compounds and 1 amino analogue were determined in strains TA100 and TA98 of Salmonella typhimurium. Reversions by p-nitroaromatics increased in the order of the acetophenone, benzaldehyde, styrene, chalcone, cinnamic acid and stilbene indicating the importance for mutagenic potency of extended conjugation to the p-nitrophenyl substituent. Highest mutagenicity was found with alpha-substituted 4-nitrostyryl derivatives of which the phenyl derivative (31 revertants per nmole in TA100) was the most active. Generally, the TA100 strain was more sensitive than TA98 to these mutagens and S9 treatment was unnecessary for activity, although 4-nitrochalcone required S9 activation. Para-nitro isomers of the cinnamic acids and chalcones were much more active than the corresponding ortho and meta isomers. The 4-aminocinnamic acid analogue was inactive suggesting that complete reduction in Salmonella of 4-nitrocinnamic acid to an active amino derivative is not response for the high mutagenicity of the former. Mutagenicity of these p-nitrostyryl compounds may be explained by the covalent interaction of the electrophilic benzylic carbon with Salmonella DNA.     

Optimum associations of tester strains for maximum detection of mutagenic compounds in the Ames test

The Ames test is now widely used as a short-term test for the detection of mutagens. Different strains are available with various genetic characteristics, and in the past decade various authors have recommended different associations of strains to give maximum detection potential. However, few studies have been done to compare the sensitivity of individual strains towards a wide range of compounds in a single study. In order to define the best association of strains for screening or regulatory purpose, we have tested 103 direct mutagens (reference genotoxins or in-house compounds) on 7 strains of Salmonella typhimurium: TA1535, TA1537, TA1538, TA97, TA98, TA100 and TA102. 126 different associations of strains have been studied in terms of sensitivity and percentage overlap. Optimum associations of 2, 3, 4 or 5 strains included strains both with and without plasmid pKM101. However, the specificity of detection is greatly diminished by the presence of plasmid pKM101 in the strain, as shown by the high degree of overlap in associations constituted entirely of strains containing the plasmid. The association of strains TA1538 and TA100 detected 86% of the chemicals tested and is therefore recommended for large-scale screening. A rate of detection of 100% was obtained when 6 strains were used. The best associations of 4 and 5 strains, which detected 97 and 99% chemicals respectively, all contained strains TA1537, TA1538 and TA102. Finally, the associations of 4 strains (TA1537, TA1538, TA100, TA102) or 5 strains (TA1535, TA1537, TA1538, TA97, TA102) seemed well adapted to the optimum detection of mutagenic compounds.     

Relationship between mutagenic potency in Salmonella typhimurium and chemical structure of amino- and nitro-substituted biphenyls

All positional isomers of mononitro- and monoaminobiphenyls and those of dinitro-, diamino- and aminonitrobiphenyls, which have one substituent on each benzene ring, were assayed for mutagenicity in Salmonella typhimurium by the Ames method. The results suggest that the structural requirements favoring mutagenic activity are the presence of substituents at the 4-position and their absence at the 2'-position. The introduction of an amino group to the 3'- or 4'-position of 4-nitrobiphenyl or a nitro group to 3'- or 4'-position of 4-aminobiphenyl enhanced the mutagenicity. Among the mutagenic compounds, 4-nitro analogues were mutagenic in strains TA98 and TA100 in the absence of a microsomal metabolic activation system. Strain TA98NR was not reverted by the direct-acting mutagens, whereas strain TA98/1,8-DNP6 was as revertible as strain TA98; these results suggest that the direct-acting mutagenicity involves the reduction of the nitro group by bacterial nitroreductase but does not involve specific esterification enzymes.     

recA-dependent inhibition of cell respiration is not induced by mitomycin C in Escherichia coli

The recently developed strain TA102, particularly suited to the detection of oxidative mutagens (Levin et al., 1983), was the most sensitive out of 9 strains of S. typhimurium his- in revealing the mutagenicity of Cr(VI) compounds (sodium dichromate, calcium chromate and chromium trioxide). The rank of sensitivity was the following: TA102, TA100, TA97, TA92, TA1978, TA98, TA1538 and TA1537, TA1535 being the only insensitive strain. Cr(III) compounds (chromic acetate, chromic nitrate and chromic potassium sulfate) were totally inactive with all strains. The direct mutagenicity of Cr(VI) was markedly decreased, through NADPH-requiring mechanisms, by rat-liver S9 fractions and, to a lower extent, by human lung S12 fractions, which supports the hypothesis of a metabolically regulated threshold in chromium pulmonary carcinogenicity.     

Structural specificity of aromatic compounds with special reference to mutagenic activity in Salmonella typhimurium--a series of chloro- or fluoro-nitrobenzene derivatives

The mutagenicity of 21 chloro- or fluoronitrobenzene compounds and 9 chloro- or fluorobenzene compounds in Salmonella typhimurium (strains TA98, TA1538, TA1537, TA100 and TA1535) was examined. The tests were carried out under the conditions of absence and presence of liver microsomal activation. 15 nitro-group compounds had mutagenic activity; above all, compounds of fluoronitrobenzene were mutagenic for both types of strain. On the other hand, chloronitrobenzene compounds were mutagenic for base-pair substitution strains only. Mutagenic activity was exhibited by all compounds having a chloro or fluoro substituent at the para and ortho position in the nitrobenzene nucleus. All compounds without a nitro substituent showed no mutagenic activity.     

Mutagenicity evaluation of the two antimalarial agents chloroquine and mefloquine, using a bacterial fluctuation test.

The two antimalarial agents chloroquine and mefloquine have been tested for mutagenicity in Salmonella typhimurium strains TA1535, TA1537 and TA1538. Chloroquine was found to revert strain TA1537 at concentrations of 100 and 250 micrograms/ml, most likely due to intercalation. No mutagenicity was found with mefloquine at concentrations up to 2.5 micrograms/ml, neither without nor with metabolic activation by Ca2+-precipitated rat liver microsomes. Higher concentrations of mefloquine and chloroquine inactivated the bacteria.     

Testing of some azo dyes and their reduction products for mutagenicity using Salmonella typhimurium TA 1538

A series of ten azo dyes as well as various single ring aromatic amines substituted on the benzene ring were tested for bacterial mutagenicity with Salmonella typhimurium TA 1538 using a soft-agar overlay method. Two dyes, sudan 2 and chrysoidin induced mutation but only in the presence of a rat liver preparation. Chrysoidin was the more active. Testing of its reduction products, aniline and 1,2,4-triaminobenzene showed a liver metabolite of the latter compound could be responsible for the mutagenic effect, having a comparable mutagenicity with 1,2-diamino-4-nitro-benzene, one of the mutagenic constituents of hair dyes. Structure-activity studies on a series of ring-substituted anilines indicated that mutagenic activity required at least two positions to be substituted with either amino or nitro groups, or one of each. The bacteria as well as the liver enzyme preparation may partake in the activation of these chemicals. The correlation between mutagenicity and carcinogenicity for this group of compounds is discussed.     

The mutagenicity on Salmonella typhimurium of nitrobenzoic acids and other wastewater components generated in the production of nitrobenzoic acids and nitrotoluenes

The wastewater contained mutagens which induced mutations in Salmonella typhimurium TA1535, TA1538, TA98 and TA100. By the use of nitroreductase-proficient and -deficient tester strains, it was possible to demonstrate that the mutagens were to a great extent aromatic nitro compounds. 30-40% of the mutagenicity could be related to the 16 identified nitroaromatic compounds. Although 13 of these induced mutations, one single compound, 3,5-dinitrobenzoic acid, was responsible for more than 80% of their total mutagenicity. p-Nitrobenzoic acid was used for further studies of the enzymatic nitroreduction leading to the formation of reactive intermediates. The bacterial enzymes and the active metabolites did not seem to be oxygen-sensitive, as the mutagenicity was decreased when anaerobic incubation was applied. The addition of dicoumarol resulted in a decreased effect, indicating that bacterial DT diaphorase or an enzyme with similar properties is responsible at least in part for the activation of this compound. Under our experimental conditions rat-liver enzymes were not able to produce any detectable amounts of mutagenic metabolites of p-nitrobenzoic acid when the nitroreductase-deficient strain TA100NR was used.     

Relationships between structures and mutagenic potencies of 16 heterocyclic nitrogen mustards (ICR compounds) in Salmonella typhimurium

16 heterocyclic nitrogen mustards (ICR compounds), which were synthesized for use as possible antitumor agents by Creech and coworkers, were tested for mutagenicity in Salmonella typhimurium strains TA1535, TA1536, TA1537, TA1538, TA98 and TA100. The compounds were incorporated into the top agar at 5 doses: 0.5, 1, 2.5, 5 and 10 micrograms/plate. All of the compounds were negative in TA1535 except ICR 449, which was positive in all 6 strains. The other 15 compounds were positive in the remaining strains with the following exceptions: ICR 371 and 355 were negative in TA100; ICR 445 was negative in TA98 and TA100; and ICR 360 was negative in TA1537, TA1538, TA98 and TA100. Good qualitative agreement was observed between the mutagenic and antitumor activities of the 16 compounds, and between the mutagenic and carcinogenic activities of the 5 compounds that have been tested for carcinogenicity by Peck and coworkers. However, no significant correlation was found between mutagenic potency in Salmonella and antitumor potency in mice for the 16 compounds. Also, for the 5 compounds that have been tested for carcinogenicity, no significant correlation was found between their mutagenic potency in Salmonella and their carcinogenic potency in mice. In Salmonella, the secondary (2 degrees) amines generally were more mutagenic than their tertiary (3 degrees) amine homologs, although the opposite result has been reported in certain eukaryotes. Relationships between structures and potencies for the different nuclei of the 16 ICR compounds are discussed, as are similarities and differences in strain sensitivities. We conclude that the Salmonella his reversion test is not a good predictor of the antitumor and carcinogenic potencies of these ICR compounds.     

Mutagenicity of benzidine and 4-aminobiphenyl after metabolic activation with isolated hepatocytes and liver 9000 × g supernatant from rat,hamster and guinea pig

The mutagenicity of benzidine and 4-aminobiphenyl towards Salmonella typhimurium strain TA1538 was measured in the presence of isolated hepatocytes from rat, hamster and guinea pig. The mutagenic potency of these compounds was also assayed with S9 (9000 × g supernatant) prepared from disrupted hepatocytes of these aryl amines was investigated.For all 3 animal species it was found that the mutagenicity of benzidine is higher with intact hepatocytes than with S9 prepared from disrupted hepatocytes. Addition of acetyl coenzyme A to the S9 fraction increased the mutagenicity of benzidine. In contrast to benzidine, the mutagenicity of 4-aminobiphenyl appeared to be lower with hepatocytes than with S9. Addition of acetyl coenzyme A to the S9 fraction decreased the mutagenicity of 4-aminobiphenyl.The mutagenic potency of 4-aminobiphenyl was almost equal in the presence of the liver preparations from the 3 different species, whereas obvious species differences were seen with benzidine.     

Mutagenicity of anthraquinone and hydroxylated anthraquinones in the Ames/Salmonella microsome system.

The mutagenicity of anthracene, anthraquinone, and four structurally similar compounds of each was evaluated in the Ames/Salmonella microsome assay. Anthraquinone was shown to be mutagenic for strains TA1537, TA1538, and TA98 in the absence of rat liver homogenate. The four anthraquinone derivatives tested were mutagenic for TA1537 exclusively. None of the anthracenes exhibited mutagenic activity.     

Production of frameshift mutations in Salmonella by phenanthridinium derivatives: enzymatic activation and photoaffinity labeling

The effect of metabolic activation on the mutagenic potential of some phenanthridinium compounds was examined in Salmonella typhimurium strains TA1538 and TA1978 . All of the compounds tested were mutagenic in TA1538, a DNA excision-repair-deficient strain, when metabolizing enzymes were included in the assay. Reversions were not detected when these compounds were examined under the same conditions in TA1978 , the isogenic strain of TA1538 proficient in DNA repair. The mutagenic activity of an azido analog of propidium iodide was also examined using photoactivation and enzymatic activation, and with both conditions, reversions were observed in TA1538 but not in TA1978 . Furthermore, the ranking of mutagenic activity of propidium azide relative to ethidium azide analogs was comparable for both types of activation. The evidence from several studies suggests that the structural requirements for mutagenic activity for this series of phenanthridinium compounds appear to be the same whether mutagenesis is induced via photoactivation or metabolic activation. The interaction with DNA resulting in covalent alteration of the DNA is implicated as the mutagenic mechanism whether the active species is generated by metabolic- or photo-activation.     

Mutagenic action of methyl 2-cyanoacrylate vapor

Alkyl 2-cyanoacrylate adhesives were tested for mutagenicity in Salmonella typhimurium strains TA98, TA100, TA1535 and TA1538. Both a normal spot test and a spot test specially designed to test volatile compounds were used. The adhesives were also tested in the plate incorporation assay. These investigations showed that methyl 2-cyanoacrylate adhesives are mutagenic in strain TA100. The spot test for volatile compounds showed that it is the vapors from the methyl 2-cyanoacrylate monomer that are responsible for the mutagenic effect. One can conclude that working with methyl 2-cyanoacrylate adhesives entails exposure to vapors with a mutagenic effect and may therefore pose a carcinogenic hazard. Because the adhesives are used in industry, their mutagenic effect has a special importance in work environment.     

Identification and characterization of SnrA,an inducible oxygen-insensitive nitroreductase in Salmonella enterica serovar Typhimurium TA1535

The biological activity of many nitrosubstituted compounds, many of which are produced commercially or have been identified as environmental contaminants, is dependent on metabolic activation catalyzed by nitroreductases. In the current study, we have cloned a nitroreductase gene, Salmonella typhimurium nitroreductase A (snrA), from S. enterica serovar Typhimurium strain TA1535, and characterized the purified gene product. SnrA is 240 amino acids in length and shares 87% sequence identity to the Escherichia coli homolog, E. coli nitroreductase A (NfsA). SnrA is the major nitroreductase in S. enterica serovar Typhimurium strain TA1535 and catalyzes nitroreduction through a ping-pong bi-bi mechanism in a NADPH and flavine mononucleotide (FMN) dependent manner. SnrA exhibits extremely low levels of FMN reductase activity but the nitroreductase activity of SnrA is competitively inhibited by exogenously added FMN. Treatment of TA1535 with paraquat resulted in induction of nitroreductase activity, suggesting that SnrA is a member of the S. enterica serovar Typhimurium SoxRS regulon associated with cellular defense against oxidative damage. Examination of the microbial genomes databases shows that SnrA homologs are widely distributed in the microbial world, being present in isolates of both Archea and Eubacteria. Southern hybridization and PCR failed to detect the snrA gene in the closely related S. enterica serovar Typhimurium strain TA1538. S. enterica serovar Typhimurium strains TA1535 and TA1538 and their derivatives are commonly used in mutagenicity testing. Differences in metabolic capacity between these two strains may have implications for the interpretation of mutagenicity data.     

Comparative genotoxic effects of the cooked-food-related mutagens Trp-P-2 and IQ in bacteria and cultured mammalian cells

As part of a major study to evaluate the mutagenicity of chemicals produced during the cooking of foods, we examined the responses of bacteria and cultured Chinese hamster cells to the compounds Trp-P-2 (3-amino-1-methyl-5H-pyrido[4,3-b]indole) and IQ (2-amino-3-methylimidazo[4,5-f]quinoline), constituents identified in cooked beef and fish. In the Ames/Salmonella tester strain TA1538, both compounds were confirmed to be extremely potent mutagens that were active at levels below 1 ng/plate in the presence of hamster-liver S9 microsomal fraction. 50-fold higher doses of both compounds were required for mutagenicity in the uvr+ tester strain TA1978. Trp-P-2 also behaved as a strong mutagen in CHO cells using the standard exogenous activation with hamster-liver S9 fraction. At concentrations below 1 microgram/ml it produced dose-dependent increases in cell killing, mutations at the hprt and aprt loci, sister-chromatid exchanges, and chromosomal aberrations. An excision-repair-deficient strain was about 2-fold more sensitive than the normal CHO cells with respect to these genotoxic effects of Trp-P-2. IQ had unexpectedly weak activity for all genetic endpoints in the CHO cells, and it produced clear-cut responses only in the repair-deficient cells and only above a concentration of 10 micrograms/ml. The toxicity that was observed with IQ was not affected by the repair capacity of the cells and was not associated with chromosomal aberrations, indicating that damage to cellular structures other than nuclear DNA was likely the predominant pathway for cell killing. Because the culture conditions normally used for CHO cell exposure were shown to be competent in producing bacterial mutagenicity with IQ, it was concluded that the active metabolite of IQ was present in the medium but was somehow ineffective in reaching the DNA of CHO cells and/or reacting with it. These results suggest that the relative mutagenic potency of compounds in Salmonella may bear no direct relationship to relative mutagenicity in CHO cells, emphasizing precaution in attempting to extrapolate microbial data to mammalian somatic cells. This study illustrates the use and merits of a multi-endpoint assay for genetic damage in CHO cells, the utility of using CHO cells that are defective in excision repair of DNA, and the importance of comparative testing between bacterial and mammalian systems.     

Comparative mutagenicity of aliphatic epoxides in Salmonella

37 aliphatic epoxides comprising 6 subclasses (unsubstituted aliphatic epoxides, halogenated aliphatic epoxides, glycidyl esters, glycidates, glycidyl ethers and diglycidyl ethers) were tested, under code, for mutagenicity in Salmonella strains TA98, TA100, TA1535 and TA1537 and/or TA97 with and without metabolic activation using a standardized protocol. The 4 halogenated aliphatic epoxides and the 4 diglycidyl ethers were all mutagenic. The 2 glycidates were negative in all strain/activation systems used while all 5 glycidyl esters were mutagenic. 3 of the 8 unsubstituted aliphatic epoxides and 11 of the 12 glycidyl ethers were mutagenic. Glycidol also was mutagenic whereas 9,10-epoxyoctadecanoic acid, 2-ethylhexyl ester was not mutagenic. Of the 28 mutagenic compounds, all but neodecanoic acid, 2,3-epoxypropyl ester and 2-ethylhexyl glycidyl ether were detected in TA100 without activation. The latter two were detected only with activation in TA100 and TA1535. The majority of the other 26 chemicals were also mutagenic in TA1535 without activation. Good intra- and interlaboratory reproducibility was seen in the results of each of the 4 chemicals tested in more than one set of experiments. The current results confirm and extend the observations of other investigators regarding structural effects on the mutagenicity of members of the aliphatic epoxide class of chemicals.     

Mutagenicity of antiamebic and anthelmintic drugs in the Salmonella typhimurium microsomal test system

4 amebicides (chloroquine diphosphate, diiodohydroxyquin, iodochlorohydroxyquin and dehydroemetine) and 6 anthelmintics (bephenium hydroxynaphthoate, 4-hexylresorcinol, mebendazole, niclosamide, pyrantel pamoate and pyrvinium pamoate) were tested for mutagenicity in the Salmonella typhimurium microsomal test system. Frameshift mutations were induced by dehydroemetine and niclosamide following activation by microsomal enzymes, while pyrvinium pamoate induced both frameshift and base-pair substitution mutations with or without metabolic activation. The urine of mice treated with dehydroemetine or pyrvinium pamoate showed no mutagenic activity. However, urine obtained from mice treated with niclosamide was mutagenic in strains TA98 and TA1538. The fluctuation assay showed chloroquine diphosphate to be mutagenic in TA1537, a strain which detects frameshift mutations.