Influence of DNA adenine methylation dam mutation and of plasmid pKM101 on the spontaneous and induced mutability of certain genes in Escherichia coli K12

Influence of dam and mismatch repair system on mutagenic and SOS-inducing activity of methyl methanesulfonate in Escherichia coli

In contrast to earlier reports (Mohn et al., 1980; Glickman, 1982), we show that E. coli dam- cells are able to mutate following MMS treatment. Since the mutagenicity of MMS has been regarded as largely dependent on induction of the SOS functions, E. coli strains bearing the recA::lacZ or umuC::lacZ fusions were used to determine the ability of MMS to induce the SOS functions in the various dam+ and dam- strains. The mutagenicity of MMS was also tested in several of these strains. The results show that (i) there is no direct correlation between SOS-inducing ability and mutagenicity potency of MMS; and (ii) most of the premutagenic lesions induced by MMS are removed from DNA of dam+ or dam- cells by the mismatch repair system. The role of strand breaks in repair of mismatches induced by alkylating agents is discussed.

Potassium chromate potentiates frameshift mutagenesis in E. coli and S. typhimurium

Possible comutagenic effects of chromate on frameshift mutagenesis were studied in bacterial assays. In these experiments, cells were treated with potassium chromate and 9-aminoacridine either singly or in combination. Results were analyzed to detect synergistic, additive and antagonistic responses. Data from these investigations show a clear potentiation of 9-aminoacridine-induced mutagenesis in the presence of chromate in S. typhimurium strain TA1537. Results from cell viability assays shows that the effect is not due to a toxicity artifact. Similar results are obtained in E. coli strains 343/358 (repair-proficient parental strain), 343/415 (recA-deficient), and 343/435 (mismatch-repair-deficient). These data indicate the neither induction of recA-protein nor inhibition of mismatch repair is involved in the action of chromate. In E. coli strain 343/447 (DNA polymerase I deficient), the potentiation was observed at lower concentrations of chromate. This finding suggests that polymerase I functions in recovery of cells from 9-aminoacridine-induced DNA damage and that its absence allows some of this damage to be dealt with in a manner which promotes mutagenesis in the presence of chromate. One possible explanation of these findings is that chromate and 9-aminoacridine react chemically to produce a unique mutagen and that damage caused by this mutagen is repaired via some excision process. However, no reaction between chromate and 9-aminoacridine could be detected by TLC under conditions similar to those in the bacterial assays, even at very high concentrations of both agents. Thus, it seems most likely that the potentiation is due to some action of chromate on repair and/or replication at sites of 9-aminoacridine intercalation. Chromate appears, then, to have significant comutagenic actions in bacterial systems.

Chromium (VI) comutagenesis: characterization of the interaction of K2CrO4 with azide

In a previous report chromate potentiated the mutagenicity of sodium azide, apparently by affecting repair and/or replication of DNA. Further evidence in support of such a mechanism for chromate potentiation is reported here. Chromate does not react directly with azide or its major mutagenic metabolite, azidoalanine, eliminating such reactions as possible mechanisms for potentiation. Further, azide was unable to potentiate the mutagenicity of chromate in Salmonella typhimurium strain TA104, which is sensitive to chromate mutagenicity but not to azide. Thus, it appears that the potentiation is not due to an action of azide in modulating chromate mutagenicity. Finally, the interaction was not altered by deficiency in recA gene product in S typhimurium GW19, nor by enhancement of SOS repair in the pKM101 containing strain TA100. Thus, induction of recA-dependent functions seems to play no role in the comutagenic actions of chromate. The simplest explanation for potentiation seems to be that chromate is able either to limit error-free recovery from azide-induced DNA damage or to promote error-prone repair or error-prone processing at sites of lesions.

Spontaneous mutagenesis: the roles of DNA repair, replication, and recombination

There appears to be no dearth of mechanisms to explain spontaneous mutagenesis. In the case of base substitutions, data for bacteriophage T4 and especially for E. coli and S. cerevisiae suggest important roles in spontaneous mutagenesis for the error-prone repair of DNA damage (to produce mutations) and for error-free repair of DNA damage (to avoid mutagenesis). Data from the very limited number of studies on the subject suggest that about 50% of the spontaneous base substitutions in E. coli, and perhaps 90% in S. cerevisiae are due to error-prone DNA repair. On the other hand, spontaneous frameshifts and deletions seem to result from mechanisms involving recombination and replication. Spontaneous insertions have been shown to be important in the strongly polar inactivation of certain loci, but it is less important at other loci. Perhaps with continued study, the term "spontaneous mutagenesis" will be replaced by more specific terms such as 5-methylcytosine deamination mutagenesis, fatty acid oxidation mutagenesis, phenylalanine mutagenesis, and imprecise-recombination mutagenesis. While most studies have concentrated on mutator mutations, the most conclusive data for the actual source of spontaneous mutations have come from the study of antimutator mutations. Further study in this area, perhaps along with an understanding of chemical antimutagens, should be invaluable in clarifying the bases of spontaneous mutagenesis.

Involvement of DNA lesions and SOS functions in 5-bromouracil-induced mutagenesis

Mutagenesis resulting from incorporation of 5-bromouracil (BU) in the DNA of E. coli K12 proceeds largely (approximately 80%) via misrepair of the lesions resulting from incorporation of the analogue. The premutational lesions are due principally to dehalogenation of incorporated BU residues, leading to formation of uracil residues, and removal of these by uracil-DNA glycosylase with formation of apyrimidinic sites. In the xthA mutant, defective in AP endonuclease, there is a several-fold increase in the frequency of BU-induced mutations, underlining the importance of AP sites in BU-induced mutagenesis. Premutational lesions undergo mutation frequency decline (MFD), which is subject to delay in the xthA mutant, pointing to some role of AP endonuclease in MFD, and further supporting involvement of AP sites in BU-induced mutagenesis. Efficient BU mutagenesis is dependent on the functions of the genes recA and umuC and non-mutated lexA protein.

The DNA repair host-mediated assay as a rapid and sensitive in vivo procedure for the determination of genotoxic factors present in various organs of mice. Some preliminary results with mitomycin C

The DNA repair host-mediated assay, in which repairable DNA damage is determined in E. coli cells present in various organs of mice exposed to genotoxic agents, was further developed to broaden the range of organs under study and to simplify the procedure of assessing differential bacterial cell survival. A pair of derivatives of E. coli K-12 strain 343/113 was constructed which differed vastly in DNA repair capacity (uvr+/rec+ vs uvrB/recA), as a means of assessing DNA damaging effects; furthermore, the strains differed in their ability to ferment lactose (delta Lac vs Lac+), so that the individual survival of both strains could be determined on a single agar medium (containing neutral red as pH indicator), on which the strains had different colony colour morphology (red, Lac+ vs white, Lac- colonies). Finally, the strains were made streptomycin-dependent, to prevent uncontrolled growth of the bacterial cells within the various organs and also to inhibit contamination of the survival agar medium by representatives of the normal intestinal microflora. The experimental procedure consisted of injecting mixtures of stationary cells of the two strains (ca. 3-5 X 10(8) viable cells per mouse) both intravenously and orally into mice, either pretreated or subsequently treated with test chemicals. Ninety minutes after injection of the bacteria, the liver, spleen, lungs, kidneys, stomach, intestine, colon, and ca. 50 microliter blood, were removed, suspended in buffer, homogenized, and the survival of the two strains determined on neutral red agar supplemented with streptomycin.(ABSTRACT TRUNCATED AT 250 WORDS)

The SOS-function-inducing activity of chemical mutagens in Escherichia coli

The SOS-function-inducing activities of 42 chemical mutagens were investigated in Escherichia coli K12. The induction of the SOS function was assayed by monitoring the beta-galactosidase activity in the sulA::lacZ fusion strain PQ37 . To correct for the inhibitory effects of test chemicals on mRNA or protein synthesis, the level of the constitutive alkaline phosphatase was assayed in parallel. Most of the mutagens reported to be mutagenic to the Ames' Salmonella tester strains showed the SOS-function-inducing activity. The inducible SOS repair may be responsible for not only base-change mutations but also frameshift mutations. However, 9-aminoacridine, ethidium bromide and 4-nitro-o-phenylenediamine did not induce the SOS function, suggesting that the mutagenesis induced by these mutagens may occur independently of SOS repair. Present results support the SOS mutagenesis model that error-prone SOS repair plays an important role in mutagenesis induced by most chemical mutagens.

Methodologies for the determination of various genetic effects in permeable strains of E. coli K-12 differing in DNA repair capacity. Quantification of DNA adduct formation, experiments with organ homogenates and hepatocytes, and animal-mediated assays

Derivatives of E. coli K-12 strain 343/113 differing in DNA repair capacity, in permeability to large molecules, and in some metabolizing activities (nitroreductase, glutathione), were constructed for the quantitative determination of the induction of various genetic effects, such as forward and back mutations, lysogenic induction of prophage lambda, and repairable DNA damage. These E. coli strains can be used in assay procedures which allow variation and control over several experimental conditions, such as oxygen tension, time, pH, temperature of incubation and growth phase of the indicator cells. Methods are described for the simultaneous determination of genetic effects and of DNA-adduct formation during mutagen treatment, i.e. by using radio-labeled compounds or by means of an enzyme-linked immunosorbent assay (ELISA). Mammalian biotransformation of xenobiotics can be investigated by including various fractions of mammalian organs in the system. Examples of the relative effectiveness of the activating potential of S9, S100 and isolated hepatocytes for dialkylnitrosamines and other carcinogens are presented. Host-mediated assays, finally, are described which, in addition to gene mutations, can also be used for the determination of repairable DNA damage in bacteria present in different organs, including the liver, spleen, lungs, kidneys, pancreas, and the blood stream of chemically treated mice. It is concluded that quantitative tests in vitro for assessment of induced mutagenic spectrum and genotoxic potency, combined with the host-mediated assay as a monitor, in vivo, of genotoxic factors present in various organs of animals, may become useful in the assessment of genotoxic (and possibly tumor-initiating) properties of chemicals for which long-term in-vivo mutagenicity and/or carcinogenicity data are not yet available.

Mutagenesis and anti-mutagenesis in Salmonella: influence of ethionine and caffeine on yields of mutations induced by 2-aminopurine and 9-aminoacridine

Ethionine, the ethyl analogue of methionine, slightly reduced the yield of reversions of the hisC3076 frameshift marker induced by 9-aminoacridine (9AA) in an excision-proficient strain of Salmonella typhimurium, but completely abolished mutagenesis by 9AA in the excision-deficient uvrB-deletion strain TA1537. No toxic effects of ethionine were apparent in either the excision-proficient or the excision-deficient strain. Because of the differential effects of ethionine on mutagenesis in the two strains, it seemed possible that an ethionine-sensitive step in the process(es) leading to fixation of 9AA-induced mutations might be compensated for by the uvrA,B,C+ excision-repair system. To further test this possibility, we used caffeine (a compound known to significantly reduce the efficacy of the excision-repair process) as a co-treatment with ethionine for cells of an excision-proficient strain exposed to 9AA. Treatment with caffeine alone or ethionine alone had very little effect on reversion yield, whereas co-treatment with the two agents abolished 9AA mutagenesis. It appeared, therefore, that either the caffeine-sensitive pathway or the ethionine-sensitive pathway needed to be functioning if 9AA-induced reversions of hisC3076 marker were to be detected. Addition of methionine to cells of the excision-deficient strain exposed to 9AA restored their ability to be mutated by 9AA, however. In a base-pair substitution back-mutation system, ethionine slightly enhanced the yields of revertants of the trpE8 marker induced by 2-aminopurine (2AP) in both an excision-proficient strain (at all 2AP dose levels tested) and an excision-deficient strain (only at the lower dose levels). In the excision-deficient strain, doses of 2AP above 300 micrograms/plate were highly toxic when ethionine was also present. It was for this reason that no 2AP-induced revertants were recovered at the higher 2AP concentrations. Treatment of the trpE8 strain with methionine also enhanced the yield of 2AP-induced revertants of this marker.

Involvement of the mismatch repair system in base analogue-induced mutagenesis

The influence of the mismatch repair system, and the role of mutS, mutR, and mutL genes, in mutagenesis induced by n2ome6Ade, n2oh6Ade and n2Pur have been investigated. From the frequency of reversion of Arg-Thr- and His- markers in AB2497, and its mut- derivatives, it was concluded that mismatches introduced by n2-ome6Ade and n2oh6Ade are better substrates for mismatch repair enzymes than that introduced by n2Pur. All these mut-gene products are more active in removing spontaneous or base analogue-induced mismatches which, when unexcised, lead to transversion of base repairs, than those which lead to transitions. Active engagements of mutL, mutR, or mutS gene products depend on the kind of mutation, the site of mutagenesis, and the inducing agent. Dam- cells are over-mutated by both n2ome6Ade and n2oh6Ade, but are hyper-sensitive to n2oh6Ade only. It is proposed that hyper-sensitivity of dam- cells is due not only to an increase in overlaping gap formation on both strands of DNA, but to a greater lability of the impaired cells. Results are presented which strongly suggest that n2ome6Ade in mut+ cells and n2oh6Ade in mut- only, can induce GC leads to TA transversions.

Genetic effects of N-methyl-N'-nitro-N-nitrosoguanidine and its homologs

Since the discovery of the mutagenic activity of N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) in 1960, this compound has become one of the most widely used chemical mutagens. The present paper gives a survey on the chemistry, metabolism, and mode of interaction of MNNG with DNA and proteins, and of the genotoxic effects of this agent on microorganisms, plants, and animals, including human cells cultured in vitro. Data on the carcinogenicity and teratogenicity of MNNG as well as on the genotoxic effects of homologs of MNNG are also presented.

Bacterial systems for carcinogenicity testing

During the past 30 years, bacterial test systems have been extensively refined in their ability to detect not only mutagenic agents but, in many cases, carcinogenic ones as well. Since many carcinogens are known to be activated within the mammalian body, major improvements in bacterial test systems were made when representative parts of mammalian metabolism were included as part of the test protocol. Presently, systems of great simplicity and convenience are available for the efficient detection of gene mutations, lysogenic induction of prophages, and differential DNA repair. These qualities render bacterial systems potentially useful in distinguishing between carcinogens and non-carcinogens, in characterizing induced mutation spectra, and possibly in quantifying mutagenic potency that may be used to predict tumor-initiating potency. Sensitive strains of Salmonella typhimurium. Escherichia coli and Bacillus subtilis with altered DNA-repair capacities have been constructed which accurately identify many carcinogens. Comparative studies have shown that techniques using these strains can be standardized to some extent and that the majority of carcinogens are active in all adequately sensitive genetic systems. Because of this redundancy, it may be sufficient to employ only one standardized set of tester strains and methodology. However, serveral classes of known carcinogens are undetected or underestimated when assayed in standard testing procedures. Some of these chemicals can be efficiently recognized as mutagens upon varying the methodology, the genetic endpoint, or the mammalian activation system. Thus, to modify and adjust the experimental protocol to the particular type of chemical under study and to calibrate the system with appropriate carcinogenic and non-carcinogenic reference compounds is advisable. It is noteworthy that chemical carcinogens which probably act by non-genotoxic mechanisms thus far remain undetected in bacterial tests. Newly developed systems which measure specific types of genetic events, such as transpositions of DNA segments and derepression of genes, presently are being tested for their ability to detect such carcinogens. A final matter of growing concern is the increasing number of environmental chemicals that are found to be mutagenic in bacteria but for which information about carcinogenic activity in vivo is insufficient. The possible use of bacteria for quantifying mutagenic potency and extrapolating this information to tumor-initiating potency can be envisaged in three ways: (i) direct extrapolation from standard in vitro tests, (ii) indirect extrapolation making use of an in vitro/in vivo comparison of induced effects (the parallelogram method) as devised by Sobels [138] on the basis of identical dose (to DNA), and (iii) host-mediated assays to assess mutagenic potency of carcinogens in selected organs of mammals...

Effects of ethionine on the replicational fidelity in V79 chinese hamster cells

The possibility that ethionine, the ethyl analog of methionine and potent liver carcinogen, exerts its action by blocking the methylation of DNA and thereby rendering the post-replicative methylation instructed error-avoidance system inoperative was investigated. While the results are not directly supportive for the existence of such a repair system in V79 Chinese hamster cells, effects of ethionine were found. Following the exposure of ethionine-treated cells to EMS an increase in cell killing and a decrease in mutation induction was observed. The base analog, 6-hydroxyaminopurine, was shown to be clearly mutagenic in the mammalian cells and in the presence of ethinine a drastic decrease in mutant frequency was observed. Ethionine itself did not appear mutagenic over the entire dose range tested (1-1000 micrograms/ml).