Physiological modification of alkylating agent induced mutagenesis. II. Influence of the numbers of chromosome replicating forks and gene copies on the frequency of mutations induced in Escherichia coli

Cell sensitivity to transplacental mutagenesis by N-ethyl-N-nitrosourea is greatest during early gestation in the Syrian hamster

The extremely high rate of cell division that occurs during early embryogenesis is hypothesized to predispose to high rates of mutation after chemical exposure. We tested this supposition experimentally. To probe the variation in susceptibility to mutation induction as a function of gestation stage, somatic cells of the developing Syrian hamster were isolated after transplacental treatment with N-ethyl-N-nitrosourea (ENU). Mutants were quantified using either 6-thioguanine (6-TG) or diphtheria toxin (DT) as selective agents. Several different approaches were used. In one, three litters were exposed on each gestation day and fetuses were removed on day 13. Maximum fetal sensitivity to ENU's genotoxic action was noted when treatment was at days 8 and 9, fewer mutants being obtained with earlier and later exposures. To compensate for the low numbers of target cells early in gestation, this experiment was repeated using larger numbers of litters exposed at the earlier time points, and the highest mutation frequency was now found to occur after treatment on gestation days 6 and 7. In the second approach, mutations were quantified in cells harvested 24 h after transplacental ENU exposure. Here again, embryos exposed at earlier times of gestation were more susceptible than those treated at later periods. Based on the total cell numbers in embryos and fetuses at each gestation day, we conclude that mutation frequency is maximal on day 6, corresponding to the primitive streak stage with extremely high rates of cell division.

DNA binding and mutagenicity of ethyl methanesulfonate in wild-type and uvrB cells of Salmonella typhimurium

The extent of DNA ethylation and the influence of excision repair on ethyl methanesulfonate (EMS) mutagenesis of Salmonella typhimurium were examined. The relationship between the dose to DNA and the exposure concentration of EMS was linear. EMS induction of his+ revertants followed exponential kinetics and did not parallel the increase in total DNA ethylation. Mutant induction was influenced by the cells' nucleotide excision repair ability. Although mutagenized to a larger extent than the wild-type (uvr+) strain at high doses, the uvrB strain was more resistant to the mutagenic effect of low doses of EMS.

Quantitative comparison of carcinogenicity, mutagenicity and electrophilicity of 10 direct-acting alkylating agents and of the initial O6:7-alkylguanine ratio in DNA with carcinogenic potency in rodents

The quantitative relationship between carcinogenicity in rodents and mutagenicity in Salmonella typhimurium was examined, by using 10 monofunctional alkylating agents, including N-nitrosamides, alkyl methanesulfonates, epoxides, beta-propiolactone and 1,3-propane sultone. The compounds were assayed for mutagenicity in two S. typhimurium strains (TA1535 and TA100) and in plate and liquid assays. The mutagenic activity of the agents was compared with their alkylating activity towards 4-(4'-nitrobenzyl)pyridine and with their half-lives (solvolysis constants) in an aqueous medium. No correlations between these variables were found, nor was mutagenic activity correlated with estimates of carcinogenicity in rodents. There was a positive relationship between carcinogenicity and the initial ratios of 7-:O6-alkylguanine formed or expected after their reaction with double-stranded DNA in vitro. The results suggest that alkylation of guanine at position O6 (or at other O atoms of DNA bases) may be a critical DNA-base modification that determines the overall carcinogenicity of these alkylating agents in rodents.

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.

Influence of DNA-repair deficiencies on MMS- and EMS-induced mutagenesis in Escherichia coli K-12

The mutagenic effects of the alkylating agents, MMS and EMS, were studied by measuring the reversion of an arg (ochre) mutation in various DNA-repair-deficient strains. When compared with the wild-type strain, EMS-induced mutagenesis was reduced in the recA13 strain but not in the lexA3 strain. MMS-induced mutagenesis was reduced to background levels in the recA13 strain and reduced to intermediate levels in the lexA3 strain. The umuC36 strain showed intermediate levels of mutagenesis with both mutagens which suggests that a substantial portion of both MMS- and EMS-induced mutagenesis depends upon this component of the error-prone "SOS" repair pathway. The uvrD101, recL152 and recF143 mutations produced increased levels of MMS-induced reversion but had no effect upon the levels of EMS-induced mutagenesis, suggesting that the pathways affected by these genes may play a role in the error-free repair of MMS but not EMS damage. In contrast, a large increase in the level of mutagenesis was noted in a delta uvrB101 mutant with EMS but not MMS. This hypermutability with EMS was also seen in uvrA6, uvrB5 and uvrC34 mutant strains and suggests a role for excision in the error-free repair of ethylation but not methylation damage to DNA.

A possible mechanism for an interaction between postreplication recombination repair and base misincorporation in mutation induction by N-methyl-N'-nitro-N-nitrosoguanidine in Haemophilus influenzae

Evidence in previous publications has suggested that treatment with monofunctional alkylating agents such as N-methyl-N'-nitro-nitrosoguanidine (MNNG) results in gaps being left in the DNA synthesized shortly after the treatment. This paper presents further evidence that suggests, though it does not conclusively prove, that there are indeed gaps. It shows that these events increase linearly with MNNG concentration, that they are formed mainly in DNA synthesized during the first hour after treatment, and that only a few are formed at later times. An hypothesis that involves the conversion by recombination repair of a single-strand base substitution, resulting from insertion of an incorrect base opposite an alkylated base, to a double-strand base substitution is proposed. It is suggested that most single-strand substitutions are removed by mismatch repair, leaving the double-strand substitutions as the main source of mutations. This hypothesis predicts that the mutation frequency will increase as the square of the exposure to MNNG, and this seems to be the case, at least at lower exposures at which complicating factors such as lengthened expression time are avoided. It also can explain a number of earlier observations on mutation fixation as detected by transformation. An attempt to show that the non-coding lesions causing the gaps were apurinic sites was unsuccessful.

Relationship between repair processes and mutation induction in bacteria

A summary is given of the main repair and replication-associated processes that can influence the induction of mutations by various mutagens in bacteria. These include both constitutive and induced, error-free and error-prone systems. The mutation yield from a treatment with a mutagen can be markedly affected by which of these systems is operating in a given bacterial species or strain. The effect of these systems on mutation induction by ultraviolet light, monofunctional alkylating agents, base analogues, and frameshift mutagens is discussed in some detail. The bearing of these studies on the practical problems of estimating hazards is briefly considered.

Forward mutations to arabinose resistance in Salmonella typhimurium strains: a sensitive assay for mutagenicity testing

The forward-mutation assay using the L-arabinose-sensitive strain SV3 of Salmonella typhimurium has been calibrated against a selected set of mutagens. Strain SV3 is sensitive to chemicals causing base-pair substitutions, frameshift mutations and deletions. New strains deficient for the excision-repair system or the lipopolysaccharide barrier or both have been selected from strain SV3. The additional mutations do not affect the independence of the assay from experimental artifacts due to physiological or lethal damage or differences in plating density. The new strains are more sensitive than SV3 to certain mutagens. Techniques for using this set of strains are presented and their relative advantages discussed.

The role of pre-replication and post-replication processes in mutation induction in Haemophilus influenzae by N-methyl-N'-nitro-N-nitrosoguanidine

Studies were carried out on the repair and fixation of premutational damage induced in Haemophilus influenzae by N-methyl-N'-nitro-N-nitrosoguanidine (MNNG). The studies employed a temperature-sensitive DNA elongation mutant (dna9) and its combinations with mutants defective in pyrimidine dimer excision (uvr1, uvr2) and in recombination (rec1). The dna9 mutant is shown to be leaky, allowing about 1% of the normal rate of DNA synthesis at the restrictive temperature. Repair of premutational lesions was detected by a decline in mutation frequency with increasing delay in DNA replication in dna9 at the restrictive temperature. This repair is unaffected by the pyrimidine dimer excision system. Mutation fixation was detected by the ability of DNA from treated and then lysed cells to transfer mutants to recipient cells by transformation. Some fixation occurred at the restrictive temperature but much less than at the non-restrictive temperature suggesting that an appreciable minority of the mutations resulted from lesions introduced near the replication fork but that the majority of mutations arise from lesions introduced at some distance from the fork, perhaps randomly. The DNA synthesized immediately after MNNG treatment is of lower molecular weight than normal and returns to normal with time. This return is blocked in the rec1 mutant, suggesting that recombination is involved. The possible role of this process in MNNG mutagenesis is discussed.

The relation of repair phenomena to mutation induction in bacteria

The relation of various processes to mutation induction by radiation and chemicals is discussed for for various species of bacteria. A variety of repair processes have been identified at the molecular level that can eliminate many kinds of potentially mutagenic lesions before they can be converted to final mutation. Fixation often but not always occurs at replication. A number of mutagens, including UV light, ionizing radiation, and a number of chemicals, induce an error-prone process, perhaps a modification of the proof-reading system, that allows bacteria to survive after potentially lethal damage at the expense of making errors. Some mutagens, notably monofunctional alkylating agents and base analogues, produce mutations by other processes. Even in these cases, repair processes play an important role. There is some evidence that error-free as well as error-prone repair processes can be induced. A brief discussion is given of the relation of these findings to the practical problems of hazards estimations.