Tests for the genotoxicity of m-AMSA, etoposide, teniposide and ellipticine in Neurospora crassa

The antitumor agents m-AMSA, etoposide, teniposide and ellipticine have been reported to be potent clastogens in mammalian cells but non- or weakly mutagenic in bacteria; these observations have been correlated to the interference of these chemicals with DNA topoisomerase II activity in the former, but not in the latter, organisms. The genotoxicity of these 4 agents was evaluated using ad-3 reverse- and forward-mutation tests in Neurospora crassa. These agents (up to 0.8 mumole/plate) did not cause reversion in conidia of the ad-3A frameshift strains N24 and 12-9-26 using the overlay plate test, as contrasted to the positive control frameshift mutagen ICR-170. Heterokaryon 12 (H-12) of N. crassa permits the recovery of all classes of forward mutation at the ad-3+ region, including multilocus deletions. Using resting conidia of H-12 in a suspension assay, ellipticine was moderately mutagenic but no increase in ad-3 mutants was noted with the other 3 agents at a dose of 100 micrograms/ml. In vegetative cultures of H-12 grown in the presence of these agents, all 4 agents were nonmutagenic at a dose of 100 micrograms/ml. The positive control mutagen ICR-170 was mutagenic in both resting conidia and growing cultures of H-12. A similarity between the topoisomerase II of N. crassa and DNA gyrase of bacteria is suggested.     

Effect of the uvs-2 allele of Neurospora crassa on the mutagenic potency of two N-hydroxylaminopurines and 2-aminopurine in the ad-3 forward-mutation test

The mutagenic potencies of 3 purine analogs were determined in the ad-3 forward-mutation test in growing cultures of heterokaryon 59 (H-59), a nucleotide excision repair-deficient (uvs-2/uvs-2) 2-component heterokaryon of Neurospora crassa. Two N-hydroxylaminopurines, 2-amino-6-N-hydroxylaminopurine (AHA) and 6-N-hydroxylaminopurine (HAP), were potent and strong mutagens, respectively, whereas 2-aminopurine (AP) was a moderate mutagen. Dose-response curves showed that AHA and HAP were about equally mutagenic at low doses but that AHA was more mutagenic than HAP at high doses. Comparison of these results in H-59 with our earlier results in heterokaryon 12 (H-12) of N. crassa, which is identical to H-59 except for being DNA-repair-proficient (uvs-2+/uvs-2+), shows that the defect in nucleotide excision repair due to uvs-2 has little or no effect on the mutagenic potencies of these 3 purine analogs. Therefore, the nucleotide excision-repair pathway in N. crassa that is deficient in H-59 does not appear to have a major role in the repair of pre-mutational lesions induced by these 3 purine analogs. On the other hand, based on the controls of these experiments, the frequency of spontaneous ad-3 mutants was 4 greater in H-59 than in H-12. This result suggests that the nucleotide excision-repair pathway in N. crassa that is inactivated by the uvs-2 mutation has a major role in the repair of lesions that would lead to spontaneous mutation at the ad-3+ region if they were not repaired.     

Effect of the homokaryotic state of the uvs-2 allele in Neurospora crassa on formaldehyde-induced killing and ad-3 mutation

Formaldehyde was tested for its killing and mutagenic activities in the ad-3 forward-mutation test in Neurospora crassa. The test was conducted in 3 two-component heterokaryons (dikaryons) of N. crassa in order to determine the effect of the uvs-2 allele, which causes a defect in nucleotide excision repair, on formaldehyde-induced killing and the induction of ad-3 mutants. These dikaryons were homokaryotic for uvs-2+ (H-12), homokaryotic for usv-2 (H-59), and heterokaryotic for uvs-2 (H-71). Formaldehyde induced killing and ad-3 mutants in H-12, but the presence of uvs-2 in the homokaryotic state (H-59) resulted in a 9-fold increase in killing and a 40-fold increase in the induction of ad-3 mutants. This increased sensitivity to formaldehyde-induced killing and mutation conferred by uvs-2 in the homokaryotic state (H-59 vs. H-12) is similar to that noted by others in Escherichia coli. Salmonella typhimurium and Saccharomyces cerevisiae. The dikaryon heterokaryotic for uvs-2 (H-71) has the same sensitivity to formaldehyde-induced ad-3 mutation as H-12, indicating that uvs-2 is recessive to uvs-2+.     

Two N-hydroxylaminopurines are highly mutagenic in the ad-3 forward-mutation test in growing cultures of heterokaryon 12 of Neurospora crassa

3 purine analogs were tested for their mutagenic activities in the ad-3 forward-mutation test in heterokaryon 12 (H-12) of Neurospora crassa. In growing cultures of H-12, the N-hydroxylaminopurines 2-amino-6-N-hydroxylaminopurine (AHA) and 6-N-hydroxylaminopurine (HAP) are potent and strong mutagens, respectively, whereas 2-aminopurine (AP) is a weak mutagen. AHA and HAP are about equally mutagenic at low doses, but AHA is more mutagenic than HAP at high doses. Despite their potent mutagenicity in growing cultures, AHA and HAP are not mutagenic in nongrowing conidia under the conditions of our experiments. AHA is the most potent mutagen tested in the ad-3 forward-mutation test in N. crassa. At the highest dose tested (30 micrograms/ml), it gave an ad-3 mutant frequency of 0.7 X 10(-2), about a 12,000-fold increase over the average spontaneous ad-3 mutant frequency. The potent mutagenicity of AHA may make it (and possibly HAP) especially useful for obtaining specific-locus mutations in other organisms.     

Comparison of the spectra of genetic damage in formaldehyde-induced ad-3 mutations between DNA repair-proficient and -deficient heterokaryons of Neurospora crassa.

The mutagenic effects of formaldehyde (FA) have been compared in DNA repair-proficient (heterokaryon 12) and DNA repair-deficient (heterokaryon 59) two-component heterokaryons of Neurospora crassa. The data from forward-mutation experiments were used to compare the spectra of FA-induced specific-locus mutations at two closely linked loci in the adenine-3 (ad-3) region and on the FA-induced inactivation of heterokaryotic conidia. Previous studies have demonstrated that specific-locus mutations at these two loci result from five major genotypic classes, namely two classes of gene/point mutations (ad-3A(R) and ad-3B(R)), and three classes of multilocus deletion mutations ([ad-3A](IR), [ad-3B](IR), and [ad-3A ad-3B](IR)). Genetic analysis of ad-3 mutants recovered from both heterokaryons after FA treatment demonstrates that predominantly gene/point mutations were found in H-12 (93.2% ad-3(R), 6.8% [ad-3](IR)) and a significantly higher frequency of multilocus deletion mutations in H-59 (62.8% ad-3(R), 37.0% [ad-3](IR)). The data from our experiments with FA on H-12 demonstrate and confirm the data from other assays that FA is a weak mutagen in this DNA repair-proficient strain. However, the data from our experiments with the DNA repair-deficient strain H-59 demonstrate that comparable concentrations of FA cause more pronounced inactivation of heterokaryotic conidia and, at the highest concentration tested, about a 35-fold higher frequency of ad-3 mutations. In addition, FA induced a 5.4-fold higher frequency of ad-3 mutations resulting from multilocus deletion mutation in H-59 than in H-12. Based on our earlier studies with X-ray-induced multilocus deletion mutations, it is this class of FA-induced ad-3 mutations that might be most expected to show deleterious heterozygous effects. The implications of the present data base from our experiments with Neurospora are that the mutagenic (and possibly the carcinogenic) effect of FA exposure might well vary in different human population subgroups.     

Effect of DNA Repair systems on antibacterial and mutagenic activity of an antitumor protein, neocarzinostatin.

Antibacterial and mutagenic effects of an antitumor protein, neocarzinostatin (NCS), on isogenic strains of Escherichia coli with normal or defective DNA repair systems were studied. Growth of the strains lacking recA gene was inhibited by NCS with much lower concentration than in the case of those possessing it, while the "differential inhibition for growth" (DIG) between the strains uvrA+ and uvrA- was not seen. NCS induces mutation in recA+ strains but not significantly in recA-, while no such difference of mutagenesis was noticed between the strains uvrA+ and uvrA-. These results suggest that NCS produces hardly excisable DNA damage which is repaired by an error-prone recombination process.     

Ionizing radiation-induced mutagenesis: radiation studies in Neurospora predictive for results in mammalian cells.

Ionizing radiation was the first mutagen discovered and was used to develop the first mutagenicity assay. In the ensuing 70+ years, ionizing radiation became a fundamental tool in understanding mutagenesis and is still a subject of intensive research. Frederick de Serres et al. developed and used the Neurospora crassa ad-3 system initially to explore the mutagenic effects of ionizing radiation. Using this system, de Serres et al. demonstrated the dependence of the frequency and spectra of mutations induced by ionizing radiation on the dose, dose rate, radiation quality, repair capabilities of the cells, and the target gene employed. This work in Neurospora predicted the subsequent observations of the mutagenic effects of ionizing radiation in mammalian cells. Modeled originally on the mouse specific-locus system developed by William L. Russell, the N. crassa ad-3 system developed by de Serres has itself served as a model for interpreting the results in subsequent systems in mammalian cells. This review describes the primary findings on the nature of ionizing radiation-induced mutagenesis in the N. crassa ad-3 system and the parallel observations made years later in mammalian cells.     

Mutation induction by rodent liver microsomal metabolities of aflatoxins B1 and G1 in Neurospora crassa.

The mutagenic activities of aflatoxins B1 and G1 were studied in the ad-3 test system of Neurospora crassa by treatment of conidia with aflatoxin and liver homogenate for 2 h. No significant increase in the ad-3 mutation frequency over the spontaneous frequency was observed when either aflatoxin or mammalian liver homogenate was omitted from the test system. The ad-3 mutation frequencies increased to between 29 and 87/10(6) survivors, which is a 73- to 217-fold increase over the average spontaneous ad-3 mutation frequency (0.4/10(6) survivors), after conidia of N. crassa were treated with 0.67 mM aflatoxin B1, hamster liver homogenate, and a NADPH generating system. A 9- to 15-fold increase in the mutation frequency over the spontaneous mutation frequency was found when 0.67 mM of aflatoxin G1 instead of aflatoxin B1 was used in the test system. Treatment of conidia with 0.44 mM aflatoxin B1 mice liver homogenate and a NADPH generating system caused a small, but significant increase in the ad-3 mutation frequencies. No significant increase in the mutation frequency was found when a single sample of human liver homogenate was used in the test system. These studies show that metabolic activation is necessary for the expression of the mutagenic activity of aflatoxins B1 and G1 in N. crassa.     

Effects of pH on the mutagenicity of sodium azide in Neurospora crassa and Salmonella typhimurium

Sodium azide at various pH values did not cause a significant increase in the frequency of forward mutation above the control frequency at the adenine-3 (ad-3) region in resting conidia and in conidia from growing cultures of heterokaryons 12 and 59 of Neurospora crassa. Conidia from ad-3 mutants were plated with sodium azide at various pH values, and no obvious increase in reverse mutation above the controls was observed. Data are presented showing that sodium azide at pH 3 is inactivating conidia by interacting with the cytoplasma rather than the nucleus, and this may be the primary reasons that no mutation at the ad-3 region was detected. The dependence of sodium azide mutagenicity on pH was investigated in histidine-requiring mutants of Salmonella typhimurium using a suspension test. There were no significant differences in the reversion frequencies among the pH values (3-8) tested. Thus, no pH dependence is associated with sodium azide mutagenicity, nor are growth and/or DNA replication required for mutagenicity by sodium azide, in S. typhimurium.     

Influence of N2 or O2 on two alkylating agents in Neurospora crassa.

Previous studies have indicated that the alkylating agent, 2-methoxy-6-chloro-9-(3-[ethyl-2-chloroethyl]aminopropylamino)acridine dihydrochloride (ICR-170), induces much more killing and mutation in conidia of Neurospora crassa treated in an atmosphere of N2 than in an atmosphere of O2. It was desirable to determine if a similar effect--more killing and mutation in N2 than in O2--could be observed with two other known alkylating agents, beta-propiolactone (BPL) and ethyl methanesulfonate (EMS), in the same test system. Conidia of a heterokaryotic strain of N. crassa were bubbled with N2 or O2 during treatment with BPL or EMS. Forward-mutation was measured in the ad-3 region by a direct method. The results indicate that N2 or O2 do not influence the lethal and mutagenic activities of BPL or EMS during treatment of conidia. Hence the influence of N2 or O2 on the lethal and mutagenic activites of ICR-170 is different from the influence of these gases on BPL or EMS using the ad-3 test system in N. crassa.     

Qualitative differences in the spectra of genetic damage in 2-aminopurine-induced ad-3 mutants between nucleotide excision-repair-proficient and -deficient strains of Neurospora crassa.

The mutagenic effects of 2-aminopurine (2AP) have been compared in the adenine-3 (ad-3) region of two-component heterokaryons of Neurospora crassa: nucleotide excision repair-proficient (uvs-2+/uvs-2+) heterokaryon 12 (H-12) and nucleotide excision repair-deficient (uvs-2/uvs-2) heterokaryon 59 (H-59). This forward-mutation, morphological and biochemical, specific-locus assay system permits the recovery of ad-3A and/or ad-3B mutants in 3 major classes: gene/point mutations, multilocus deletion mutations, and unknowns, and 3 different subclasses of multiple-locus mutations. Previous studies (Brockman et al., Mutation Res., 218 (1989) 1-11) showed that 2AP treatment of growing cultures of H-12 and H-59 gave no difference between ad-3 forward-mutation frequencies over a wide range of 2AP concentrations in each strain. In the present experiments, genetic analyses of ad-3 mutants recovered from these experiments has demonstrated qualitative differences between the spectra of the 3 main classes of ad-3 mutations. In H-12, 84.2% (203/241) resulted from gene/point mutation, 11.6% (28/241) from multilocus deletion mutation, and 4.1% (10/241) were unknowns. In contrast, in H-59, 43.0% (99/230) resulted from gene/point mutation, 55.7% (128/230) from multilocus deletion mutation, and 1.3% (3/230) were unknowns. In addition, quantitative differences were also found between the spectra of ad-3 mutations in 1 subclass of multiple-locus mutations, but not 2 additional subclasses. The first subclass consisted of 1.7% (4/241) and 9.6% (22/230) gene/point mutations with a closely linked recessive lethal mutation, in H-12 and H-59, respectively. The second two subclasses consisted of (a) 0.4% (1/241) and 0.4% (1/230) multilocus deletion mutations with a closely linked recessive lethal mutation, and (b) 13.3% (32/241) and 15.2% (35/230) gene/point mutations with a separate recessive lethal mutation elsewhere in the genome, in H-12 and H-59, respectively. Data from studies by others have shown that 2AP inhibits adenosine deaminase, resulting in nucleotide precursor pool inbalance, and that 2AP can saturate the mismatch repair system. As a consequence of either effect of 2AP, the spectrum of 2AP-induced mutation could include frameshift mutations and chromosome aberrations such as multilocus deletions in addition to base-pair substitutions. The defect in DNA repair due to the uvs-2 allele, which has been shown to be a deficiency in pyrimidine dimer excision (Worthy and Epler, 1974), most probably has some other excision-repair deficiency (Macleod and Stadler, 1986; Baker et al., 1991).(ABSTRACT TRUNCATED AT 400 WORDS)     

Lethal and mutagenic effects of 8-methoxypsoralen-induced lesions on plasmid DNA

The genotoxic effect of 8-methoxypsoralen damages (monoadducts and crosslinks) on plasmid DNA was studied. pBR322 DNA was treated with several concentrations of 8-methoxypsoralen plus fixed UVA light irradiation. After transformation into E. coli cells with different repair capacities (uvrA, recA and wild-type), plasmid survival and mutagenesis in ampicillin- and tetracycline-resistant genes were analysed. Results showed that crosslinks were extremely lethal in all 3 strains; indeed, it seemed that they were not repaired even in proficient bacteria. Monoadducts were also found to be lethal although they were removed to some extent by the excision-repair pathway (uvrA-dependent). Damaged plasmid DNA appeared to induce mutagenic repair, but only in the wild-type strain. In order to study the influence of the SOS response on plasmid recovery, preirradiation of the host cells was also performed. Preirradiation of the uvrA or wild-type strains significantly increased plasmid recovery. Consistent with the expectations of SOS repair, no effect was observed in preirradiated recA cells. Plasmid recovery in the excision-deficient strain was mainly achieved by the mutagenic repair of some fraction of the lesions, probably monoadducts. The greatest increase in plasmid recovery was found in the wild-type strain. This likely involved the repair of monoadducts and some fraction of the crosslinks. We conclude that repair in preirradiated repair-proficient cells is carried out mainly by an error-free pathway, suggesting enhancement of the excision repair promoted by the induction of SOS functions.     

Methyl cinnamate derivatives enhance UV-induced mutagenesis due to the inhibition of DNA excision repair in Escherichia coli B/r

UV-induced mutagenesis in Escherichia coli B/r WP2 was enhanced by certain derivatives of methyl cinnamate which themselves were not mutagenic. Methyl ferulate, methyl isoferulate and methyl sinapate showed this effect markedly. Such an enhancement effect was absent with the derivatives of cinnamic acid and ethyl cinnamate and was not observed in Escherichia coli WP2s uvrA. Methyl sinapate also enhanced 4NQO-induced mutation and suppressed liquid-holding recovery in the above repair-proficient strain. The presence of methyl sinapate in plating agar medium decreased the survival of UV-irradiated cells of a recombination-repair-deficient strain, CM571 recA. However, the effect was not observed with those of WP2s uvrA. In an in vitro experiment in which the removal rate of thymine dimers was measured, methyl sinapate clearly inhibited this repair event. From these results, we conclude that methyl sinapate inhibits DNA excision repair, thus enhancing UV mutagenicity.     

Mutagenic potency and specificity of procarbazine in the ad-3 forward-mutation test in growing cultures of heterokaryon 12 of Neurospora crassa.

Procarbazine (Natulan) was tested for its mutagenic potency and specificity in the ad-3 forward-mutation test in heterokaryon 12 (H-12) of Neurospora crassa. In these experiments, procarbazine was a weak mutagen when present in growing cultures but nonmutagenic when conidial suspensions (nongrowing conidia) were treated. A total of 208 ad-3 mutants recovered after exposure of growing cultures of H-12 to 1 mg of procarbazine/ml, and 2 ad-3 mutants of spontaneous origin, were characterized genetically. These tests distinguish among gene/point mutations (ad-3R) at the ad-3A or ad-3B locus, multilocus deletion mutations ([ad-3]IR) covering one or more loci in the ad-3 and immediately adjacent regions, and 3 different classes of multiple-locus mutations: gene/point ad-3 mutations with a recessive lethal mutation elsewhere in the genome (ad-3R + RL), gene/point mutations with a closely linked recessive lethal mutation (ad-3R + RLCL), and multilocus deletion mutations with a closely linked recessive lethal mutation ([ad-3]IR + RLCL). All of the procarbazine-induced ad-3 mutants resulted from gene/point mutations; 92.2% (200/217) resulted from gene/point mutations at the ad-3A or ad-3B locus, and 3.7% (8/217) resulted from gene/point mutations with a recessive lethal mutation elsewhere in the genome. Identical percentages (15.4% [20/130] and 15.4% [12/78]) of the sigma ad-3BR and sigma ad-3AR mutants were leaky, and a high percentage (71.5% [93/130]) of the sigma ad-3BR mutants had nonpolarized complementation patterns. These results indicate that procarbazine-induced ad-3 mutants of Neurospora crassa are composed solely of gene/point mutations (ad-3R) that resulted, predominantly or exclusively, from base-pair substitutions. The Neurospora specific-locus data on procarbazine-induced ad-3 mutants are compared with data from similar experiments with the mouse using the morphological specific-locus assay; marked similarities were found between the mutagenic effects of procarbazine in the 2 specific-locus assays.     

Mutagenicity of some substituted 1-phenyl-3,3-dimethyltriazenes. I. The salmonella/mammalian microsome assay and repair test

The mutagenic activity and related biological properties of Br-, Cl-, NO2- and CH3-derivatives of 1-(phenyl)-3,3-dimethyltriazene were investigated in Salmonella/microsome assays with standard and preincubation metabolic activation and in the repair test using Salmonella and E. coli B/r. In the repair test, the CH3-derivative was slightly positive in the E. coli recA and uvrA repair system, the NO2-derivative had a killing effect on Salmonella typhimurium uvrB-deficient strains. In Salmonella mutagenicity assays, all tested triazene derivatives reverted frameshift tester strains, especially TA1537. The highest number of frameshift mutations was induced by the CH3-derivative in the presence of a standard metabolic activation system; direct mutagenicity of this derivative was weak, reaching about the same level of activity as seen after preincubation. The only test compound that induced mutations of the base-substitution type was the NO2-derivative; this derivative showed the highest mutagenicity when activated by preincubation.     

Mutagenicity of cigarette smoke condensate in Neurospora crassa

The mutagenicity of cigarette smoke condensate (CSC) was investigated in Neurospora crassa in the presence and absence of S9 mix prepared from Aroclor-1254-induced rat liver. CSC from the University of Kentucky Reference Cigarette 1R1 was assayed in a forward-mutation test at the adenine-3 (ad-3) region in resting conidia of 2-component heterokaryons. In the absence of S9 mix, CSC exhibited direct-acting mutagenicity. CSC was also mutagenic in the presence of S9 mix, but higher doses were required than in the absence of S9 mix. The dose range, survival curves, and mutation-induction curves were not significantly different when CSC was used in the presence of unheated or heat-inactivated S9. There was a positive association between killing and mutagenicity, and CSC killed conidia of N. crassa by a cytoplasmic, rather than by a nuclear, mechanism. The mutagenic potency of CSC was similar in a repair-sufficient and a nucleotide excision repair-deficient heterokaryon of N. crassa. CSC did not exhibit a photodynamic effect for killing, and CSC caused more killing at high pH than at low pH. In addition, CSC caused more killing at 37 degrees C than at 25 degrees C and also caused more killing in higher concentrations (20%) of solvent (DMSO) than in low concentrations (1%). This is the first report of the presence of potent direct-acting mutagens in CSC.     

Influence of a uvrD mutation on survival and repair of X-irradiated Escherichia coli K-12 cells.

The presence of a uvrD mutation increased the X-ray sensitivities of E. coli wild-type and polA strains, but had no effect on the sensitivities of recA and recB strains, and little effect on a lexA strain. Incubation of irradiated cells in medium containing 2,4-dinitrophenol or chloramphenicol decreased the survival of wild-type and uvrD cells, but had no effect on the survival of recA, recB and lexA strains. Alkaline sucrose gradient sedimentation studies indicated that the uvrD strain is deficient in the growth-medium-dependent (Type III) repair of DNA single-strand breaks. These results indicate that the uvrD mutation inhibits certain rec+lex+-dependent repair processes, including the growth-medium-dependent (Type III) repair of X-ray-induced DNA single-strand breaks, but does not inhibit other rec+lex+-dependent processes that are sensitive to 2,4-dinitrophenol and chloramphenicol.     

Further studies on the lethal and mutagenic effects of 8-methoxypsoralen-induced lesions on plasmid DNA

Our previous results on the genotoxic effect of 8-methoxypsoralen-induced lesions on pBR322 suggested an important involvement of an inducible error-free repair pathway in the repair of plasmid lesions. We present herein further results obtained in order to explore that possibility, together with a more general report on the subject. pBR322 treated with increasing concentrations of 8-MOP plus fixed UVA light irradiation was used to transform several E. coli strains differing in their repair capacities, and plasmid survival and mutagenesis were determined. Survival results suggested that crosslinks were completely lethal in pBR322 whereas monoadducts were partially removed from plasmid DNA mainly through an error-free excision pathway. A mutagenic repair pathway did not show a significant contribution to the total repair process. Cell preirradiation stimulated plasmid recovery in recA+ strains, including the umuC strain, thus confirming our previous results indicating that an inducible error-free repair had occurred. Globally, our results showed a strong requirement on the excision pathway for the repair of psoralen-damaged plasmid DNA. In contrast, the recA dependent pathway was needed only for SOS induction. After a theoretical correction of the data for estimating the effect only due to 8-MOP adducts, a different pattern of repair mechanisms appeared to be involved.     

Frameshift mutagenesis by chloroquine in Escherichia coli and Salmonella typhimurium

Chloroquine can be detected as a direct-acting mutagen in plate-incorporation assays using the excision-deficient Salmonella typhimurium strain TA97, but very much more effectively using the repair-proficient Escherichia coli strain DG1669 which carries the lacZ19124 marker. When tested at concentrations of 200-1000 micrograms/plate with strain DG1669, the mutagenicity of chloroquine is enhanced by the addition of Aroclor-induced rat-liver S9. Further experiments indicated that chloroquine-induced reversion frequencies were essentially identical in wild-type, recA, umuC and uvrC derivatives of DG1669, as well as in strains carrying the mutation enhancing plasmid pKM101, over a wide range of doses (0-1200 micrograms/plate). These results suggest that neither excision repair nor SOS-type repair are important in chloroquine-induced frameshift mutagenesis.     

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.