Role of the uvrE gene product and of inducible O6-methylguanine removal in the induction of mutations by N-methyl-N'-nitro-N-nitrosoguanidine in Escherichia coli

Plasmid-dependent co-mutation in Streptomyces coelicolor A3(2)

A spontaneous chromosomalmutation(plc A(-)) in Streptomyces coelicolor A3(2) made the inborn plasmid SCP1 susceptible to curing by UV irradiation. Lack of the SCP1 plasmid (in SCP1(-) strains) prevented the occurrence of the co-mutation process after MNNG treatment, although it left the susceptibility to the lethal effect of the mutagen virtually unaffected. SCP1(-) strains were, however, ultrasensitive to the lethal effect of UV. Curing a plc A(-) strain of its SCPI plasmid made it refractory to co-mutation by MNNG and sensitive to the lethal effect of UV; reinfected by the plasmid, the strain resumed both the co-mutation proficiency and the UV-resistance. The occurrence on the SCP1 plasmid of a gene comparable to the uvrE gene of E. coli (Nevers and Spatz 1975) was assumed.

Simultaneous induction of multiple mutations by N-methyl-N'-nitro-N-nitrosoguanidine in the yeast Saccharomyces cerevisiae

Contrary to what happens in bacteria, mutations induced by nitrosoguanidine in yeast are not accompanied by an excess of mutations in nearby genes. We have investigated nitrosoguanidine mutagenesis in three regions of the yeast genome: the contiguous DNA segments HIS4A, HIS4B and HIS4C, located on chromosome III; ADE1 and CDC15 separated by about 3 map units on chromosome I; and CAN1, some 50 map units away from the centromere on chromosome V. Revertants at HIS4C never suffered mutations at HIS4A or HIS4B. Reversion at CDC15 did not affect the frequency of mutation at ADE1. No tsm mutations, leading to thermonsensitivity, were found in the immediate vicinity of the locus CAN1 after selecting for canavanine resistant mutants. However, as expected from nitrosoguanidine mutagenesis of replication points and the fixed pattern of chromosome replication, the induced tsm mutations seem not to map randomly over the yeast genome; in fact, two out of the three groups of such tsm mutations studied are located in the same chromosome arm as CAN1, indicating that these two regions are replicated at the same time as CAN1. Replication synchrony is less than perfect, since the tsm mutations of each group affect many different genes.

Comparative study of mutagenesis by O6-methylguanine in the human Ha-ras oncogene in E. coli and in vitro

Single residues of O6-methylguanine (O6-meG) were introduced into the first or second position of codon 12 (GGC; positions 12G1 or 12G2, respectively) or the first position of codon 13 (GGT; position 13G1) of the human Ha-ras oncogene in phage M13-based vectors. After transformation of E.coli, higher mutant plaque frequencies (MPF) were observed at 12G1 and 13G1 than at 12G2 if O6-alkylguanine-DNA alkyltransferase (AGT) had been depleted, while similar MPF were observed at all three positions in the presence of active AGT. Taken together, these observations suggest reduced AGT repair at 12G2. Kinetic analysis of in vitro DNA replication in the same sequences using E. coli DNA polymerase I (Klenow fragment) indicated that variation in polymerase fidelity may contribute to the overall sequence specificity of mutagenesis. By constructing vectors which direct methyl-directed mismatch repair to the (+) or the (-) strand and comparing the MPF values in bacteria proficient or deficient in mismatch repair and/or AGT, it was concluded that, while mutS-mediated mismatch repair did not remove O6-meG from O6-meG:C pairs, this repair mechanism can affect O6-meG mutagenesis by repairing G:T pairs generated through AGT-induced demethylation of O6-meG:T replication intermediates.

Mammalian cells share a common pathway for the relief of DNA replication arrest by O6-alkyl guanine, incorporated 6-thioguanine and UV photoproducts

We previously reported the cloning of a mammalian gene that restores UV resistance to a postreplication recovery defective and mex- Indian muntjac mutant cell line, SVM, by improving daughter-strand DNA replication on a UV-damaged template. The improved replication was, however, found to be error-prone, as judged by a hypermutable phenotype (Bouffler et al. (1990) Somatic Cell Mol. Genet., 16, 507-516). We now report that this gene also increases the resistance of SVM to the cytotoxic effects of methyl- and ethyl-nitrosourea, though not to dimethyl sulphate, by a similar postreplication recovery process. The gene does not increase the activity of O6-alkylguanine-DNA-alkyltransferase in the cell. We conclude that at least one mechanism of postreplication recovery in mammalian cells allows UV photoproducts and O6-alkylguanine lesions to be tolerated by the replication complex. The fact that the gene also confers resistance to 6-thioguanine suggests that, once incorporated, this base analogue can disrupt normal DNA replication and that a single mechanism can allow replication to proceed beyond 3 diverse DNA lesions.

Use of data from bacteria to interpret data on DNA damage processing in mammalian cells

The most important reason for determining the changes in base sequence in the processing of DNA damage is to determine mechanisms. Currently, much more is known about these mechanisms in prokaryotes, partly because the experiments are easier and quicker to do in bacteria, and partly because of the wealth of well characterized bacterial mutants deficient in various DNA repair pathways. This paper summarizes some information on the mechanisms in bacteria that are involved in the induction by various agents of base change mutations, 1- and 2-base deletions or additions that cause frameshifts, and more complicated insertions and deletions that involve up to tens of base pairs. For gross DNA rearrangements such as large deletions involving hundreds or thousands of base pairs, there is actually more information available in mammalian cells than in bacterial cells. It is suggested that deletions of several kilobases or more in bacteria are not easy to detect because they have a high probability of deleting both the gene under study and an adjacent essential gene, forming a nonviable cell. In mammalian cells, the large size (30-40-kb pairs) of the average gene, including both introns and exons, means that a large deletion is more likely to be confined to a single gene and less likely to lead to a nonviable cell.

Isolation and characterization of a UV-sensitive mutator (mutB1) mutant of Haemophilus influenzae

The mutB1 mutant of Haemophilus influenzae is very sensitive to UV radiation but only slightly sensitive to methylmethane sulfonate or N-methyl-N'-nitro-N-nitrosoguanidine. Cultures of mutB1 cells contain high numbers of spontaneous mutants and show hypermutability after exposure to the latter mutagen. Normally high-efficiency transforming markers, as well as low-efficiency ones, transform mutB1 recipients at similarly low efficiencies. Significant host cell reactivation was observed when mutB1 cells were exposed to UV-damaged phage; however, these mutants showed a decrease in phage recombination. This mutant did not degrade its DNA following exposure to UV. It is speculated that the mutB1 mutation is similar to the Escherichia coli uvrD mutation.

Chronic ethanol consumption inhibits repair of dimethylnitrosamine-induced DNA alkylation

Chronic ethanol consumption causes a DNA repair deficiency. This was demonstrated in Sprague-Dawley rats injected with 14C-labeled dimethylnitrosamine after being pair-fed isocaloric, ethanol, or carbohydrate control diets for 4 weeks. Hepatic DNA was isolated from rats killed at intervals over a 36 hour period after administration of the nitrosamine and concentrations of alkylated guanine derivatives were measured. While N7-methylguanine was lost at equivalent rates from the DNA of both diet groups, 06methylguanine, a promutagenic lesion, persisted at higher levels for longer periods of time in the DNA from the alcohol-fed animals.

Effect of the direction of DNA replication on mutagenesis by N-methyl-N'-nitro-N-nitrosoguanidine in adapted cells of Escherichia coli

The methylating agent N-methyl-N'-nitro-N-nitrosoguanidine preferentially induces G:C to A:T transitions at DNA base pairs with the G in one particular strand of the cI gene in a lambda prophage, in this case the nontranscribed strand, in Escherichia coli cells in which the adaptive response is induced. The same preference is found for the cI gene inserted in the genome in the inverse orientation, so the differential effect is not caused by the direction of motion of the DNA replicating fork.

The in vivo mutagenic frequency and specificity of O6-methylguanine in phi X174 replicative form DNA

A bacteriophage phi X174-based site-specific mutagenesis system for the study of the in vivo mutagenic frequency and specificity of carcinogen-induced modification in DNA is presented. A (-)-strand primer containing O6-methylguanine in a specific site was hybridized to a single-stranded region in gene G of phi X gapped duplex DNA. The hybrid was enzymatically converted to replicative form DNA and was used to transform Escherichia coli cells. All gene G mutants generated by the modification were rescued by genetic complementation. An amber mutation in lysis gene E of the (+) strand of the replicative form DNA prevented lytic growth of wild-type phage derived from this strand. In each mutant-containing infective center produced from the transformed cells, gene G mutant phage were present in a 3:1 ratio compared to wild type. Thus, in vivo, O6-methylguanine in replicating phi X DNA has a mutagenic frequency of 75%. When repair of O6 methylguanine occurred, it was prereplicative. The mutations were due exclusively to the misincorporation of thymine.

Alkylation mutagenesis in Saccharomyces cerevisiae: lack of evidence for an adaptive response

We have found no evidence for an adaptive response for either lethality or mutagenesis following treatment of Saccharomyces cerevisiae with N-methyl-N'-nitro-N-nitrosoguanidine (MNNG). The rad6 and rad52 mutants of S. cerevisiae are highly defective in MNNG and ethyl methanesulfonate induced mutagenesis of both stationary and exponential phase cells. These and other observations indicate that the mechanisms of repair of alkylation damage and mutagenesis differ markedly between S. cerevisiae and Escherichia coli.

Adaptive response and enhancement of N-methyl-N'-nitro-N-nitrosoguanidine mutagenesis by chloramphenicol in Streptomyces fradiae

Streptomyces fradiae expressed an adaptive response to treatment with small doses of N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) that caused a reduction in mutagenesis by treatment with larger doses of MNNG. Treatment of S. fradiae with high levels of MNNG in the presence of chloramphenicol caused enhancement of mutagenesis, independent of the adaptive response.

Chemical carcinogens and proto-oncogens activation

The induction of proto-oncogens H-ras-1 by nitrosomethylurea (Sukumar et al. (1983), 306, 658-661) is discussed in term of lack of DNA-repair of lesions induced in DNA by this alkylating agent, particularly O6-methylguanine residues and apurinic/apyrimidinic sites.

Mutagenesis by N-nitroso compounds in Salmonella typhimurium TA102 and TA104: evidence for premutagenic adenine or thymine DNA adducts

Mutagenesis induced by the N-nitroso compounds: N-nitrosomethylurea, N-nitrosoethylurea, N-nitrosodi-n-propylamine and N-nitrosopyrrolidine was measured in Salmonella typhimurium TA100, TA102 and TA104. TA100 detects damage mainly at G-C base pairs while TA102 and TA104 can detect damage at A-T base pairs. In general all strains were similarly sensitive, except that TA104 was much less sensitive to high doses of N-nitroso-N-methylurea. In TA104 a significant percentage of the revertants induced by all agents except NMU resulted from point mutations at A-T base pairs, indicating that adenine or thymine DNA adducts are important premutagenic adducts formed by certain N-nitroso compounds.

Mechanism of mutagenesis by O6-methylguanine

O6-methylguanine (O6meG) lesions of double-stranded DNA have been associated with mutation and neoplastic transformation. These lesions can, in principle, be produced by at least three different mechanisms: direct alkylation of G X C base pairs in double-stranded DNA; alkylation of guanine residues in single-stranded regions of DNA associated with replication forks; and alkylation of the DNA precursor pool followed by incorporation of O6-methyl deoxyguanosine triphosphate (O6-medGTP) during DNA replication. DNA biosynthesis subsequent to all three events will generate predominantly O6-meG X T base pairs as O6meG preferentially pairs with T. We show here that O6meG X T base pairs are mutagenic; that transalkylase repair has a direct role in the generation of mutations induced by alkylated pool nucleotides; and that the Escherichia coli mismatch repair system is capable of repairing mutagenic G X T intermediates.

Induction of mutation in Micrococcus radiodurans by N-methyl-N'-nitro-N-nitrosoguanidine

Micrococcus radiodurans was highly resistant to the lethal effect of N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) but it was sensitive to the mutagenic action of this chemical. The induction of mutation was not significantly modified by the culture growth phase. This last finding leads to the assumption that the mutation takes place at some distance from the replication fork. Moreover, a low concentration of MNNG induced mutations that were added to those subsequently obtained from a second exposure to a higher concentration of the alkylating agent. Thus, M. radiodurans does not seem to have an inducible error-free repair system for alkylation damage. Furthermore, incubation in the presence of chloramphenicol did not modify the mutation rate, indicating that protein synthesis is not involved in the mutagenic process.

Mutagenesis, by methylating and ethylating agents, in mutH, mutL, mutS, and uvrD mutants of Salmonella typhimurium LT2

Salmonella typhimurium LT2 mutH, mutL, mutS, and uvrD mutants were especially sensitive to mutagenesis by both the recA+-dependent mutagen methyl methane sulfonate and the recA+-independent mutagen ethyl methane sulfonate, but not to mutagenesis by agents such as 4-nitroquinoline-1-oxide and UV irradiation. Similarly, these mutator strains were very sensitive to mutagenesis by the methylating agents N-methyl-N'-nitro-N-nitrosoguanidine and N-methyl-N-nitrosourea. The increased susceptibility to mutagenesis by small alkylating agents due to mutH, mutL, mutS, and uvrD mutations was not accompanied by an increased sensitivity to killing by these agents. Various models are discussed in an effort to explain why strains thought to be deficient in methyl-instructed mismatch repair are sensitive to mutagenesis by methylating and ethylating agents.

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.

The role of DNA polymerase in base substitution mutagenesis on non-instructional templates

In vitro DNA synthesis on phi X174 or M13 templates with non-instructional lesions such as UV dimers or AP (apurinic/apyrimidinic) sites terminates one base before the site of the lesion when synthesis is catalyzed by T4 DNA polymerase or E. coli polymerase I. E. Coli polymerase I also produces termination bands at the site of AP lesions. Substitution of Mn2+ for Mg2+ and increasing the concentration of dNTP's results in elongation of the newly synthesized strand opposite the site of the lesion and beyond. Purine deoxynucleoside triphosphates are utilized for insertion opposite lesions to a greater extent than are pyrimidine deoxynucleoside triphosphates. Deoxy ATP is used almost exclusively for elongation opposite AP sites with pol I-Klenow fragment in the presence of Mg2+. We suppose that these results illustrate the previously observed greater affinity of polymerases under template-free conditions for purine nucleotides. We also suppose that the results can be used to account for mutagenic base selection on noninstructional DNA templates. If purines are preferentially selected by polymerases, then treatments which inactivate pyrimidines will lead to an excess of transitions whereas inactivation of purines will produce more transversions. Data in the literature support this hypothesis.

A system in mouse liver for the repair of O6-methylguanine lesions in methylated DNA

An activity from mouse liver with catalyzes the disappearance of O6-methylguanine from DNA methylated with methylnitrosourea has been partially purified by ammonium sulfate fractionation and DNA-cellulose chromatography. The activity does not require divalent metal ions and is not affected by EDTA. It is specific for the repair of O6-methylguanine lesions and does not affect the removal of 7-methylguanine, 7-methyladenine or 3-methyladenine. The disappearance of O6-methylguanine is linear with respect to the concentration of protein and is dependent on incubation temperature. The kinetics and substrate dependence experiments suggest that the protein factor is product-inactivated. Amino acid analysis of hydrolysates of protein obtained after incubation of methylated DNA with the protein factor indicates the presence of radiolabeled S-methyl-L-cysteine, suggesting that during the repair of O6-methylguanine from methylated DNA, the methyl group is transferred to a sulfhydryl of a cysteine residue of a protein. This represents the first such demonstration in a mammalian system.

Removal of O6-methylguanine from DNA of normal and xeroderma pigmentosum-derived lymphoblastoid lines

The ability to excise (repair) UV-induced pyrimidine dimers in Escherichia coli is not related to its ability to remove N-methyl-N'-nitro-N-nitrosoguanidine (MNNG)-induced O6-methylguanine (O6-MeG) from DNA. It was therefore surprising that certain xeroderma pigmentosum cell lines, deficient in dimer excision, were also unable to remove O6-MeG. We find that removal of O6-MeG occurs rapidly with a half life of less than 1 h. Two cell types can be distinguished: mex+, which remove O6-MeG residues produced by incubation with 0.5 microgram ml-1 MNNG, and mex- cells, which are unable to remove the adduct. Xeroderma pigmentosum-derived lymphoblastoid lines of complementation groups A, C or D may be either mex+ or mex-. The biochemical mechanism for the removal of O6-MeG in human cells is distinct from the excision of adducts produced by compounds such as N-acetoxy-N-2-acetylaminofluorene (AAAF) or by UV irradiation but it is not clear whether the distinction between mex+ and mex- lines is genetic or epigenetic.