Isolation of mutants of Escherichia coli with increased resistance to alkylating agents: mutants deficient in thiols and mutants constitutive for the adaptive response

Glutathione in bacteria

Glutathione metabolism and its role in vital functions of bacterial cells are considered, as well as common features and differences between the functions of glutathione in prokaryotic and eukaryotic cells. Particular attention is given to the recent data for the role of glutathione in bacterial redox-regulation and adaptation to stresses.

Linkage map of Escherichia coli K-12, edition 10: the traditional map

This map is an update of the edition 9 map by Berlyn et al. (M. K. B. Berlyn, K. B. Low, and K. E. Rudd, p. 1715-1902, in F. C. Neidhardt et al., ed., Escherichia coli and Salmonella: cellular and molecular biology, 2nd ed., vol. 2, 1996). It uses coordinates established by the completed sequence, expressed as 100 minutes for the entire circular map, and adds new genes discovered and established since 1996 and eliminates those shown to correspond to other known genes. The latter are included as synonyms. An alphabetical list of genes showing map location, synonyms, the protein or RNA product of the gene, phenotypes of mutants, and reference citations is provided. In addition to genes known to correspond to gene sequences, other genes, often older, that are described by phenotype and older mapping techniques and that have not been correlated with sequences are included.

Thermostable archaeal O6-alkylguanine-DNA alkyltransferases

Archaea represent some of the most ancient organisms on earth, and they have relatively uncharacterized DNA repair processes. We now show, using an in vitro assay, that extracts of two Crenarchaeota (Sulfolobus acidocaldarius and Pyrobaculum islandicum) and two Euryarchaeota (Pyrococcus furiosus and Thermococcus litoralis) contain the DNA repair protein O6-alkylguanine-DNA alkyltransferase (ATase). The ATase activities found in the archaea were extremely thermostable, with half-lives at 80 degreesC ranging from 0.5 hr (S. acidocaldarius) to 13 hr (T. litoralis). The temperature optima of the four proteins ranged from approximately 75 to approximately 100 degreesC, although activity was seen at 37 degreesC, the temperature optimum of the Escherichia coli and human ATases. In all cases, preincubaton of extracts with a short oligonucleotide containing a single O6-methylguanine residue caused essentially complete loss of ATase activity, suggesting that the alkylphosphotriester-DNA alkyltransferase activity seen in some prokaryotes is not present in Archaea. The ATase from Pyrobaculum islandicum had an apparent molecular mass of 15 kDa, making it the smallest of these proteins so far described. In higher organisms, ATase is responsible for the repair of toxic and mutagenic O6-alkylguanine lesions in alkylated DNA. The presence of ATase in these primitive organisms therefore suggests that endogenous or exogenous exposure to agents that generate appropriate substrates in DNA may be an early event in evolution.

Ada protein-RNA polymerase sigma subunit interaction and alpha subunit-promoter DNA interaction are necessary at different steps in transcription initiation at the Escherichia coli Ada and aidB promoters

The methylated form of the Ada protein (meAda) binds the ada and aidB promoters between 60 and 40 base pairs upstream from the transcription start and activates transcription of the Escherichia coli ada and aidB genes. This region is also a binding site for the alpha subunit of RNA polymerase and resembles the rrnB P1 UP element in A/T content and location relative to the core promoter. In this report, we show that deletion of the C-terminal domain of the alpha subunit severely decreases meAda-independent binding of RNA polymerase to ada and aidB, affecting transcription initiation at these promoters. We provide evidence that meAda activates transcription by direct interaction with the C-terminal domain of RNA polymerase sigma70 subunit (amino acids 574-613). Several negatively charged residues in the sigma70 C-terminal domain are important for transcription activation by meAda; in particular, a glutamic acid to valine substitution at position 575 has a dramatic effect on meAda-dependent transcription. Based on these observations, we propose that the role of the alpha subunit at ada and aidB is to allow initial binding of RNA polymerase to the promoters. However, transcription initiation is dependent on meAda-sigma70 interaction.

Hydrogen peroxide induces protection against N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) effects in Escherichia coli

Cross-adaptive response is defined as the capacity of cells to become resistant to a lethal agent when pretreated with a different lethal substance. In the present paper, the cross-adaptive response between hydrogen peroxide and N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) was studied in Escherichia coli repair mutants. Our results suggest that high doses of H2O2 induces protection against the lethal effects of MNNG in wild-type strain, ada, ogt, ada-ogt, aidB and alkA mutants. On the other hand, the MNNG induced mutagenesis is reduced by H2O2 pretreatment in wild-type and ogt mutant strains, but not in ada mutant. Furthermore, the protecting effect induced by H2O2 is time dependent: it decreases 15 min after the pretreatment and, after 30 min, is almost abolished. This reduction in the protecting effect is followed by an augmentation in the mutation frequency when MNNG is added 30 min after H2O2 pretreatment. This cross-adaptive response may be due to a modification of the MNNG alkylation pattern in the oxidized DNA.

Generation of an endogenous DNA-methylating agent by nitrosation in Escherichia coli

Escherichia coli ada ogt mutants, which are totally deficient in O6-methylguanine-DNA methyltransferases, have an increased spontaneous mutation rate. This phenotype is particularly evident in starving cells and suggests the generation of an endogenous DNA alkylating agent under this growth condition. We have found that in wild-type cells, the level of the inducible Ada protein is 20-fold higher in stationary-phase and starving cells than in rapidly growing cells, thus enhancing the defense of these cells against DNA damage. The increased level of Ada in stationary cells is dependent on RpoS, a stationary-phase-specific sigma subunit of RNA polymerase. We have also identified a potential source of the mutagenic agent. Nitrosation of amides and related compounds can generate directly acting methylating agents and can be catalyzed by bacteria] enzymes. E. coli moa mutants, which are defective in the synthesis of a molybdopterin cofactor required by several reductases, are deficient in nitrosation activity. It is reported here that a moa mutant shows reduced generation of a mutagenic methylating agent from methylamine (or methylurea) and nitrite added to agar plates. Moreover, a moa mutation eliminates much of the spontaneous mutagenesis in ada ogt mutants. These observations indicate that the major endogenous mutagen is not S-adenosylmethionine but arises by bacterially catalyzed nitrosation.

Structure and transcriptional regulation of the Escherichia coli adaptive response gene aidB

Expression of the Escherichia coli aidB gene is induced in vivo by alkylation damage in an ada-dependent pathway and by anaerobiosis or by acetate at pH 6.5 in an ada-independent fashion. In this report, we present data on aidB gene structure, function, and regulation. The aidB gene encodes a protein of ca. 60 kDa that is homologous to several mammalian acyl coenzyme A dehydrogenases. Accordingly, crude extracts from an aidB-overexpressing strain showed isovaleryl coenzyme A dehydrogenase activity. aidB overexpression also reduced N-methyl-N'-nitro-N-nitrosoguanidine-induced mutagenesis. Both ada- and acetate/pH-dependent induction of aidB are regulated at the transcriptional level, and the same transcriptional start point is used for both kinds of induction. Ada protein plays a direct role in aidB regulation: methylated Ada is able to bind to the aidB promoter region and to activate transcription from aidB in an in vitro transcription-translation system using crude E. coli extracts.

The Escherichia coli AlkB protein protects human cells against alkylation-induced toxicity

Escherichia coli can ameliorate the toxic effects of alkylating agents either by preventing DNA alkylation or by repairing DNA alkylation damage. The alkylation-sensitive phenotype of E. coli alkB mutants marks the alkB pathway as an extremely effective defense mechanism against the cytotoxic effects of the SN2, but not the SN1, alkylating agents. Although it is clear that AlkB helps cells to better handle alkylated DNA, no DNA alkylation repair function could be assigned to the purified AlkB protein, suggesting that AlkB either acts as part of a complex or acts to regulate the expression of other genes whose products are directly responsible for alkylation resistance. However, here we present evidence that the provision of alkylation resistance is an intrinsic function of the AlkB protein per se. We expressed the E. coli AlkB protein in two human cell lines and found that it confers the same characteristic alkylation-resistant phenotype in this foreign environment as it does in E. coli. AlkB expression rendered human cells extremely resistant to cell killing by the SN2 but not the SN1 alkylating agents but did not affect the ability of dimethyl sulfate (an SN2 agent) to alkylate the genome. We infer that SN2 agents produce a class of DNA damage that is not efficiently produced by SN1 agents and that AlkB somehow prevents this damage from killing the cell.

The isolation of mutagen-sensitive nuv mutants of Aspergillus nidulans and their effects on mitotic recombination

More than 200 mutants of Aspergillus nidulans were isolated as hypersensitive to the monofunctional alkylating agent MNNG and/or UV-irradiation (designated nuv mutants). Of these, 23 were selected for further characterization. All were markedly hypersensitive to both MNNG and the quasi-UV-mimetic mutagen 4-NQO. The hypersensitive phenotype of each mutant was shown to result from mutation of a single gene. The nuv mutants exhibited a diverse range of growth responses on solid media containing various concentrations of MNNG or 4-NQO. This suggested that they represented many nonallelic mutations. Analysis to determine the dominance/recessiveness of the nuv mutations with respect to hypersensitivity revealed that most were fully recessive, although several appeared to be semidominant. A novel system to assay homologous mitotic recombination using simple plating tests was developed. The system was exploited to determine the effects of the nuv mutations on mitotic recombination. Of the 23 mutations tested, 10 caused a hypo-recombination phenotype and three a hyper-recombination phenotype, while 10 appeared to have no effect on recombination. The hypo-rec effect of one of the mutations, nuv-117, appeared to be semidominant. Transcomplementation analysis between seven of the nuv mutations defined at least six nonallelic loci.

Resistance to cisplatin in an E. coli B/r NalR mutant

The effects of various mutations in DNA-repair processes have been reported to either enhance or decrease bacterial sensitivity to cis-diamminedichloroplatinum(II) (cis-DDP). In the search for other mutations affecting bacterial sensitivity to this antitumor compound, we tested the E. coli B/r BS80 mutant, which is resistant to nalidixic acid (NalR). This mutation maps in the topoisomerase II gene (gyrA subunit) and leads to cross-resistance to cis-DDP. The mechanism underlying the resistance phenotype was only partly due to decreased DNA platination. BS80 was cross-resistant to mitomycin C and, to a lesser extent, to UV light, while it was normally sensitive to MNNG. The mechanisms involved in cis-DDP and mitomycin C resistance were independent of uvrA (excision repair) and recA (SOS repair and recombination) gene expression. In contrast, UV resistance was dependent upon recA gene expression. Both the reversion to NalS in BS80 and the transduction of NalR in the parental wild type (F26) did not modify cis-DDP toxicity; in addition, platinated plasmids equally survived in BS80 and F26 strains. Hence, it is possible that selection of the NalR phenotype induced other mutation(s) than gyrA responsible for cis-DDP, mitomycin C and UV resistance and/or that lesions with a different toxic potential were introduced by cis-DDP into the BS80 and F26 chromosomes.

Acidification of Escherichia coli and Salmonella typhimurium cytoplasm reduces the mutagenic effect of N-methyl-N'-nitro-N-nitrosoguanidine

Preliminary acidification of the cytoplasm of E. coli cells growing at pH 6.9 by adding to the medium 50 mM of sodium acetate or propionate reduced the mutagenic effect of N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) to almost the spontaneous level. In experiments with S. typhimurium the protective effects of cytoplasm acidification against the mutagenic effect of MNNG was observed at pH 5.5 and was absent at a medium pH of 6.9. Alkalinization of the cytoplasm by adding 80 mM of methylamine to the growth medium at pH 8.1 did not modify the effect of MNNG on the cells of E. coli and S. typhimurium. Alkalinization of the cytoplasm of E. coli B/r and K12 was followed by a reduction of the intracellular non-protein SH group level by 25 and 50%, respectively. It is supposed that the protective effect of acidification may be due to a decline in the productivity of mutagenically more active intermediates of MNNG when the pH is reduced and the associated fall of the level of intracellular non-protein thiols occurs. The above situation may serve as a model for studying the effects of MNNG and other alkylating agents on cells differing in physiological status.

Effects of sodium selenite and caffeine on mutagenesis induced by N-methyl-N-nitrosourea, N-methyl-N'-nitro-N-nitrosoguanidine and aflatoxin B1 in S. typhimurium

Pre-treatment, co-treatment, and post-treatment procedures were comparatively used in order to assess the modulation of mutagenicity in S. typhimurium his- strains. Pre-treatment of bacteria with sodium selenite had no effect on sodium azide mutagenicity. Irrespective of the procedure used neither selenite nor caffeine had any influence on the S9-mediated mutagenicity of aflatoxin B1. In contrast, the mutagenicity of N-methyl-N-nitrosourea (MNU) and N-methyl-N'-nitro-N- nitrosoguanidine (MNNG) was variably affected, depending on the sequence of exposures of target bacterial cells to mutagens and modulators. In particular, pre-treatment of bacteria with either selenite or caffeine or their combination generally resulted in a potentiation of MNU and MNNG mutagenicity. However, co-incubation of these alkylating agents and test modulators with bacterial cells yielded an evident inhibition of mutagenicity, the methylxanthine being more effective in this case. Caffeine exhibited an an antimutagenic effect towards MNU also when assayed in a post-treatment procedure. Thus, in dependence on the test conditions, selenite and caffeine could act in the same mutagenicity assay as co-mutagens, antimutagens or agents without effect on mutagenesis. These opposite trends reflect the complexity of the mechanisms of action of both mutagens and modulators tested, and underscore the variable outcome of their interactions, also depending on topological and chronological factors. The data reported emphasize the need for a multiple methodological approach in studies investigating the modulation of mutagenicity.

Isolation and characterization of additional genes influencing resistance to various mutagens in the yeast Saccharomyces cerevisiae

Screening of a multi-copy vector-based yeast genomic library in haploid cells of wild-type Saccharomyces cerevisiae yielded transformants hyper-resistant to various chemical mutagens. Genetical analysis of the yeast insert DNAs revealed three genes SNG1, SNQ2, and SNQ3 that confer the phenotype hyper-resistance to MNNG, to 4-NQO and triaziquone, and to mutagens 4-NQO, MNNG, and triaziquone, respectively. Integration of the gene disruption-constructs into the haploid yeast genome yielded viable null-mutants with a mutagen-sensitive phenotype. Thus, copy number of these non-essential yeast genes determines the relative resistance to certain chemical mutagens, with zero copies yielding a phenotype of mutagen sensitivity and multiple copies one of mutagen hyper-resistance, respectively.

Molecular analysis of Bacillus subtilis ada mutants deficient in the adaptive response to simple alkylating agents

Previously, we isolated and characterized six Bacillus subtilis ada mutants that were hypersensitive to methylnitroso compounds and deficient in the adaptive response to alkylation. Cloning of the DNA complementing the defects revealed the presence of an ada operon consisting of two tandem and partially overlapping genes, adaA and adaB. The two genes encoded proteins with methylphosphotriester-DNA methyltransferase and O6-methylguanine-DNA methyltransferase activities, respectively. To locate the six mutations, the ada operon was divided into five overlapping regions of about 350 bp. The fragments of each region were amplified by polymerase chain reaction and analyzed by gel electrophoresis to detect single-strand conformation polymorphism. Nucleotide sequences of the fragments exhibiting mobility shifts were determined. Three of the mutants carried sequence alterations in the adaA gene: the adaA1 and adaA2 mutants had a one-base deletion and insertion, respectively, and the adaA5 mutant had a substitution of two consecutive bases causing changes of two amino acid residues next to the presumptive alkyl-accepting Cys-85 residue. Three mutants carried sequence alterations in the adaB gene: the adaB3 mutant contained a rearrangement, the adaB6 mutant contained a base substitution causing a change of the presumptive alkyl-accepting Cys-141 to Tyr, and the adaB4 mutant contained a base substitution changing Leu-167 to Pro. The adaB mutants produced ada transcripts upon treatment with low doses of alkylating agents, whereas the adaA mutant did not. We conclude that the AdaA protein functions as the transcriptional activator of this operon, while the AdaB protein specializes in repair of alkylated residues in DNA.

Protective effects of sodium selenite on killing and mutation by N-methyl-N'-nitro-N-nitrosoguanidine in E. coli

Sodium selenite was found to protect Escherichia coli cells against killing and mutagenic effects of N-methyl-N'-nitro-N-nitrosoguanidine (MNNG). Such protective effects were not observed when cells were treated with N-methyl-N-nitrosourea (MNU). The protection by sodium selenite was not controlled by the ada gene, which is responsible for the repair of alkylated damage in DNA. A reduction of the amount of glutathione was found when cells were treated with sodium selenite, and glutathione is known to be involved in the methylation of DNA by MNNG, not by MNU. Reduced methylation by MNNG due to the reduction of the amount of glutathione caused by abundant sodium selenite was suggested to be the mechanism of protection.

Increased spontaneous mutation and alkylation sensitivity of Escherichia coli strains lacking the ogt O6-methylguanine DNA repair methyltransferase

Escherichia coli expresses two DNA repair methyltransferases (MTases) that repair the mutagenic O6-methylguanine (O6MeG) and O4-methylthymine (O4MeT) DNA lesions; one is the product of the inducible ada gene, and here we confirm that the other is the product of the constitutive ogt gene. We have generated various ogt disruption mutants. Double mutants (ada ogt) do not express any O6MeG/O4MeT DNA MTases, indicating that Ada and Ogt are probably the only two O6MeG/O4MeT DNA MTases in E. coli. ogt mutants were more sensitive to alkylation-induced mutation, and mutants arose linearly with dose, unlike ogt+ cells, which had a threshold dose below which no mutants accumulated; this ogt(+)-dependent threshold was seen in both ada+ and ada strains. ogt mutants were also more sensitive to alkylation-induced killing (in an ada background), and overexpression of the Ogt MTase from a plasmid provided ada, but not ada+, cells with increased resistance to killing by alkylating agents. The induction of the adaptive response was normal in ogt mutants. We infer from these results that the Ogt MTase prevents mutagenesis by low levels of alkylating agents and that, in ada cells, the Ogt MTase also protects cells from killing by alkylating agents. We also found that ada ogt E. coli had a higher rate of spontaneous mutation than wild-type, ada, and ogt cells and that this increased mutation occurred in nondividing cells. We infer that there is an endogenous source of O6MeG or O4MeT DNA damage in E. coli that is prevalent in nondividing cells.

The sensitivities of SV40-transformed human fibroblasts to monofunctional and DNA-crosslinking alkylating agents

4 repair-deficient (Mer-) and 2 repair-proficient (Mer+) lines of SV40-transformed human fibroblasts were assayed for colony-forming ability after treatment with MNNG, methyl methanesulfonate (MMS), 1,3-bis-(2-chloroethyl)-1-nitrosourea (BCNU), and 1-(2-chloroethyl)-3-(2-hydroxyethyl)-1-nitrosourea (HECNU). The sensitivities to MMS, BCNU and HECNU of these SV40-transformed lines were similar to those of comparably treated human tumor cells observed previously. However, unlike human tumor lines, whose post-MNNG survival is strongly dependent upon Mer phenotype, SV40-transformed lines showed a lack of dependence of post-MNNG colony-forming ability on Mer phenotype. No differences in glutathione levels that might explain these differences were detected. The amounts of SV40-specific DNA and RNA among the lines were found to vary widely, but no correlation with Mer phenotype was found.

Palindromic unit highly repetitive DNA sequences exhibit species specificity within Enterobacteriaceae

Palindromic units (PU, or REP for repetitive extragenic palindrome) constitute a family of DNA sequences of 40 nucleotides which is highly repeated in the genome of Escherichia coli. We analysed the presence of PU sequences in 99 different bacterial genomes by cross-hybridization. When PU sequences were used as a probe, only DNA from Enterobacteriaceae closely related to E. coli exhibited an appreciable hybridization signal: Shigella sonnei, Shigella boydii, Salmonella enteritica serotype Typhimurium, Citrobacter freundii and Levinea malonatica. Furthermore, these bacteria could be divided into two groups which corresponded to a slight difference in their PU sequence: the E. coli group includes S. sonnei and S. boydii; the S. enteritica serotype Typhimurium group includes C. freundii and L. malonatica.

Expression of the ogt gene in wild-type and ada mutants of E. coli

O6-alkylguanine (O6-AlkG) DNA alkyltransferase (ATase) and alkylphosphotriester (AlkP) ATase activity have been quantitated individually in extracts of various E. coli strains by means of ATase specific DNA substrates. O6-AlkG ATase activity was higher than AlkP ATase activity in the wild-type strains F26, AB1157 and SB229 and in the ada- mutants PJ1, PJ3, PJ5 and PJ6 indicating a 5-70 times higher level of expression of the ogt gene than the ada gene. The ada- mutant strains BS23, BS73 and GW5352 expressed O6-AlkG ATase but not AlkP ATase activity indicating expression only of the ogt gene. Southern analysis of DNA from F26, BS23, BS73, PJ1 and GW5352 showed a consistent pattern of hybridisation to an ogt probe but not to an ada probe. Exposure of E. coli to adaptive doses of N-methyl-N-nitro-N-nitroso-guanidine (MeNNG) caused an increase in AlkP ATase activity in F26, AB1156, SB229, PJ1, PJ3, PJ5 and PJ6. O6-AlkG ATase activity also increased in F26, AB1157 and SB229 but decreased to almost undetectable levels in all other strains examined except PJ3 where it remained constant. MeNNG increased ada mRNA abundance in F26 but no ada mRNA was detected in BS23, BS73 or GW5352: there was no evidence for increased ogt mRNA in any of the strains examined. In a limited survey, other bacterial strains have been shown to possess an ogt-like ATase activity.

Enhancement and inhibition of mutation by o-vanillin in Escherichia coli

2-Hydroxy-3-methoxybenzaldehyde (omicron-vanillin), the antimutagenic effect of which has been reported on mutagenesis induced by 4-nitroquinoline 1-oxide (4NQO) in Escherichia coli WP2s, enhanced N-methyl-N'-nitro-N-nitrosoguanidine (MNNG)-induced mutagenesis in the same strain. A remarkable enhancement of mutagenesis provoked by N-methyl-N-nitrosourea (MNU) was also observed by the addition of omicron-vanillin. No enhancing effect was observed on mutagenesis induced by other mutagens such as methyl methanesulfonate (MMS), dimethylsulfate, N-ethyl-N'-nitro-N-nitrosoguanidine (ENNG), N-ethyl-N-nitrosourea (ENU), ethyl methanesulfonate, diethylsulfate, 4NQO and furylfuramide (AF-2). On the contrary, omicron-vanillin greatly suppressed AF-2- and 4NQO-induced mutagenesis and showed a slight suppressing effect against mutagenesis induced by MMS, ENNG and ENU. One possible explanation for the enhancing effect of omicron-vanillin on the mutagenesis induced by MNNG or MNU in E. coli WP2s may be inhibition of an inducible adaptive response. Among 7 derivatives of omicron-vanillin, 2-hydroxy-3-ethoxy-benzaldehyde, omicron-hydroxybenzaldehyde and m-methoxybenzaldehyde showed an enhancing effect on MNNG-induced mutagenesis.

Characterization of the major DNA repair methyltransferase activity in unadapted Escherichia coli and identification of a similar activity in Salmonella typhimurium

Escherichia coli has two DNA repair methyltransferases (MTases): the 39-kilodalton (kDa) Ada protein, which can undergo proteolysis to an active 19-kDa fragment, and the 19-kDa DNA MTase II. We characterized DNA MTase II in cell extracts of an ada deletion mutant and compared it with the purified 19-kDa Ada fragment. Like Ada, DNA MTase II repaired O6-methylguanine (O6MeG) lesions via transfer of the methyl group from DNA to a cysteine residue in the MTase. Substrate competition experiments indicated that DNA MTase II repaired O4-methylthymine lesions by transfer of the methyl group to the same active site within the DNA MTase II molecule. The repair kinetics of DNA MTase II were similar to those of Ada; both repaired O6MeG in double-stranded DNA much more efficiently than O6MeG in single-stranded DNA. Chronic pretreatment of ada deletion mutants with sublethal (adapting) levels of two alkylating agents resulted in the depletion of DNA MTase II. Thus, unlike Ada, DNA MTase II did not appear to be induced in response to chronic DNA alkylation at least in this ada deletion strain. DNA MTase II was much more heat labile than Ada. Heat lability studies indicated that more than 95% of the MTase in unadapted E. coli was DNA MTase II. We discuss the possible implications of these results for the mechanism of induction of the adaptive response. A similarly active 19-kDa O6MeG-O4-methylthymine DNA MTase was identified in Salmonella typhimurium.

Comparison of killing of gram-negative and gram-positive bacteria by pure singlet oxygen

Gram-negative and gram-positive bacteria were found to display different sensitivities to pure singlet oxygen generated outside of cells. Killing curves for Salmonella typhimurium and Escherichia coli strains were indicative of multihit killing, whereas curves for Sarcina lutea, Staphylococcus aureus, Streptococcus lactis, and Streptococcus faecalis exhibited single-hit kinetics. The S. typhimurium deep rough strain TA1975, which lacks nearly all of the cell wall lipopolysaccharide coat and manifests concomitant enhancement of penetration by some exogenous substances, responded to singlet oxygen with initially faster inactivation than did the S. typhimurium wild-type strain, although the maximum rates of killing appeared to be quite similar. The structure of the cell wall thus plays an important role in susceptibility to singlet oxygen. The outer membrane-lipopolysaccharide portion of the gram-negative cell wall initially protects the bacteria from extracellular singlet oxygen, although it may also serve as a source for secondary reaction products which accentuate the rates of cell killing. S. typhimurium and E. coli strains lacking the cellular antioxidant, glutathione, showed no difference from strains containing glutathione in response to the toxic effects of singlet oxygen. Strains of Sarcina lutea and Staphylococcus aureus that contained carotenoids, however, were far more resistant to singlet oxygen lethality than were both carotenoidless mutants of the same species and other gram-positive species lacking high levels of protective carotenoids.

Absence of DNA damage-mediated induction of human methyltransferase specific for precarcinogenic O6-methylguanine

The ability of cultured normal human fetal liver and kidney epithelial cells to repair the premutagenic and precarcinogenic O6-methylguanine (O6-MeGua) DNA adduct was determined by directly monitoring its loss in cellular DNA and quantitating the number of O6-MeGua-DNA-methyltransferase (O6-MT) molecules per cell. Following treatment of the epithelial cells with the direct acting carcinogen N-methyl-N-nitrosourea (MNU), the loss of the O6-MeGua adduct was biphasic, exhibiting a half-life of 2.0 and 1.5 h in the liver and kidney cells, respectively. The activity of O6-MT in the liver and kidney epithelial cells in culture was 0.19 pmol/mg protein or 18,500 molecules/cell. The activity of O6-MT was maintained throughout the life of the cultures, i.e., 20 subpassages or 50 cumulative population doublings for the liver and kidney. In order to ascertain whether human fetal epithelial cells exhibit an induction of O6-MT, the cell cultures were treated with single and multiple conditioning doses of N-methyl-N-nitro-N-nitroso-guanidine (MNNG) or gamma-irradiated and assayed for the amount of O6-MT. A 1 h exposure of cells to 2, 4, and 8 microM MNNG resulted in an 80-100% decrease of the initial O6-MT activity which was restored to the constitutive levels within 48 and 72 h post-treatment. Rat hepatoma cells, used as a positive control, increased their levels of O6-MT to 2.8-fold the constitutive levels following treatment with MNNG. Treatment of the human liver and kidney epithelial cells with chronic low doses of MNNG exhibited O6-MT levels identical to untreated cells. The O6-MT activity in epithelial cells remained unaffected upon pre-irradiation with 1.2 or 2.5 Gy of gamma-irradiation.

An adaptive response of Vibrio cholerae strain OGAWA 154 to furazolidone

Since furazolidone is an antimicrobial drug, any possibility of its evoking an adaptive response appears to be very important. This response was studied in Vibrio cholerae cells as a model system. In order to determine this response, a dose-response relation of these cells to furazolidone and the kinetics of inactivation of the drug were studied. The study of the adaptive response of these cells to furazolidone reveals that cells treated with a low concentration of furazolidone for a particular period were 100% more resistant to the lethal effects of a subsequent challenging dose than control cultures. Variation of the challenging dose level showed better survival of adapted cells than control cells. A time-dependent response study reveals a maximum response at 15-30 min, and a gradual fall thereafter.

Polyclonal antibodies against O6-methylguanine-DNA methyltransferase in adapted bacteria

The similarity of the adaptive response and the methyltransferase component in bacterial strains from different phylogenic groups was investigated. An adaptive response with induction of transferase activity was found for the first time in the soil bacteria P. aeruginosa and X. maltophilia. Polyclonal antibodies against the E. coli ada protein were used to investigate the structural similarity of the transferases from several bacterial strains with adaptive responses and inducible transferase activity. These antibodies cross-reacted with transferase from M. luteus and P. aeruginosa but not with proteins from other related bacteria, and not with human transferase. The phylogenic relationships of bacteria with adaptive responses suggest that the response likely was present in the common ancestor of eubacteria. The restricted antibody cross-reactivity may reflect the dual role of the E. coli ada protein not only in DNA repair but in positive gene regulation.

Mutants of Aspergillus nidulans with increased resistance to the alkylating agent, N-methyl-N'-nitro-N-nitrosoguanidine

The isolation and characterisation of mutants of Aspergillus nidulans showing resistance to MNNG is described. Such isolates were stable through prolonged subculture in the absence of the selective agent, and resistance segregated as an allele of a single gene in meiotic and mitotic analysis. MNNG-resistant strains showed an increase in resistance to EMS and UV irradiation but no cross-resistance to MMS was detected. Possible mechanisms of resistance to alkylating agents are discussed.

A second DNA methyltransferase repair enzyme in Escherichia coli

The Escherichia coli ada-alkB operon encodes a 39-kDa protein (Ada) that is a DNA-repair methyltransferase and a 27-kDa protein (AlkB) of unknown function. By DNA blot hybridization analysis we show that the alkylation-sensitive E. coli mutant BS23 [Sedgwick, B. & Lindahl, T. (1982) J. Mol. Biol. 154, 169-175] is a deletion mutant lacking the entire ada-alkB operon. Despite the absence of the ada gene and its product, the cells contain detectable levels of a DNA-repair methyltransferase activity. We conclude that the methyltransferase in BS23 cells is the product of a gene other than ada. A similar activity was detected in extracts of an ada-10::Tn10 insertion mutant of E. coli AB1157. This DNA methyltransferase has a molecular mass of about 19 kDa and transfers the methyl groups from O6-methylguanine and O4-methylthymine in DNA, but not those from methyl phosphotriester lesions. This enzyme was not induced by low doses of alkylating agent and is expressed at low levels in ada+ and a number of ada- E. coli strains.

N-methyl-N'-nitro-N-nitrosoguanidine-induced resistance to ionizing radiation

N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) pretreatments increase the resistance of Escherichia coli to gamma-radiation. The increased resistance is dependent on functional polA, recA, recB, recC, and lexA genes and is partly dependent on recN. The MNNG-induced resistance is additive to resistance induced by pretreatment with gamma-radiation but not by increases induced by hydrogen peroxide. The MNNG-induced resistance occurs in adaptive response mutants and at pretreatment levels of MNNG that do not activate cells to reactivate UV-inactivated lambda phage. The MNNG-induced resistance appears to be distinct from other inductions to gamma-radiation resistance.

Further characterization of an E. coli strain resistant to the toxic and mutagenic action of cis-diamminedichloroplatinum(II)

An increased resistance to the toxic and mutagenic activity of the antitumor drug cis-diamminedichloroplatinum(II) (cis-DDP) in the E. coli strain BS21 compared to its wild-type parent, F26, has been reported. This resistance was neither due to different binding of cis-DDP to DNA nor to adaptive DNA repair (Germanier et al., 1984). In the present work, we found that mutation of the uvrA, recA and polA genes did not abolish the resistance of BS21 to the toxic action of cis-DDP. The lower mutability of BS21 was not influenced by the polA mutation, while uvrA greatly reduced and recA eliminated the mutagenic activity of cis-DDP in both strains. Treatment of BS21 and F26 with equal doses of cis-DDP produced the same initial number of platinum-DNA lesions. Little excision repair was detected in vivo in either strain during 6-h post-treatment incubation, the F26 strain being the most efficient of the two for this process. In contrast, F26 and BS21 were transformed identically by pBR322 DNA which had been treated with cis-DDP in vitro. Analysis of the platinum-DNA adducts which were formed between cis-DDP and salmon sperm DNA in the buffer conditions of this experiment suggests that plasmid DNA contains 80% monofunctional adducts and 20% bifunctional bis-guanine adducts. These data indicate that the selective toxicity and mutagenicity of these two strains in vivo are neither a result of different numbers of Pt-DNA lesions nor of their repair. The selectivity disappeared when the two bacterial strains were transformed by pBR322 DNA containing identical platinum-DNA lesions, suggesting that the biochemical events which process platinum-DNA lesions are the same in both strains. Hence, it appears that cis-DDP may form qualitatively different platinum-DNA adducts in the BS21 and F26 strains which are responsible for the different toxicity and mutagenicity.

Glutathione in Escherichia coli is dispensable for resistance to H2O2 and gamma radiation

Escherichia coli devoid of glutathione (because of transposon insertions in the gshA gene) has normal resistance to H2O2, cumene hydroperoxide, heat, or ionizing radiation. Intracellular glutathione thus does not protect E. coli from such lethal oxidative damage. The use of gshA::Tn10 mutants also revealed a glutathione-independent, H2O2-inducible resistance to N-ethylmaleimide.

Glutathione levels and cytotoxicity of a thiol activated alkylating agent in human and mouse cells

The effects of cellular GSH levels on the cytotoxicity of MNNG and mitomycin C were examined in normal and BSO treated mouse C3H10T1/2 cells. MNNG was less cytotoxic in the GSH depleted cells (less than 10% of normal) whereas the cytotoxicity of mitomycin C was not influenced by thiol status. This is compatible with the alkylating agent MNNG requiring thiols for activation to the methylating electrophile. Conversely, thiols have little if any effect in modulating the activity of mitomycin C. When naturally thiol deficient human fibroblasts were compared with BSO treated fibroblasts depleted to a similar GSH level (less than 10% of normal), only the BSO depleted cells were less effected by MNNG. The GSH deficient cells showed the same MNNG dose response as normal human fibroblasts. These studies indicate that a naturally acquired thiol status and an equivalent induced thiol status need not behave the same and this needs consideration when evaluating the role of thiols in influencing cellular response to chemotherapeutic agents.

Isolation of glutathione-deficient mutants of the yeast Saccharomyces cerevisiae

Glutathione-deficient (gsh-) mutants of the yeast Saccharomyces cerevisiae were isolated after UV treatment using MNNG as selective agent. For genetic and biochemical characterization 5 mutant strains were chosen which exhibited considerably decreased residual GSH contents varying from 2 to 6% of the wild-type levels. All 5 isolates showed a 2:2 segregation of the gsh-:GSH+ phenotypes alluding to a monogenic recessive mutation. Complementation analysis indicates that all gsh- mutants belong to one complementation group.

Two classes of Bacillus subtilis mutants deficient in the adaptive response to simple alkylating agents

Six mutant strains of Bacillus subtilis hypersensitive to N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) were shown to be deficient in the adaptive response to MNNG and termed ada mutants (Morohoshi and Munakata 1985). All the mutations mapped between the attSPO2 and lin loci on the chromosome. The mutant and wild-type (ada+) cells contained similar constitutive levels of O6-methylguanine-DNA methyltransferase activity. Pretreatment with low concentrations of MNNG increased the activity about nine-fold in the ada+ cells, while it uniformly decreased the activity in the ada cells. The pretreatment of three mutants (ada-3, ada-4, and ada-6) as well as ada+, augmented the activity of methylpurine-DNA glycosylase and rendered the cells resistant to the lethal and mutagenic effects of N-propyl- or N-butyl-N'-nitro-N-nitrosoguanidine. With the rest of the mutant strains (ada-1, ada-2, and ada-5), neither of such responses was elicited by the pretreatment. Thus, the former ada strains seem to have a defect in the gene specifically involved in the induction of the methyltransferase, while the latter ada strains have a defect in the gene controlling the adaptive response as a whole.

Glutathione: a protective agent in Salmonella typhimurium and Escherichia coli as measured by mutagenicity and by growth delay assays

Cultures of some aerobically grown strains of Salmonella typhimurium and Escherichia coli contain up to 24 microM extracellular glutathione (GSH) [Owens RO, Hartman PE (1985): Environ Mutagen 7(Suppl 3): 47] in addition to having intracellular GSH concentrations in the millimolar range. The addition of 26 microM GSH to cultures of Salmonella typhimurium strain TA1534 partially protected the bacteria from the toxic effects causing growth delay by 54 microM N-methyl-N'-nitro-N-nitrosoguanidine (MNNG). When MNNG was preincubated with equimolar GSH, the mutagenicity of the MNNG was neutralized. The addition of micromolar GSH to cultures of an Escherichia coli GSH- strain protected the cells from growth inhibition by micromolar concentrations of mercuric chloride, methyl mercuric chloride, silver nitrate, cisplatin, cadmium chloride, cadmium sulfate, and iodoacetamide. In the cases of mercuric chloride, cisplatin, MNNG, silver nitrate, and iodoacetamide, reaction products with GSH were detected by paper chromatography. In contrast to reduced GSH, micromolar concentrations of oxidized glutathione (GSSG) provided little or no protection and formed no detectable reaction products. Export of GSH by enteric bacteria may provide an important defense mechanism against exogenous toxic agents otherwise active in the micromolar range.

Cloning of Micrococcus luteus 3-methyladenine-DNA glycosylase genes in Escherichia coli

The 3-methyladenine-DNA glycosylase (m3ADG) excises 3-methyladenine (m3A) residues formed in DNA after treatment with alkylating agents. In Escherichia coli, the repair of this type of damage depends on the products of the genes tagA and/or alkA, which code for m3ADG I (20 kDa) and II (30 kDa), respectively. The tagA- and alkA--single mutants are sensitive to alkylating agents, the double mutant much more so. We have cloned two genes of Micrococcus luteus that can partly substitute the function of the E. coli tagA- and alkA- genes. An M. luteus genome bank was made by shotgun cloning of EcoRI + BamHI-digested DNA into pBR322. Two hybrid plasmids were identified that confer methylmethane sulfonate (MMS) resistance to the tagA- ada+ mutant and a capacity to reactivate MMS-treated bacteriophage lambda. Each hybrid plasmid directed the synthesis of 21-kDa m3ADG in E. coli tagA- ada-, which were not inhibited by 4 mM m3A. However, the restriction maps of the two cloned genes were different, and they showed no sequence homology as judged by the lack of cross hybridization.

Chemical adaptation of M. luteus induces repair functions for O-alkylated DNA pyrimidines

A partially purified extract prepared from adapted M. luteus cells contains repair functions for oxygen methylated pyrimidine residues present in alkylated DNA. The removal of O2-MeT is mediated by a DNA glycosylase enzyme whereas disappearance of O4-MeT is effected by a methyltransferase in a manner similar to the in situ repair of O6-MeG. O4-MeT methyltransferase enzyme is unusually heat resistant. Synthesis of these repair proteins, which are distinctly different from the previously known inducible 3-MeA DNA glycosylase and O6-MeG methyltransferase activities, forms a part of the adaptive response.