A loss of uvrA function decreases the induction of the SOS functions recA and umuC by mitomycin C in Escherichia coli

Nucleotide excision repair and recombination are engaged in repair of trans-4-hydroxy-2-nonenal adducts to DNA bases in Escherichia coli

One of the major products of lipid peroxidation is trans-4-hydroxy-2-nonenal (HNE). HNE forms highly mutagenic and genotoxic adducts to all DNA bases. Using M13 phage lacZ system, we studied the mutagenesis and repair of HNE treated phage DNA in E. coli wild-type or uvrA, recA, and mutL mutants. These studies revealed that: (i) nucleotide excision and recombination, but not mismatch repair, are engaged in repair of HNE adducts when present in phage DNA replicating in E. coli strains; (ii) in the single uvrA mutant, phage survival was drastically decreased while mutation frequency increased, and recombination events constituted 48 % of all mutations; (iii) in the single recA mutant, the survival and mutation frequency of HNE-modified M13 phage was slightly elevated in comparison to that in the wild-type bacteria. The majority of mutations in recA^- strain were G:C → T:A transversions, occurring within the sequence which in recA⁺ strains underwent RecA-mediated recombination, and the entire sequence was deleted; (iv) in the double uvrA recA mutant, phage survival was the same as in the wild-type although the mutation frequency was higher than in the wild-type and recA single mutant, but lower than in the single uvrA mutant. The majority of mutations found in the latter strain were base substitutions, with G:C → A:T transitions prevailing. These transitions could have resulted from high reactivity of HNE with G and C, and induction of SOS-independent mutations.

Comparative mutagenesis of Escherichia coli strains with different repair deficiencies irradiated with 222-nm and 254-nm ultraviolet light

Photoinactivation and reversion to tryptophan prototrophy were studied in four Escherichia coli strains with different repair deficiencies. Cells were irradiated with 222-nm wavelength UV emitted by an excimer lamp and with 254-nm wavelength UV emitted by a low-pressure mercury lamp. Strain DSM 9494 (trp(-)uvrA(+)) turned out to be most resistant while the strain DSM 9495 (trp(-)uvrA(-)), which is defective in nucleotide-excision repair (NER) was most sensitive to both wavelengths. UV-fluence rates for a respective inactivation were twice as high for 222-nm wavelength UV than for 254-nm UV. No clear difference in efficiency of inactivation could be observed between the two wavelengths in strains DSM 9496 (trp(-)uvrA(+) pKM101) and DSM 9497 (trp(-)uvrA(-) pKM101). In general, more revertants were induced by 254-nm wavelength UV, which corroborates the hypothesis that a higher amount of DNA damage was induced by this wavelength than by 222-nm UV, except for DSM 9497 where no clear difference could be observed regarding the number of revertants induced by both wavelengths. This strain DSM 9497 has a high sensitivity to certain oxidative mutagens compared with other strains, which is indicative of formation of reactive oxygen species during irradiation with 222-nm wavelength UV.

Precise temporal modulation in the response of the SOS DNA repair network in individual bacteria

The SOS genetic network is responsible for the repair/bypass of DNA damage in bacterial cells. While the initial stages of the response have been well characterized, less is known about the dynamics of the response after induction and its shutoff. To address this, we followed the response of the SOS network in living individual Escherichia coli cells. The promoter activity (PA) of SOS genes was monitored using fluorescent protein-promoter fusions, with high temporal resolution, after ultraviolet irradiation activation. We find a temporal pattern of discrete activity peaks masked in studies of cell populations. The number of peaks increases, while their amplitude reaches saturation, as the damage level is increased. Peak timing is highly precise from cell to cell and is independent of the stage in the cell cycle at the time of damage. Evidence is presented for the involvement of the umuDC operon in maintaining the pattern of PA and its temporal precision, providing further evidence for the role UmuD cleavage plays in effecting a timed pause during the SOS response, as previously proposed. The modulations in PA we observe share many features in common with the oscillatory behavior recently observed in a mammalian DNA damage response. Our results, which reveal a hitherto unknown modulation of the SOS response, underscore the importance of carrying out dynamic measurements at the level of individual living cells in order to unravel how a natural genetic network operates at the systems level.

Inhibitory effect of curcumin on SOS functions induced by UV irradiation

The antigenotoxic effects of curcumin, including the inhibition of SOS induction and mutagenesis by UV light, were investigated in Salmonella typhimurium TA1535/pSK1002 and Escherichia coli K-12 strains. Induction of the SOS gene (umuC) expression was assayed by measuring accumulated beta-galactosidase activity. We found that curcumin blocked umuC induction promoted by UV irradiation in a dose-dependent manner. Also, with another SOS response, Weigle reactivation, we observed that curcumin effectively inhibited phage reactivation by UV irradiation. Furthermore, we tested the effect of curcumin on UV mutagenesis. We showed that mutagenesis induced by UV irradiation was suppressed by the addition of curcumin. Together these results indicate that curcumin acts as an inhibitor of SOS functions including UV mutagenesis.

Differences in DNA damage induced by mutagenic and nonmutagenic 4-aminoazobenzene derivatives in Escherichia coli

DNA lesions produced in Escherichia coli AB2500 (uvrA) exposed to the carcinogen N-hydroxy-3-methoxy-4-aminoazobenzene (N-OH-3-MeO-AAB) or the noncarcinogen N-hydroxy-2-methoxy-4-aminoazobenzene (N-OH-2-MeO-AAB) were investigated by alkaline sucrose gradient sedimentation and 32P-postlabeling analysis. Alkali-labile sites appeared to be formed equally in cells treated with both aminoazobenzene derivatives. 32P-Postlabeling analysis revealed that the 3-MeO-AAB-DNA adduct level was 25-fold higher than that for 2-MeO-AAB-DNA adducts. In addition to major adducts, 4 minor spots were detected in N-OH-3-MeO-AAB-treated cells, while only one major adduct was found in N-OH-2-MeO-AAB-treated cells. The mutagenicities and cytotoxicities were also determined with E. coli with different repair capacities; we found that repair of 3-MeO-AAB damages is strongly dependent on the UVR repair system. Moreover, N-OH-3-MeO-AAB, but not N-OH-2-MeO-AAB, could induce recA and umuC gene expression, which was higher in uvrA strains than in the wild type.

Spontaneous transposition in the bacteriophage lambda cro gene residing on a plasmid

A new mutagenesis assay system based on the phage lambda cro repressor gene residing on a plasmid was developed. The assay detects mutations in cro that decrease the binding of the repressor to the OR operator in an OR PR-lacZ fusion present in a lambda prophage. Mutations arose spontaneously during growth of E. coli cells harboring cro plasmids at a frequency of 3-6 x 10(-6). Analysis of some 200 cro mutants from several 'wild-type' strains revealed a substantial fraction of 25-70% insertion events caused by transposition of IS elements. Most of the insertions were caused by IS1, but IS5 insertions were observed too. In strains harboring Tn10, IS10 was responsible for most insertions. Restriction nuclease digestion analysis revealed a preference for insertion of IS10 into the C-terminal half of cro, despite the absence of sequences which are known hot spots for Tn10 insertions. The frequency of IS1 insertions into cro decreased 25-60-fold and that of IS10 insertions decreased 200-fold in cells carrying the recA56 mutation, suggesting that RecA is involved in transposition of these elements. During the logarithmic phase of growth, the mutation frequency was constant for at least 22 generations; however, upon continuous incubation at the stationary phase, the mutation frequency gradually increased, yielding a 3-fold increase in the frequency of insertion and a 4-5-fold increase in point mutation. Genomic Southern analysis of chromosomal IS elements in cells which underwent a transposition from the chromosome into the cro plasmid revealed that the number and distribution of IS1 and IS5 were usually unaltered compared to cells which did not undergo a transposition event. In contrast, essentially each IS10 transposition was accompanied by multiple events which led to changes in the number and distribution of chromosomal IS10 elements.

Cross-adaptive response in Escherichia coli caused by pretreatment with H2O2 against formaldehyde and other aldehyde compounds

A cross-adaptive response (CAR), defined as a reduction of the effects of an agent by pretreatment with another agent, was demonstrated when E. coli WP2 cells were pretreated with hydrogen peroxide (H2O2) followed by challenging treatment with aldehyde compounds. Pretreatment with a sublethal dose (60 microM) of H2O2 for 30 min made WP2 cells resistant to the killing effects of formaldehyde (FA), and 4 other mutagenic aldehydes: glutaraldehyde, glyoxal, methyl glyoxal and chloroacetaldehyde. CAR was also observed in WP2uvrA (uvrA-) and ZA12 (umuC-) cells, but not in ZA60 (recA-) and CM561 (lexA- (Ind-] cells. A role of recA and lexA in CAR was further suggested by the lack of beta-galactosidase induction in recA- and lexA- cells by H2O2. CAR and beta-galactosidase induction, however, were found to be separate events since CAR was recovered by introducing the recA+ gene into lexA- cells, but no induction of beta-galactosidase by H2O2 was observed in cells with the same gene transfer. These results suggest that H2O2 has the capacity to induce a function which reduces the killing effects of aldehydes, and the function is controlled by the recA gene without involvement of SOS response.

Plasmid pSK1002-mediated mutator effect and SOS response and SOS mutagenesis of 2,5-dichloronitrobenzol in Salmonella typhimurium

The plasmid pSK1002 (umuC'-'lacZ) could increase the number of revertants induced by methyl methanesulfonate (MMS) and 4-nitroquinoline-N-oxide (4NQO) in Salmonella typhimurium TA 1535 (his-). The values induced by N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) were not different irrespective of the presence or absence of plasmid. However, the plasmid pKM101-mediated mutagenesis-enhancing effect was much greater than that mediated by pSK1002 as induced by the 3 mutagens mentioned above. Moreover, the plasmid pSK1002 could induced umu-mediated SOS response in the presence of any of these 3 mutagens or of mitomycin C, and a dose-response relationship was evident. It shows that pSK1002 (umuC'-'lacZ) has a dual biological effect, namely a mutator effect and the effect of inducing the SOS response. Besides, this study has proved SOS mutagenesis of 2,5-dichloronitrobenzol (2,5-DCNB) because of the dual indicator nature of pSK1002. Therefore, it is probable that pSK1002 could be further developed and applied in studying the relation between the SOS response and mutagenesis and in identifying environmental SOS mutagens.

Inhibitory effect of cadmium and mercury ions on induction of the adaptive response in Escherichia coli

Cadmium and mercury ions inhibited the promotion of ada and alkA gene expression in the adaptive process induced by methylating agents such as N-methyl-N-nitrosourea (MNU), methyl methanesulfonate (MMS) and methyl iodide in Escherichia coli. In fact, the induction of O6-methylguanine-DNA methyl-transferase (MGTase) by MNU was suppressed in E. coli in the presence of these metal ions. These ions potentiated mutagenesis induced by methylating agents such as MNU and MMS, but not that induced by ethylating agents, UV irradiation, or N4-aminocytidine. These comutagenic effects were observed in wild-type and umuC36 strains of E. coli but not in the ada-5 strain, which is unable to induce the adaptive response. These results suggest that the comutagenic effects of Cd2+ and Hg2+ are due to inhibition of ada and alkA gene expression promoted by methylated MGTase.

'Rec-lac test' for detecting SOS-inducing activity of environmental genotoxic substance

beta-Galactosidase activities (beta-GA) of E. coli strains carrying the fusion gene of recA and lacZ, GE94 and its DNA repair-deficient derivatives such as KY946[uvrA], KY945[recA] and KY943[lexA] treated with UV, 4NQO, MNNG and MMC were examined. The beta-GA, reflecting the SOS-inducing activity, of GE94 and KY946 treated with these compounds increased significantly with a clear dose-response relationship, and reached a maximum level within 60 min, while no response was seen in KY945 and KY943. Using KY946 and KY945 as a positive and a negative indicator, respectively, the SOS-inducing activity of oxidative mutagens, i.e., hydrogen peroxide (H2O2), formaldehyde, tert-butyl hydroperoxide, cumene hydroperoxide and streptonigrin, was investigated. Clear dose-dependent increases in beta-GA were observed in KY946 treated with all oxidative mutagens tested, but not in KY945. Significant increases in beta-GA were observed with a lower concentration of H2O2 and a shorter incubation time of 4NQO in this assay than in the umu-test. The assay, called 'Rec-lac test' by us, may be useful to detect environmental genotoxic substances.

A specific inhibition of induction of adaptive response by o-vanillin, a potent comutagen

The present study revealed that potent comutagenic activity of o-vanillin in mutagenesis by N-methyl-N-nitrosourea (MNU) is a consequence of inhibition of the transcriptional promoting capacity of methylated O6-methylguanine-DNA methyltransferase (MGTase). As evidence of this, in the presence of o-vanillin, there were (i) dose-dependent decreases in MGTase induced in E. coli exposed to MNU, (ii) inhibition of ada gene promotion determined in the ada'-lacZ' assay system, and (iii) minimal inhibition of the methyl transfer capacity of MGTase in a subcellular system.

Genotoxicity of quinoxaline 1,4-dioxide derivatives in Escherichia coli and Salmonella typhimurium

The mutagenicities of 5 quinoxaline 1,4-dioxide (QdO) derivatives were tested by 2 bacterial assays using forward mutation with Escherichia coli WP2uvrA/pKM101 and reverse mutation with Salmonella typhimurium TA100 and TA98. Potent mutagenic activities of all QdOs tested were observed in both mutation assays. Mutagenicities of these compounds were varied by addition of S9 mix. Their SOS-inducing activities were examined with a 'Rec-lac test' that has been newly developed by us for detecting genotoxins. A high level of SOS-inducing activity was observed in all samples tested. These results suggest that the mutagenicity of QdOs results from the error-prone repair involved in SOS responses.

Effect of metal ions on the adaptive response induced by N-methyl-N-nitrosourea in Escherichia coli

Induction of the adaptive response was quantified by analysis of beta-galactosidase released after the treatment of Escherichia coli CHS26/pYM3 (ada'-lacZ') with N-methyl-N-nitrosourea (MNU). Of the 15 metal ions examined, only Cd++ and Hg++ inhibited induction of the adaptive response with neither severe suppression of cell growth nor inhibition of the induction of the SOS response by MNU. Mutagenicity of MNU was potentiated by the presence of these metal ions in an E. coli strain. These results suggest that the inhibition mechanism involves a specific interaction of Cd++ or Hg++ with O6-methyl-guanine-DNA methyltransferase.

Structural requirement for the induction of the adaptive response in Escherichia coli among N-(substituted alkyl)-N-nitrosoureas

Using E. coli CSH26 transformed with a plasmid carrying an alkA'-lacZ' fused gene, a series of N-(substituted alkyl)-N-nitrosoureas were subjected to a colorimetric assay to evaluate their capacity to induce the adaptive response, an inducible DNA-repair network in E. coli. Some of these derivatives induced the response in greater or lesser degrees, while others did not. Several structural requirements for the induction were disclosed. The capacity of these derivatives to induce the SOS response, which is another inducible DNA-repair network, was also evaluated using E. coli transformed with a plasmid carrying a umuC'-lacZ' fused gene. Since all the derivatives induced the SOS response, the structural requirements for the adaptive response disclosed in this study are substantially related to the molecular mechanism involved in the adaptive response.

Characterization of recombination-deficient mutants of Bacillus subtilis

An isogenic set of "prophage-free," DNA repair-proficient and -deficient strains of Bacillus subtilis were characterized phenotypically. The mutant strains were provisionally classified into four categories on the basis of their sensitivity to DNA-damaging agents, their ability to release phage after lysogenization followed by damage to chromosomal DNA, and their impairment in genetic exchange. The properties of double Rec- mutants showed that recF and addA belong to different epistatic groups, whereas recF, recL, and recH fall into the same group. More than one pathway for genetic exchange might be operative in B. subtilis.

Harman and norharman induce SOS responses and frameshift mutations in bacteria

Norharman and harman, beta-carboline derivatives with comutagenic activity in Salmonella typhimurium, were examined for their activity to induce SOS responses in S. typhimurium using the umu-test and mutations in Escherichia coli. The inducibility of the umuC gene by norharman and harman was assayed by measuring the levels of beta-galactosidase activity in tester cells harbouring the umuC'-'lacZ fusion gene on a plasmid. In the umu-test, both norharman and harman weakly induced umuC gene expression at 25-100 and 50-150 micrograms/ml, respectively. In the mutation test using reversion from trpE9777 to Trp+, harman was relatively more potent than norharman in inducing the mutations. These results indicate that norharman and harman induce SOS responses as well as reversion of trpE9777 frameshift mutation in bacteria.

Induction of phr gene expression by irradiation of ultraviolet light in Escherichia coli

To measure the degree of phr gene induction by DNA-damaging agents, the promoter region was fused to the coding region of the lacZ gene in plasmid pMC1403. The new plasmids were introduced into Escherichia coli cells having different repair capabilities. More efficient induction of phr gene expression was detected in a uvrA- strain as compared with the wild-type strain. In addition, obvious induction was detected in uvrA- cells treated by 4-nitroquinoline 1-oxide and mitomycin C. Nalidixic acid, an inhibitor of DNA gyrase, also induced phr gene expression. In contrast, little induced gene expression was noted in UV-irradiated lexA- and recA- strains. It is suggested from these results that induction of the phr gene is one of the SOS responses. Possible nucleotide sequences which could be considered to constitute an SOS box were found at the regulator region of the phr gene.

Induction of umu gene expression by cross-links and other DNA lesions

The induction of umu gene expression by DNA cross-links was investigated in various strains of E. coli with different DNA-repair capacities. Expression was measured by quantifying enzymatic activity of beta-galactosidase produced under regulation of the umu promoter carried on a plasmid carrying the umuC-lacZ gene fusion. The treatment with MMC induced gene expression more efficiently in a wild-type strain when compared with an excision-repair-deficient strain (uvrA). In contrast, PUVA and cis-Pt treatment induced higher levels of the gene expression in the uvrA strain than in the wild-type strain, as did other DNA-damaging agents including 4NQO, MNNG and MMS. None of these chemicals induced umu expression in either lexA and recA strains. The mechanisms of the induction of umu expression by DNA cross-links in relation to DNA damage and repair are discussed.

Induction of SOS responses in Escherichia coli by 5-fluorouracil

The inducibility of SOS responses by 5-fluorouracil (5-FU), which has been used as an antitumor drug, was studied in Escherichia coli cells which have different DNA repair capacities for UV lesions. Expression of the umuC gene was apparently induced by 5-FU in the wild-type and uvrA strains, but not in lexA and recA strains. The inducibility of the umuC gene by 5-FU, the metabolite of which inhibits thymidylate synthetase, was abolished in cultures containing deoxythymidine monophosphate which is converted from deoxyuridine monophosphate by thymidylate synthetase. These results suggest that 5-FU may exert its SOS inducibility by inhibiting thymidylate synthetase and then disturbing DNA metabolism but not by incorporating 5-FU residues into RNA. Further, 5-FU weakly induced mutations in E. coli.

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.

Induction of SOS functions by nitrogen dioxide in Escherichia coli with different DNA-repair capacities

The effect of gaseous nitrogen dioxide (NO2) on cytotoxicity, induction of synthesis of UmuC and RecA proteins, and mutagenesis was studied in Escherichia coli strains with different capacities of DNA repair. Gaseous NO2 (90, 180 microliter/l) killed Escherichia coli. The recA mutant was most sensitive, the lexA mutant moderately sensitive, and the uvrA mutant and the wild-type the least sensitive. When 90 microliter/l NO2 gas was bubbled into bacterial suspensions for 30 min at a flow rate of 100 ml/min, the induction of umuC gene expression increased in the wild-type strain. NO2 also induced the recA gene expression in the wild-type strain. The synthesis of neither RecA nor UmuC proteins was induced in the recA and lexA mutants. We further investigated the NO2 mutagenesis in the cells treated with bubbling of NO2 gas. NO2 caused mutation to Trp+ of WP2.

Analysis of mutagenic DNA repair in a thermoconditional mutant of Saccharomyces cerevisiae. III. Dose-response pattern of mutation induction in UV-irradiated rev2ts cells

Recent studies regarding the influence of cycloheximide on the temperature-dependent increase in survival and mutation frequencies of a thermoconditional rev2 mutant lead to the suggestion that the REV2-coded mutagenic repair function is UV-inducible. In the present study we show that stationary-phase rev2ts cells are characterized by a biphasic linear-quadratic dose-dependence of mutation induction ("mutation kinetics") of ochre alleles at 23 degrees C (permissive temperature) but linear kinetics at the restrictive temperature of 36 degrees C. Mathematical analysis using a model based on Poisson statistics and a further mathematical procedure, the calculation of "apparent survival", support the assumption that the quadratic component of the reverse mutation kinetics investigated can be attributed to a UV-inducible component of mutagenic DNA repair controlled by the REV2 gene.