Comparative mutagenesis and interaction between near-ultraviolet (313- to 405-nm) and far-ultraviolet (254-nm) radiation in Escherichia coli strains with differing repair capabilities.

Comparative mutagenesis and possible synergistic interaction between broad-spectrum (313- to 405-nm) near-ultraviolet (black light bulb [BLB]) radiation and 254-nm radiation were studied in Escherichia coli strains WP2 (wild type), WP2s (uvrA), WP10 (recA), WP6 (polA), WP6s (polA uvrA), WP100 (uvrA recA), and WP5 (lexA). With BLB radiation, strains WP2s and WP6s demonstrated a high level of mutagenesis, whereas strains WP2, WP5, WP6, WP10, and WP100 did not demonstrate significant mutagenesis. In contrast, 254-nm radiation was mutagenic in strains WP2, WP2s, WP6, and WP6s, but strains WP5, WP10, and WP100 were not significantly mutated. The absence of mutagenesis by BLB radiation in lexA and recA strains WP10, WP5, and WP100 suggests that lex+ rec+ repair may play a major role in mutagenesis by both BLB and 254-nm radiation. The hypothesis that BLB radiation selectively inhibits rec+ lex+ repair was tested by sequential BLB-254-nm radiation. With strain WP2, a fluence of 30 J/m2 at 254 nm induced trp+ revertants at a frequency of 15 X 10(-6). However, when 10(5) J/m2 or more of BLB radiation preceded the 254-nm exposure, no trp+ revertants could be detected. A similar inhibition of 254-nm mutagenesis was observed with strain WP6 (polA). However, strains WP2s (uvrA) and wP6s (polA uvrA) showed enhanced 254-nm mutagenesis when a prior exposure to BLB radiation was given.     

Enhancing effects of cinoxate and methyl sinapate on the frequencies of sister-chromatid exchanges and chromosome aberrations in cultured mammalian cells

Sister-chromatid exchanges (SCEs) induced by mitomycin C (MMC), 4-nitroquinoline-1-oxide (4NQO) or UV-light in cultured Chinese hamster ovary cells (CHO K-1 cells) were enhanced by cinoxate (2-ethoxyethyl p-methoxycinnamate) or methyl sinapate (methyl 3,5-dimethoxy 4-hydroxycinnamate). Both substances are cinnamate derivatives and cinoxate is commonly used as a cosmetic UV absorber. Methyl sinapate also increased the frequency of cells with chromosome aberrations in the CHO K-1 cells treated with MMC, 4NQO or UV. These increasing effects of methyl sinapate were critical in the G1 phase of the cell cycle and the decline of the frequencies of UV-induced SCEs and chromosome aberrations during liquid holding was not seen in the presence of methyl sinapate. Both compounds were, however, ineffective in cells treated with X-rays. In cells from a normal human embryo and from a xeroderma pigmentosum (XP) patient, MMC-induced SCEs were also increased by the post-treatment with methyl sinapate. The SCE frequencies in UV-irradiated normal human cells were elevated by methyl sinapate, but no SCE-enhancing effects were observed in UV-irradiated XP cells. Our results suggest that the test substances inhibit DNA excision repair and that the increase in the amount of unrepaired DNA damage might cause the enhancement of induced SCEs and chromosome aberrations.     

Rat liver S9 preparations contain comutagenic activity

Rat liver S9 preparations contain material which causes enhancement of UV mutagenesis in Escherichia coli WP2. This comutagenic activity is present in S9 preparations from both uninduced and Aroclor-induced rats. Strains of E. coli which are defective in the uvr-dependent excision repair pathway fail to show comutagenic action by S9. The comutagenic material is heat-labile and non-dialyzable, suggesting that it might be protein. This differs from the small amount of mutagenic material present in rat liver S9, as the latter is dialyzable and can be demonstrated in the repair-deficient strain E. coli WP2s (uvrA).     

Radioadaptive enhancement of the repair of UV-induced postreplication gaps in Escherichia coli cells deficient in DNA repair

In UV-irradiated E. coli WP2 uvrA, deficient in excision repair of DNA with pyrimidine dimers, gamma-irradiation in low doses (radioadaptation) before UV-irradiation leads to the intensification of postreplication repair of DNA. This process in WP2 uvrA polA and uvrA lexA mutants is less than in WP2 uvrA cells, but in WP2 uvrA recA both postreplication repair and its radioadaptive intensification are absent. In E. coli AB1157 excising pyrimidine dimers the radioadaptive intensification of postreplication repair of DNA is expressed almost to the same extent as in WP2 uvrA. In GW2100 umuC mutant, deficient in DNA polymerase V, postreplication repair of DNA is expressed, but its radioadaptive intensification is absent, while in AB2463 recA13 both postreplication repair of DNA and radioadaptive intensification of postreplication repair of DNA are absent. The above data suggest that DNA polymerase I and LexA protein are needed for radioadaptive intensification of postreplication repair of DNA in uvrA strain, and DNA polymerase V is needed for radioadaptive intensification in E. coli AB1157, and that RecA protein is required for postreplication repair and radioadaptive intensification of postreplication repair of DNA.     

Inactivation of Escherichia coli by Near-Ultraviolet Light and 8-Methoxypsoralen: Different Responses of Strains B/r and K-12

A series of Escherichia coli K-12 AB1157 strains with normal and defective deoxyribonucleic acid repair capacity were more resistant to treatment with 8-methoxypsoralen (8-MOP) and near-ultraviolet light (NUV) than a comparable series of strains from the B/r WP2 family although sensitivities to 254-nm ultraviolet light were closely similar. The difference was most marked with strains deficient in both excision and postreplication repair (uvrA recA). The hypothesis that the internal level of 8-MOP was lower in K-12 than B/r uvrA recA derivatives was ruled out on the basis of fluorometric determinations of 8-MOP content and the similar inactivation curves for phage T3 treated intracellularly within the two strains. The demonstration of liquid holding recovery with AB2480 but not WP100 (both recA uvrA strains) and the somewhat greater resistance of the former strain to inactivation by captan revealed the presence in the K-12 strain of a deoxyribonucleic acid repair system independent of the recA(+) and uvrA(+) genes. The presence of this repair system did not, however, affect the survival of T3 phage treated with 8-MOP plus NUV and probably has a relatively small effect on survival of AB2480 under normal conditions. Experiments in which 8-MOP monoadducts were converted to cross-links by a second NUV exposure in the absence of 8-MOP indicated that the level of potentially cross-linkable monoadducts immediately after 8-MOP + NUV is about eightfold lower in K-12-than in B/r-derived strains. It is therefore suggested that the photoproduct yield in the former is well below that in the latter. In agreement with this is the observation that, during the first 10 min after treatment, deoxyribonucleic acid synthesis was just over five times more sensitive to inhibition by 8-MOP plus NUV in WP100 than in AB2480. We assume that 8-MOP in K-12 bacteria is hindered in some way from adsorbing to cellular (though not to phage T3) deoxyribonucleic acid. Consistent with this, 8-MOP has been shown to act as an inhibitor of a component of repair of 254-nm ultraviolet light damage in WP2 but not in AB1157.     

Evaluation of drug candidates in a battery of short-term genetic toxicology assays: overview

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.     

Mutation induction and killing of Escherichia coli by DNA adducts and crosslinks: a photobiological study with 8-methoxypsoralen.

Low doses of 350 nm radiation (NUV) in the presence of 8-methoxypsoralen (8-MOP) induce predominantly mono-adducts in bacterial DNA. Further exposure to NUV in the absence of 8-MOP converts a proportion of these mono-adducts to interstrand cross-links. Using this approach the relative effects of adducts and cross-links on bacteria with different repair capacities was studied. Escherichia coli WP100 uvrA recA, believed to be totally deficient in the ability to repair 8-MOP plus NUV damage to DNA, was inactivated on average by a single photon event occurring with a quantum efficiency of about 0.03. We conclude that the inactivating lesion is probably a single mono-adduct. E. coli WP2 uvrA, deficient in excision endonuclease activity, may be inactivated by a very small number of cross-links, probably one. These conclusions are consistent with present knowledge of the repair capabilities of these bacteria. Conversion of mono-adducts to cross-links in WP2 uvrA (which occurs with a quantum efficiency of around 0.3) greatly increases lethality but results in a reduction of the induced mutation frequency presumably because cross-links are (almost) invariably lethal. In the repair-proficient strain WP2 both adducts and cross-links can be repaired but the latter are more likely than the former to lead to either death or mutation.     

New mutation (mmrA1) in Escherichia coli K-12 that affects minimal medium recovery and postreplication repair after UV irradiation

After UV irradiation, Escherichia coli uvrA mutant cells show higher survival on minimal than on rich growth medium, i.e., they show minimal-medium recovery. This effect of rich growth medium on survival is not observed in a uvrA mutant carrying an mmrA1 mutation, and the uvrA mmrA strain showed the same survival rate on minimal and rich growth media as the uvrA strain did on minimal medium plates. The mmrA1 mutation was isolated as a hidden mutation from a uvrA polA mutant strain and shown to map at 84.3 min on the E. coli K-12 linkage map. In contrast to the uvrA strain, the repair of DNA daughter strand gaps was not inhibited in the uvrA mmrA strain by rich growth medium after irradiation. However, the uvrA and uvrA mmrA strains were similar in their ability to repair DNA when compared in minimal medium. These data are consistent with the idea that the mmr gene product is not involved directly in the repair of UV radiation-induced DNA damage, but rather allows rich growth medium to inhibit a portion of postreplication repair.     

Inhibition of deoxyribonucleic acid repair in Escherichia coli by caffeine and acriflavine after ultraviolet irradiation.

The effects of caffeine and acriflavine on cell survival, single-strand deoxyribonucleic acid break formation, and postreplication repair in Escherichia coli wild-type WP2 and WP2 uvrA strains after ultraviolet irradiation was studied. Caffeine (0.5 mg/ml) added before and immediately after ultraviolet irradiation inhibited single-strand deoxyribonucleic acid breakage in wild-type WP2 cells. Single-strand breaks, once formed, were no longer subject to repair inhibition by caffeine. At 0.5 to 2 mg/ml, caffeine did not affect postreplication repair in uvrA strains. These data are consistent with the survival data of both irradiated WP2 and uvrA strains in the presence and absence of caffeine. In unirradiated WP2 and uvrA strains, however, a high caffeine concentration (greater than 2 mg/ml) resulted in gradual reduction of colony-forming units. At a concentration insufficient to alter survival of unirradiated cells, acriflavine (2 microgram/ml) inhibited both single-strand deoxyribonucleic acid breakage and postreplication repair after ultraviolet irradiation. These data suggest that although the modes of action for both caffeine and acriflavine may be similar in the inhibition of single-strand deoxyribonucleic acid break formation, they differ in their mechanisms of action on postreplication repair.     

Radioadaptive enhancement of the repair of UV-induced postreplication gaps in Escherichia coli DNA

Postreplication DNA repair (PRR) in UV-irradiated Escherichia coli WP2 uvrA (tryptophan-dependent strain) and K12 AB1886 uvrA6 pre-irradiated by gamma-rays in low doses (radioadaptation, the first stress effect) has been investigated. PRR was found to be more effective after incubation in the growth medium (for 45-60 min) than in non-radioadapted cells: the repair of postreplication gaps increased by 6-15%. If cells of WP2 uvrA strain were incubated after UV-irradiation in media lacking tryptophan or casamin acids (the second stress effect), PRR was seen to increase as early as within 15 min of incubation and it is more effective than at the first stress. After a 30-60 min incubation the double stress effect leads to an increase in postreplication gap repair by 23-45%. In this case almost all the gaps prove to be repaired. The second stress alone exerts no influence on PPR efficiency. It is supposed that a preliminary radioadaptation may stimulate synthesis of a protein (proteins) of the SOS-response (presumably DNA polymerase V). The second stress effect apparently induces synthesis of an unknown factor (or depreesses synthesis of a MmrA-like protein), and this in cooperation with a protein newly synthesized during radioadaptation significantly increases the efficiency of PPR.     

Mutagenicity of methyl-, ethyl-, propyl- and butylnitrosourea towards Escherichia coli WP2 strains with varying DNA repair capabilities.

Methyl- (MNUA), ethyl- (ENUA), propyl- (PNUA) and butylnitrosourea (BNUA) have been tested for toxicity and mutation in a liquid suspension assay towards Escherichia coli WP2 and some of its repair deficient derivatives. A comparison of survival rates after nitrosourea exposure between WP2 and WP2 uvrA showed no difference between the two strains but a consistent difference in potency between the various nitrosoureas studied. Toxicity increased in the order MNUA less than PNUA less than ENUA less than BNUA. ENUA and PNUA induced a greater number of trp+ revertants in both strains than did MNUA and BNUA, particularly at low survival rates. None of these differences in biological potency could be accounted for by differences in rates of hydrolysis. ENUA, PNUA and BNUA were non-mutagenic towards WP2 lexA, WP2 recA and WP2 uvrA lexA, whereas MNUA did induce mutations. Ethyl methanesulphonate (EMS) was able to mutate WP2 lexA. These results are discussed in the light of current theories regarding the mechanism of action of these compounds.     

Sodium arsenite inhibits spontaneous and induced mutations in Escherichia coli

Sodium arsenite at a non-toxic concentration was found to inhibit strongly mutagenesis induced by ultraviolet light (UV), 4-nitroquinoline-1-oxide (4NQO), furylfuramide (AF-2) and methyl methane-sulfonate (MMS) as well as spontaneous mutation in the reversion assay of E. coli WP2uvrA/pKM101. The effect was not, however, seen in the case of the mutagenesis induced by N-methyl-N'-nitro-N-nitrosoguanidine (MNNG). In order to elucidate the mechanism of the mutation-inhibitory effect of sodium arsenite, its action on umuC gene expression and DNA-repair systems was investigated. It was found that sodium arsenite depressed beta-galactosidase induction, corresponding to the umuC gene expression. For UV-irradiated E. coli strains possessing different DNA-repair capacities, sodium arsenite decreased the UV survival rates of WP2, WP2uvrA[uvrA] and WP67[uvrA polA], increased those of SOS-uninducible strains having either the recA+ or uvrA+ such as CM571 [recA], CM561 [lexA(Ind-)] and CM611[uvrA lexA (Ind-)], and did not affect that of the uvrA recA double mutant, WP100. From these results, we assume that sodium arsenite may have at least two roles in its antimutagenesis: as an inhibitor of umuC gene expression, and as an enhancer of the error-free repairs depending on the uvrA and recA genes.     

Antimutagenic and mutagenic activities of some terpenes in the bacterial reverse mutation assay

The mutagenic and antimutagenic effects of linalool, linalyl acetate and beta-caryophyllene were evaluated by the bacterial reverse mutation assay on Salmonella typhimurium TA 98 and TA 100, and on Escherichia coli WP2uvrA strains. Neither linalool nor beta-caryophyllene showed mutagenicity, but linalyl acetate induced a statistically significant increase in the number of revertant colonies in WP2uvrA, both with and without S9 mixture. Linalool was devoid of antimutagenic activity against 2-nitrofluorene (2NF), sodium azide (SA), methyl methane sulfonate (MMS) and 2-aminoanthracene (2AA). In contrast, beta-caryophyllene showed a strong antimutagenic activity against 2NF: at the maximum concentration tested (6.40mg/plate) the number of 2NF-induced revertant colonies was reduced by 83.9%. beta-Caryophyllene also showed to counteract the mutagenicity of SA (in TA 100), MMS and 2AA (in WP2uvrA): the effect was weak against SA (inhibition lower than 25%) and moderate against MMS and 2AA (up to 30.5%). The antimutagenic activity of beta-caryophyllene observed here suggests further studies to evaluate its possible chemopreventive properties.     

A 50 Hz, 14 mT magnetic field is not mutagenic or co-mutagenic in bacterial mutation assays

We used bacterial mutation assays to assess the mutagenic and co-mutagenic effects of power frequency magnetic fields (MF). For the former, we exposed four strains of Salmonella typhimurium (TA98, TA100, TA1535, TA1537) and two strains of Escherichia coli (WP2 uvrA, WP2 uvrA/pKM101) to 50Hz, 14mT circularly polarized MF for 48h. All results were negative. For the latter, we treated S. typhimurium (TA98, TA100) and E. coli (WP2 uvrA, WP2 uvrA/pKM101) cells with eight model mutagens (N-ethyl-N'-nitro-N-nitrosoguanidine, 2-(2-furyl)-3-(5-nitro-2-furyl) acrylamide, 4-nitroquinoline-N-oxide, 2-aminoanthracene, N(4)-aminocytidine, t-butyl hydroperoxide, cumen hydroperoxide, and acridine orange) with and without the MF. The MF induced no significant, reproducible enhancement of mutagenicity. We also investigated the effect of MF on mutagenicity and co-mutagenicity of fluorescent light (ca. 900lx for 30min) with and without acridine orange on the most sensitive tester strain, E. coli WP2 uvrA/pKM101. Again, we observed no significant difference between the mutation rates induced with and without MF. Thus, a 50Hz, 14mT circularly polarized MF had no detectable mutagenic or co-mutagenic potential in bacterial tester strains under our experimental conditions. Nevertheless, some evidence supporting a mutagenic effect for power frequency MFs does exist; we discuss the potential mechanisms of such an effect in light of the present study and studies done by others.     

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.     

The amyloid peptide induces early genotoxic damage in human preneuron NT2

The antimutagenic activities of benzalacetone (4-phenyl-3-buten-2-one) and its structurally-related compounds were evaluated through their use as post-treatments for the UV-induced mutagenesis in Escherichia coli WP2s (uvrA) and the gamma-induced mutagenesis in Salmonella typhimurium TA2638, the latter of which is sensitive to oxidants. Structure-activity relationships were studied between IC(50) activity values, i.e. the dose (micromol/ml) at which the mutation frequency is reduced to 50% of the control, and electronic and hydrophobicity properties of the studied molecules. Benzalacetone and benzalacetone analogs, cinnamaldehyde and trans-1,1,1-trifluoro-4-phenyl-3-buten-2-one (TF), inhibited both forms of mutagenesis, but methyl cinnamate, cinnamic acid and cinnamamide did not. The IC(50) values of TF, for UV-induced mutagenesis and gamma-induced mutagenesis, were 0.028 and 0.045 micromol/ml, respectively, and one order of magnitude lower than those of cinnamaldehyde and benzalacetone. The three antimutagenic analogs listed in order of decreasing activity are: TF>cinnamaldehyde>benzalacetone. This order is proportional to the electron-withdrawing property of the terminal group attached to an alpha,beta-unsaturated carbonyl moiety in the side chain that is known to play an important role in the antimutagenicities of benzalacetone and related compounds. In UV-induced mutagenesis in E. coli WP2s, mono-substituted benzalacetones - the ring-substituents of which have electron-withdrawing properties - showed antimutagenic activity that correlated with their electronic property. In gamma-induced mutagenesis in S. typhimurium TA2638, the antimutagenic activities of mono-substituted benzalacetones were proportional to the substituent hydrophobicities (pi). The different effects on both the mutation-induced systems is suggested to be related to the relative permeability of the cell membranes and the different sensitivities to mutagens between E. coli WP2s and S. typhimurium TA2638. In addition, the antimutagenic activity against gamma-induced mutagenesis could be due to the ability of parent compounds or their derivatives to scavenge long-lived organic radicals; the radicals have been described to be generated as a result of the X-irradiation of cells by Koyama et al. [Mutat. Res. 421 (1998) 45].     

Genetic and kinetic evidence for different types of postreplication repair in Escherichia coli B.

The changes in molecular weight of deoxyribonucleic acid (DNA) synthesized after ultraviolte irradiation of Escherichia coli WP28 uvrA, and strains additionally mutant at polA, exrA, recA, and exrA and polA loci, were examined by alkaline sucrose gradient centrifugation. In a repari=deficient uvrA recA strain, the frequency of breaks in newly synthesized DNA was equal to that for pyrimidine dimers in parental DNA. Measurements of the amounts and rates of postreplication repair of these breaks indicate that (i) repair is two to three times faster when DNA polymerase I is present, although (ii) almost all breaks are repaired regardless of DNA polymerase I activity. (iii) Increased ultraviolet doses lead to an increase in the proportion of breaks remaining unrepaired in uvrA recA, UVRA exrA, and uvrA exrA polA strains. The numbers of unrepaired breaks resemble the numbers expected if repair of one lesion is prevented by proximity of a second lesion.     

Mutagenicity studies on fenitrothion in bacteria and mammalian cells

The mutagenicity of fenitrothion was determined in strains of Salmonella typhimurium and Escherichia coli. Fenitrothion was found to be non-mutagenic in Salmonella typhimurium strains of TA98, TA1535 and TA1537 and in Escherichia coli WP2uvrA both with and without S9 mix, while weak mutagenicity was observed only in Salmonella typhimurium TA100 and enhanced by the addition of S9 mix. The mutagenicity observed in the TA100 strain was not expressed in a nitroreductase-deficient strain, TA100 NR, and decreased in a transacetylase-deficient strain, TA100 1,8-DNP6. The mutagenicity of fenitrothion was also examined by a gene mutation assay using the gene for hypoxanthine-guanine phosphoribosyltransferase (hgprt) in V79 Chinese hamster lung cells. Fenitrothion did not induce any increment of 6-thioguanine-resistant mutant cells at doses ranging from 0.01 to 0.3 mM regardless of the presence or absence of S9 mix. These results suggest that reduction of fenitrothion by a bacterial nitroreductase of TA100 to an active form is essential for the expression of the mutagenicity of fenitrothion in TA100 and that a bacterial transacetylase of TA100 also has an important role in the process of mutagenic activation.     

Ethidium bromide and SYBR Green I enhance the genotoxicity of UV-irradiation and chemical mutagens in E. coli

Ethidium bromide (EtBr) and SYBR Green I are nucleic acid gel stains and used commonly in combination with UV-illumination. EtBr preferentially induces frameshift mutations but only in the presence of an exogenous metabolic activation system, while SYBR Green I is a very weak mutagen that induces frameshift mutations. We found that EtBr and SYBR Green I, without an added metabolic activation system, strongly potentiated the base-substitution mutations induced by UV-irradiation in E. coli B/r WP2 cells. Each DNA stain alone showed no mutagenicity to the strain. Base-substitutions induced by 3-chloro-4-(dichloromethyl)-5-hydroxy-2(5H)-furanone (MX) and 4-nitroquinoline-1-oxide were similarly potentiated by EtBr and SYBR Green I. SYBR Green I had a much greater effect. No enhancing effects were observed on mutations induced by mitomycin C, cisplatin, transplatin, cumene hydroperoxide, base analogs, and alkylating agents. Another DNA stain, acridine orange, showed similar enhancing effects on UV- and MX-mutagenicity, but no effect was observed for 4',6-diamidino-2-phenylindole (DAPI). UV- and MX-induced mutations in E. coli WP2s (uvrA), which is defective in nucleotide excision repair activity, were not potentiated by the addition of EtBr, SYBR Green I, or acridine orange. Those nucleic acid stains might inhibit the nucleotide excision repair of DNA damaged by UV and MX treatment.     

Intermediate frequency magnetic fields do not have mutagenic, co-mutagenic or gene conversion potentials in microbial genotoxicity tests

We used bacterial mutation and yeast genotoxicity tests to evaluate the effects of intermediate frequency (IF; 2 kHz, 20 kHz and 60 kHz) magnetic fields (MFs) on mutagenicity, co-mutagenicity and gene conversion. We constructed a Helmholtz type exposure system that generated vertical and sinusoidal IF MFs, such as 0.91 mT at 2 kHz, 1.1 mT at 20 kHz and 0.11 mT at 60 kHz. Mutagenicity, co-mutagenicity and gene conversion assays were performed for each of the three MF exposure conditions. Mutagenicity testing was performed in four strains of Salmonella typhimurium (TA98, TA100, TA1535 and TA1537) and two strains of Escherichia coli (WP2 uvrA and WP2 uvrA/pKM101) to cover a wide spectrum of point mutations. For co-mutagenicity tests, we used four sensitive test strains (TA98, TA100, WP2 uvrA and WP2 uvrA/pKM101) with five chemical mutagens (t-butyl hydroperoxide (BH, a hydroxyl free radical precursor), 2-(2-furyl)-3-(5-nitro-2-furyl) acrylamide (AF2) and N-ethyl-N'-nitro-N-nitrosoguanidine (ENNG, DNA reactive reagents), benz[a]pyrene (BaP) and 2-aminoanthracene (2AA, DNA reactive promutagens). Gene conversion testing was performed in the yeast test strain, Saccharomyces cerevisiae XD83. We also examined the effects on the repair process of DNA damage by UV irradiation. No statistically significant effects were observed between exposed and control groups in any of the genotoxicity tests, indicating that the IF MFs (0.91 mT at 2 kHz, 1.1 mT at 20 kHz or 0.11 mT at 60 kHz) do not have mutagenic or co-mutagenic potentials for the chemical mutagens tested under these experimental conditions. Our findings also indicate that these IF MFs do not induce gene conversion or affect the repair process of DNA damage in eukaryotic cells.