Inhibition of UV-induced G1 arrest by exposure to 50 Hz magnetic fields in repair-proficient and -deficient yeast strains.

PURPOSE: To assess the possibility that extremely low frequency (ELF) magnetic fields obstruct the damage repair process, the gene conversion frequency and cell cycle kinetics in a DNA repair-proficient and nucleotide excision repair (NER)-deficient strain of diploid yeast Saccharomyces cerevisiae. MATERIALS AND METHODS: DNA repair- or NER-deficient cells were irradiated with sublethal doses of ultraviolet light (UV) radiation followed by exposure to 50 Hz magnetic fields up to 30 mT for 48 h. After exposure, colony-forming ability was scored as revertants in which gene conversion had restored the functional allele of the ARG4 gene conversion hotspot. Cell cycle analysis was performed using flow cytometry. RESULTS: Gene conversion rate was increased by the combined exposure in DNA repair-proficient cells, whereas it remained unchanged between UV alone and the combined exposure in NER-deficient cells. The UV-induced G1 arrest was inhibited by exposure to 30 mT ELF magnetic fields in both repair-proficient and -deficient cells. CONCLUSIONS: The results suggest that exposure to high-density (30 mT) ELF magnetic fields decreases the efficiency of NER by suppressing G1 arrest, which in turn led to enhancement of the UV-induced gene conversion.

Induction of micronuclei in mice exposed to static magnetic fields.

The aim of this experiment was to investigate whether static magnetic fields (SMFs) have cytogenetic effects in mouse bone marrow cells. The frequency of micronuclei was significantly increased by exposure of mice to 3.0 T for 48 and 72 h and 4.7 T for 24, 48 and 72 h. The increase in micronucleus frequency was dose dependent at all times. Micronucleus frequency at 4.7 T was higher than at 3.0 T. We consider that the increased numbers of micronuclei may be attributable to a stress reaction caused by SMFs or a direct clastogenic/spindle disturbance effect of SMFs.

Involvement of eddy currents in the mutagenicity of ELF magnetic fields.

Possible carcinogenic and/or mutagenic activity of extremely low frequency magnetic fields was examined using somatic mutation and recombination test system of Drosophila melanogaster. An X-linked semi-dominant DNA repair defective mutation mei-41(D5) was introduced into the conventional mwh/flr test system to enhance mutant spot frequency. Virgin females of w mei-41(D5)/FM6; flr/TM6 were crossed with w mei-41(D5)/Y; mwh jv; spa(pol) males. The F(1) third instar larvae were exposed to a 50Hz, 20mT sinusoidal AC magnetic field for 24h. After moulting from pupal cases, their wings were examined under a bright field microscope to detect hair spots with mwh or flr mutant morphology. The exposure caused a statistically significant enhancement in somatic recombination spot frequency. Mutant spots arising due to chromosomal non-disjunction or terminal deletion also increased but the frequency of spots resulting from point mutation was not altered. The enhancement in the recombination spot frequency was suppressed to the control level when a culture medium without electrolytes was used during exposure. When larvae were exposed to a magnetic field in an annular dish, flies from the outer ring showed more mutant spots compared to those from the inner ring. These results suggest that the detected mutagenic activity was that of the induced eddy current, rather than that of the magnetic field itself.

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.

Mutagenicity and co-mutagenicity of static magnetic fields detected by bacterial mutation assay.

Possible mutagenic and co-mutagenic effects of strong static magnetic fields were estimated using bacterial mutagenicity test. Mutagenic potential of static magnetic fields up to 5T (T:1T=10,000 G) was not detected by the bacterial mutagenicity test using four strains of Salmonella typhimurium (TA98, TA100, TA1535 and TA1537) and Escherichia coli WP2 uvrA either in the pre-incubation method or in the plate incorporation method. In the co-mutagenicity test, E. coli WP2 uvrA cells were treated with various chemical mutagens and were simultaneously exposed to a 2T or a 5T static magnetic field. Mutation rate in the exposed group was significantly higher than that in the non-exposed group when cells were treated with N-ethyl-N'-nitro-N-nitrosoguanidine (ENNG), N-methyl-N'-nitro-N-nitrosoguanidine (MNNG), ethylmethanesulfonate (EMS), 4-nitroquinoline-N-oxide (4-NQO), 2-amino-3-methyl-3H-imidazo[4,5-f]quinoline (IQ) or 2-(2-furyl)-3-(5-nitro-2-furyl) acrylamide (AF-2). The mutagenicity of 2-aminoanthracene (2-AA), 9-aminoacridine (9-AA), N4-aminocytidine and 2-acetoamidofluorene (2-AAF) was not affected by the magnetic field exposure. Possible mechanisms of the co-mutagenicity of magnetic fields are discussed.

Increase in the mitotic recombination frequency in Drosophila melanogaster by magnetic field exposure and its suppression by vitamin E supplement.

In order to estimate possible mutagenic and/or carcinogenic activity of electromagnetic fields, wing spot tests were performed in Drosophila melanogaster. A DNA repair defective mutation mei-41D5 was introduced into the conventional mwh/flr test system to enhance mutant spot frequency. Third instar larvae were exposed to a 5-Tesla static magnetic field for 24 h, and after molting, wings were examined under a microscope to detect hair spots with mutant morphology. The exposure caused a statistically significant enhancement of somatic recombination compared with the unexposed control. This enhancement was suppressed to the control level by supplement of vitamin E, a non-specific antioxidant. It is inferred that the magnetic field enhanced the genotoxic effect of spontaneously produced free radicals, possibly by affecting the lifetime of the radicals. Enhancement of non-disjunction, terminal deletions and gene mutations were not detected.

Pseudomonas fluorescens KKL101, a benzoic acid degrader in a mixed culture that degrades biphenyl and polychlorinated biphenyls.

A mixed culture we had isolated, which degrades biphenyl/polychlorinated biphenyls, is composed of two strains, Pseudomonas fluorescens KKL101 and Pseudomonas sp. strain KKS102. KKS102 produces benzoic acid as a dead-end metabolite in the degradation of biphenyl. In this study we showed that KKL101 grew on benzoic acid as a sole source of carbon. This indicated a role of KKS102 in the growth of KKL101 and KKL101 for the growth of KKS102 in the mixed culture.

Cloning and sequencing of two tandem genes involved in degradation of 2,3-dihydroxybiphenyl to benzoic acid in the polychlorinated biphenyl-degrading soil bacterium Pseudomonas sp. strain KKS102.

Two genes involved in the degradation of biphenyl were isolated from a gene library of a polychlorinated biphenyl-degrading soil bacterium, Pseudomonas sp. strain KKS102, by using a broad-host-range cosmid vector, pKS13. When a 3.2-kilobase (kb) PstI fragment of a 29-kb cosmid DNA insert was subcloned into pUC18 at the PstI site downstream of the lacZ promoter, Escherichia coli cells carrying this recombinant plasmid expressed 2,3-dihydroxybiphenyl dioxygenase activity. Nucleotide sequencing of the 3.2-kb PstI fragment revealed that there were two open reading frames (ORFI [882 base pairs] and ORFII [834 base pairs], in this gene order). Results of analysis of Tn5 insertion mutants and unidirectional deletion mutants suggested that the ORFI coded for 2,3-dihydroxybiphenyl dioxygenase. When the sequence of ORFI was compared with that of bphC of Pseudomonas pseudoalcaligenes KF707 (K. Furukawa, N. Arima, and T. Miyazaki, J. Bacteriol. 169:427-429, 1987), the homology was 68%, with both strains having the same Shine-Dalgarno sequence. The result of gas chromatography-mass spectrometry analysis of the metabolic product suggested that the ORFII had meta cleavage compound hydrolase activity to produce benzoic acid. DNA sequencing suggested that these two genes were contained in one operon.

Effects of the Sex Ratio Organism on IN VITRO Differentiation of Drosophila Embryonic Cells.

Effects of Sex Ratio Organism (SRO) on the development of embryonic cells of Drosophila melanogaster were studied in vitro. The single embryo culture technique and a histochemical staining method were employed to distinguish male embryo cultures from female ones. SRO-infected Zw(n) females were crossed to wild-type males, and their embryos were separately dissociated and cultured in vitro. Cell growth and differentiation in each culture were examined by phase contrast microscopy and described both qualitatively and quantitatively. Neurons, imaginal disk cells and plasmatocyte-like cells barely differentiated in male (Zw( n)/Y) cultures, whereas muscle and fatbody cells were not so severely affected. These symptoms were generally specific to male cultures. Female (Zw(n)/+) cultures were relatively normal. These observations are consistent with the results of mosaic analysis. The susceptibility of cells to SRO is discussed in relation to the dose of X chromosome in the cells.

Isolation and characterization of flightless mutants in Drosophila melanogaster.

Since animal behaviour is executed through neuronal circuits including sensory receptors and muscle, genes vital for their development and differentiation must be found among mutants having behavioural anomaly. After mutagenesis with ethyl methanesulphonate (EMS), we screened for X-linked flightless mutants of Drosophila melanogaster by using column-type flight tester. Approximately 10(4) individuals were screened and 21 mutant genes were isolated. Chromosomal mapping and complementation experiments revealed that they belong to 15 cistrons randomly located on X chromosome, three cistrons having more than two alleles. Two of the isolated mutants (fltO2 and fltH, which are recessive both behaviourally and morphologically) were analysed with the mosaic fate mapping technique, and both were found to have their primary foci in mesodermal region of blastoderm, suggesting that the genes exert their primary effect in indirect flight muscle. Electronmicroscopic studies on the muscles from four alleles of the fltO2 cistron revealed an abnormality in myofibrillar arrangement. A possible deficit within Z-band components is discussed in relation to wings-up B mutants. The indirect flight muscle of flltH was also examined, and it was found that sarcomere length and diameter of myofibrils were abnormal. It was postulated that a possible factor which controls size of myofibrils is defective in this mutant. These examples indicate the advantage of combining ultrastructural examination with genetic mosaic mapping technique.