Intragenic mitotic recombination induced by ultraviolet and gamma rays in radiosensitive mutants of Saccharomyces cerevisiae yeasts

The effect of UV- and gamma-irradiation on the survival and intragenic mitotic recombination (gene conversion) of 5 radiosensitive mutants was studied in comparison with the wild type. The level of spontaneous conversion was similar for RAD, rad2 and rad15, mutations xrs2 and xrs4 increasing and rad54 significantly decreasing it. The frequency of conversion induced by UV-light was greater in rad2, rad15 and xrs2 mutants and lower in xrs4, as compared to RAD. Gamma-irradiation caused induction of gene conversion with an equal frequency in RAD, rad2, rad15. Xrs2 and xrs4 mutations slightly decreased gamma-induced conversion. In rad54 mutant, UV-and gamma-induced conversion was practically absent. In the wild type yeast, a diploid strain is more resistant than a haploid, whereas in rad54 a diploid strain has the same or an increased sensitivity, as compared to a haploid strain (the "inverse ploidy effect"). This effect and also the block of induced mitotic recombination caused by rad54 indicate the presence in the yeast Saccharomyces cerevisiae of repair pathways of UV- and gamma-induced damages acting in diploid cells and realised by recombination. The data obtained as a result of many years' investigation of genetic effects in radiosensitive mutants of yeast are summarised and considered.     

Genetic effects of the decay of tritium incorporated into Saccharomyces cerevisiae yeast cells. I. Lethal effect of 3H-alanine on cells of different ploidy and radiosensitivity]

Lethal effect of 3H-alanine on cells of different ploidy (1n-4n) and radiosensitivity wild type haploid and diploid strains; xrs2 mutant and diploid strains homozygous for this mutation is studied. It is shown that the efficiency of the inactivation per tritium decay depends on ploidy, radiosensitivity and geometric dimension of cells and nuclei. Do for all the studied strains is slightly higher than Do for these strains in case of external beta-irradiation. RBE of beta-ray tritium with respect to beta-ray of 32P is 1.5.     

Reparation after the action of 8-methoxypsoralen and light (lambda=365 nm) on radiosensitive mutants of Saccharomyces cerevisiae yeasts]

The method of repeated irradiation allowed to study kinetics of excision of mono-adducts induced by 8-methoxypsoralen (8-MOP) plus light (lambda=365 nm) in DNA of UV-sensitive mutants rad4 and rad15 and X-ray sensitive mutants rad54, xrs2, xrs4. The survival of the mutant rad4 was not practically increased after incubation in complete liquid medium for 3 hours at 28 degrees C before the repeated irradiation. These data suggest that the mutant rad4 is characterized by nearly complete absence of the mono-adduct excision. The survival of mutants rad15 and rad54 in the same environment was increased less effectively than the survival of the control radioresistant strain, but the mutants xrs2 and xrs4 did not differ from the control strain. Possible causes of differences in survival between radiosensitive strains are discussed. The increased sensitivity of the excision defective strain (rad4) and of the postreplicative recombination defective strains (xrs2, xrs4, rad54) to the lethal effect of 8-MOP plus light (lambda=365 nm) suggests that two systems of reparation take part in the removal of photoproducts induced by 8-MOP in DNA of yeast cells.     

Genetic effects of the decay in Saccharomyces cerevisiae yeast cells of the radionuclide products of nuclear fuel fission. II. Lethal mutagenic effects and the nature of mutations induced by an equilibrium mixture of 90Sr-90Y and 89Sr

The lethal and mutagenic effects and the nature of mutations induced by 90Sr-90Y and 89Sr in cells of the yeast Saccharomyces cerevisiae were studied. The lethal efficiency was determined for 89Sr as (7,6 +/- 1,05) X 10(-5) decay-1, for 90Sr-90Y-(3,3 +/- 1,6) X 10(-4) decay-1. The mutagenic efficiency for ade1 and ade2 genes was determined for 89Sr as (8,3 +/- 2,5) X 10(-9) decay-1, for 90Sr-90Y-(2,9 +/- 1,5) X 10(-8) decay-1. For ade2 locus, the spectrum of mutations induced by 89Sr was a follows: one deletion, 17% of frameshifts and 83% of base pair substitutions--51% of transversions, 22% of GC-AT transitions and 10% of AT-GC transitions. The data of the present work suggest that 90Sr-90Y and 89Sr are very efficient physical mutagens. The relative mutagenic efficiency (RME) was estimated for radionuclides studied.     

Mutants of the yeast Saccharomyces cerevisiae characterized by enhanced induced mutagenesis. III. Effect of the him mutation on the effectiveness and specificity of UF-induced mutagenesis

We have studied the influence of him1-1, him2-1, him3-1 and himX mutations on induction frequency and specificity of UV-induced adenine-dependent mutations in the yeast Saccharomyces cerevisiae. Him mutations do not render haploid cells more sensitive to the lethal action of UV-light; however, in him strains adenine-dependent mutations (ade1, ade2) were induced more frequently (1.5--2-fold), as compared to the HIM strain. An analysis of the molecular nature of ade2 mutants revealed that him1-1, him2-1 and himX mutations increase specifically the yield of transitions (AT----GC and GC----AT), whereas in the him3-1 strain the yield of transversions was enhanced as well. We suggest him mutations analysed to affect specific repair pathway for mismatch correction.     

Effect of radiosensitivity gene irradiation and genotype on yeast cytoduction]

Effects of gamma- and UV-irradiation of one parent on progenies of yeast crosses are studied. In the crosses of the type: p+ a alc4 X p-alpha ade2 (alc4--nuclear respiration deficiency) zygotic colonies (white) and haploid p+ cytoductant colonies (red) were scored. The irradiation of p+ parent increases the cytoductant frequency up to 17%. When both parents are radiation-sensitive (xrs1, allelic to rad54), the frequency of cytoduction reaches 90%. Crosses p+xrs1 X p-xrs+ or p+xrs+ X p-xrs1 are similar to crosses of wild types. The same results were observed in the case of xrs4 mutants as well as for UV-irradiation. In the progenies of crosses irradiated p+ a alc4 (xrs+ or xrs1) X non-irradiated p-alpha ade2 ura (xrs+ or xrs1) the genotype of red colonies was analysed: in most cases (82.3%-99.8%) they were p+ alpha ade2 ura, that was true haploid cytoductants. It is concluded that irradiation induces particular damages of yeast nuclei leading to a block of normal karyogamy and thus to a cytoductant formation. The highest frequency of cytoduction was observed in crosses of radiosensitive mutants.     

Genetic effects of the decay of radionuclide products of nuclear fission in Saccharomyces cerevisiae cells. Lethal effects of the decay of 89Sr incorporated in cells of different ploidies and radiosensitivity

Decay of 89Sr incorporated in yeast cells produces a pronounced inactivating effect. The transmutation mainly contributes (about 80%) to cell inactivation. Haploid cells are more sensitive to 89Sr disintegration than diploid and tetraploid ones. A radiosensitive mutant XRS2, that is particularly sensitive to the transmutation effect of radionuclides, has proved to be sensitive to 89Sr transmutation as well. At the same time, another radiosensitive mutant, rad 54, does not virtually differ from the wild-type strain by its sensitivity to 89Sr decay.     

Biochemical and cytogenetical characterization of Chinese hamster ovary X-ray-sensitive mutant cells xrs 5 and xrs 6. VI. The correlation between UV-induced DNA lesions and chromosomal aberrations, and their modulations with inhibitors of DNA repair synthesis

The role of UV-induced DNA lesions and their repair in the formation of chromosomal aberrations in the xrs mutant cell lines xrs 5 and xrs 6 and their wild-type counterpart, CHO-K1 cells, were studied. The extent of induction of DNA single-strand breaks (SSBs) and DNA double-strand breaks (DSBs) due to UV irradiation in the presence or absence of 1-beta-D-arabinofuranosylcytosine (ara-C) and hydroxyurea (HU) was determined using the alkaline and neutral elution methods. Results of these experiments were compared with the frequencies of induced chromosomal aberrations in UV-irradiated G1 cells treated under similar conditions. Xrs 6 cells showed a defect in their ability to perform the incision step of nucleotide repair after UV irradiation. Accumulation of breaks 2 h after UV irradiation in xrs 6 cells in the presence of HU and ara-C remained at the level of incision breaks estimated after 20 min, which was about 35% of that found in wild-type CHO-K1 cells. In UV-irradiated CHO-K1 and xrs 5 cells, more incision breaks were present after 2 h compared with 20 min post-treatment with ara-C, a further increase was evident when HU was added to the combined treatment. The level of incision breaks induced under these conditions in xrs 5 was about 80% of that observed in CHO-K1 cells. UV irradiation itself did not induce any detectable DNA strand breaks. Accumulation of SSBs in UV-irradiated cells post-treated with ara-C and HU coincides with the increase in the frequency of chromosomal aberrations. These data suggest that accumulated SSBs when converted to DSBs in G1 give rise to chromosome-type aberrations, whereas strand breaks persisting until S-phase result in chromatid-type aberrations. Xrs 6 appeared to be the first ionizing-radiation-sensitive mutant with a partial defect in the incision step of DNA repair of UV-induced damage.     

Genetic effects of cosmic radiation in Drosophila melanogaster]

To examine possible effects of space radiation on living organism, we have analyzedtwo types of mutations, sex-linked recessive lethal mutations and somatic mutations, in fruit fly of the species Drosophila melanogaster. Drosophila strains used were wild type strains and a radiation-sensitive strain mei-41. Two different developmental stages of samples were sent into space; young adult males to analyze sex-linked recessive lethal mutations and about 30hr-old larvae to detect somatic mutations in wing epidermal cells. For wild type and mei-41 strains each, about 200 adult male flies and about 6,000 larvae were loaded on space shuttle Endeavour. The male flies returned from space were mated to virgin female flies of a tester strain, and the presence of the lethal mutations was analyzed at F2 generation. The frequencies of sex-linked recessive lethal mutations in flight groups were 2 and 3 times higher for wild type Canton-S and mei-4 1, respectively, than those in ground control groups. Most larvae sent to space emerged as adult flies within about 10 days after the landing. The presence of wing-hair somatic mutations, which give morphological change in hairs growing on the surface of wing epidermal cells, was analyzed under microscope. In wild type strain Muller-5, the frequency of wing hair mutant spots in flight group was about 1.5-fold higher than that in ground control, and in Canton-S-derived wild type strain the frequencies were similar between the two groups. By contrast, for mei-41 strain the mutation frequency was lower in flight group than in control group. The observed higher frequency of lethal mutations in the flight group might be due to a possibility that radiation effects on reproductive cells could be greatly enhanced under micro gravity. However, if this would be the case, we do not have appropriate explanation for the apparent absence of such synergistic effects on somatic wing-hair mutation system.     

Lethal effect of the decay of 64Cu and 67Cu incorporated in mammalian cells

A high lethal efficiency was observed when the decay of two radioactive isotopes of copper, 64Cu or 67Cu, occurred in mammalian cells. The lethal efficiency is of the same order for both isotopes in spite of their different decay processes. As the lethal event could be attributed to an injury inside the cellular DNA only, these results suggest that, even if present in the DNA in trace amounts solely, copper atoms are essential chromatin components. Their behaviour differs depending on whether the cell is a tumor or a non tumor cell because their lethal efficiencies are different (0.5 and 0.1 respectively).     

Repair of cisplatin-DNA adducts in mutants for genes controlling spontaneous and induced mutagenesis in Saccharomyces cerevisiae

Sensitivity to the lethal action of the anticancer substance cisplatin was studied in the yeast mutants himl, hsm2, hsm3, and hsm6, deficient for repair of spontaneous and induced mutations. The himl and hsm3 mutants were as resistant to the agent under study as the wild-type strain. The survival of the double mutant rad2 hsm3 was higher than that of the single mutant rad2. The hsm2 and hsm6 mutants were more cisplatin-sensitive than the wild type. Cisplatin was shown to have high mutagenic and recombinogenic effects on yeast cells.     

Efficient mutation induction by 125I and 131I decays in DNA of human cells

To examine the role of radiation energy deposition in DNA on cellular effects, we investigated the ability of 125IdUrd and 131IdUrd to kill cells and induce mutations at the hprt locus. We employed human lymphoblastoid cells proficient (TK6) or deficient (SE30) in the ability to incorporate a thymidine analog into DNA by way of the thymidine kinase (TK) scavenger pathway. Iodine-125 releases a shower of low-energy Auger electrons upon decay which deposit most of their energy within 20 nm of the decay site, whereas 131I is a high-energy beta/gamma emitter that is generally considered to emit sparsely ionizing radiation. Although 125IdUrd incorporated into cellular DNA was very effective at producing toxic and mutagenic effects in TK6 cells, virtually no effect was seen in TK-deficient cells incubated with similar levels of 125IdUrd in the extracellular medium. In response to 131IdUrd treatment, 0.45 X 10(-6) mutants were induced per centigray dose deposited within the nucleus in TK-proficient cells, whereas few mutations were induced in TK-deficient cells at doses up to 38 cGy from 131I decays occurring in the medium. The differences in biological response between TK6 and SE30 cells cannot be explained by differential radiosensitivity or IdUrd sensitization of the cell lines involved. We conclude that both 125I and 131I decays occurring while incorporated into DNA are more effective at inducing cell killing and mutations in human cells than either nonincorporated decays or low-LET radiations. These results suggest that localized energy deposition is an important factor in producing biologically important damage by both of these isotopes, and that residual lesions following the decay of DNA-incorporated radioisotopes may contribute to the toxic and mutagenic effects observed in TK-proficient cells. Furthermore, they emphasize that certain beta/gamma-emitting isotopes such as 131I may be particularly hazardous when incorporated into DNA.     

Induction of forward mutations in mutationally defective yeast

Summary The 3 rev loci that reduce ultraviolet light (UV)-induced reversion in S. cerevisiae had a similar effect on forward mutation to auxotrophy induced by a single 400 erg/mm² UV dose: rev1-1, rev2-1 and rev3-1 reduced average frequencies of auxotrophs to 4%, 64% and 4% that in wild type and reduced frequencies of mutants at ade1 or ade2 to 19%, 88% and 2% wild type, respectively. The rev2-1 strain exhibited high frequencies of spontaneous mutation. It is suggested that rev1-1 and rev3-1 block steps in a general UV mutation mechanism controlling forward and reverse mutation throughout the genome. The small effect of rev2-1, compared to the effect of rev1-1 or rev3-1, is consistent with previously obtained data on UV reversion and could be due to a specificity for induced mutation involving only certain types of UV damage or, on the other hand, it may be related to mutator activity. Although rev caused varying degrees of sensitivity to ethylmethanesulfonate (EMS), there was little or no significant effect on mutation induced by a single 30 min. dose of 3% EMS. Auxotroph frequencies were 79%, 109% and 94% wild type, whild frequencies at ade1 or ade2 were 82%, 56% and 51% wild type in the respective strains. It is suggested that steps blocked by rev, although they may participate in repair of lethal EMS damage, do not themselves generate EMS-induced mutations.     

Resolution of the fluorescence decay of the two tryptophan residues of lac repressor using single tryptophan mutants.

We have studied the time-resolved intrinsic tryptophan fluorescence of the lac repressor (a symmetric tetramer containing two tryptophan residues per monomer) and two single-tryptophan mutant repressors obtained by site-directed mutagenesis, lac W201Y and lac W220Y. These mutant repressor proteins have tyrosine substituted for tryptophan at positions 201 and 220, respectively, leaving a single tryptophan residue per monomeric subunit at position 220 for the W201Y mutant and at position 201 in the W220Y mutant. It was found that the two decay rates recovered from the analysis of the wild type data do not correspond to the rates recovered from the analysis of the decays of the mutant proteins. Each of these residues in the mutant repressors displays at least two decay rates. Global analysis of the multiwavelength data from all three proteins, however, yielded results consistent with the fluorescence decay of the wild type lac repressor corresponding simply to the weighted linear combination of the decays from the mutant proteins. The effect of ligation by the antagonistic ligands, inducer and operator DNA, was similar for all three proteins. The binding of the inducer sugar resulted in a quenching of the long-lived species, while binding by the operator decreased the lifetime of the short components. Investigation of the time-resolved anisotropy of the intrinsic tryptophan fluorescence in these three proteins revealed that the depolarization of fluorescence resulted from a fast motion and the global tumbling of the macromolecule. Results from the simultaneous global analysis of the frequency domain data sets from the three proteins revealed anisotropic rotations for the macromolecule, consistent with the known elongated shape of the repressor tetramer. In addition, it appears that the excited-state dipole of tryptophan 220 is alighed with the long axis of the repressor.     

Cytological characterization of Chinese hamster ovary X-ray-sensitive mutant cells xrs 5 and xrs 6. I. Induction of chromosomal aberrations by X-irradiation and its modulation with 3-aminobenzamide and caffeine

We have studied two X-ray-sensitive mutants xrs 5 and xrs 6 (derived from the CHO-K1 cell line), known to be defective in repair of double-strand breaks, for cell killing and frequency of the chromosomal aberrations induced by X-irradiation. The survival experiments showed that mutants are very sensitive to X-rays, the D0, for the wild-type CHO-K1 was 6-fold higher than D0 value for the mutants. The modal number of chromosomes (2 n = 23) and the frequency of spontaneously occurring chromosomal aberrations were similar in all 3 cell lines. X-Irradiation of synchronized mutant cells in G1-phase significantly induced both chromosome- and chromatid-type of aberrations. The frequency of aberrations in xrs mutants was 12-fold more than in the wild-type CHO-K1 cells. X-Irradiation of G2-phase cells also yielded higher frequency of aberrations in the mutants, namely 7-8-fold in xrs 5 and about 3.5-fold in xrs 6 compared to the wild-type CHO-K1 cells. There was a good correlation between relative inability to repair of DNA double-strand breaks and induction of aberrations. The effect of 3-aminobenzamide (3AB), an inhibitor of poly(ADP-ribose) synthetase on the frequency of X-ray-induced chromosomal aberrations in these 3 cell lines was also studied. 3AB potentiated the frequency of aberrations in G1 and G2 in all the cell types. In the mutants, 3AB had a potentiating effect on the frequency of X-ray-induced chromosomal aberrations only at low doses. X-Ray-induced G2 arrest and its release by caffeine was studied by cytofluorometric methods. The relative speed with which irradiated S-G2 cells progressed into mitosis in the presence of caffeine was CHO-K1 greater than xrs 5 greater than xrs 6. Caffeine could counteract G2 delay induced by X-rays in CHO-K1 and xrs 5 but not in xrs 6. Large differences in potentiation by caffeine were observed among these cells subjected to X-rays and caffeine post-treatment for different durations. These responses and possible reasons for the increased radiosensitivity of xrs mutants are discussed and compared to ataxia telangiectasia (A-T) cells and a radiosensitive mutant mouse lymphoma cell line.     

Mutagenesis on cloned yeast genes. The mutation of the yeast gene comprising the plasmid and chromosome

The cells of Saccharomyces cerevisiae were transformed by plasmid pYG-007 treated in vitro with o-methylhydroxylamine. The plasmid consists of a portion of the bacterial plasmid with genes of resistance to ampicillin, chloramphenicol and tetracycline, 2 mkm yeast DNA and yeast genes ADE2 and LEU2. The collection of mutants containing a mutant allele of ADE2 gene within the plasmid was obtained. Interallelic complementation and that induced by suppression were studied in these ade 2 mutants. It was shown that all these induced ade 2 mutations were base-pair substitutions. Using the mechanism of conversion we managed to transfer the plasmid ade 2 mutations into the chromosome. Three pairs of strains carrying similar mutation in plasmid and chromosome were created. Analysis of frequency of reversions induced by UV-light and hydroxylaminopurine in the mutant ade2 locus comprised in the plasmid and chromosome showed that the former induced reversions in plasmid alleles less effectively than the latter.     

Mutagenesis in cloned yeast genes. The effect of mutation rad2 on the frequency of gene mutation in plasmid and chromosome

The influence of rad2 mutation blocking incision of pyrimidine dimers on frequency of UV-light and 6-hydroxylaminopurine (6-GAP)-induced adenine-independent revertants was studied in the strains of Saccharomyces cerevisiae containing the same mutant allele of gene ADE2 in episomic plasmid and in chromosome. It was shown that the strains carrying the ade2 mutation in chromosome and in plasmid did not differ in sensitivity to lethal action of UV-light and 6-GAP. However, in the plasmid rad2 strain reversions were induced by UV-light more frequently (approximately 100 times), as compared to the chromosome strain. We observed no significant differences between reversion frequencies in plasmid and chromosome RAD strains. The tendency to enhanced 6-GAP-induced mutagenesis, less sharply expressed, was observed in the chromosome rad2 strain, as compared to the plasmid one. However, the plasmid RAD strain was characteristic of higher reversion frequency induced by 6-GAP, as compared to the chromosome strain. The possible mechanisms of these phenomena are discussed.     

Biochemical and cytogenetical characterization of Chinese hamster ovary X-ray-sensitive mutant cells xrs 5 and xrs 6. V. The correlation of DNA strand breaks and base damage to chromosomal aberrations and sister-chromatid exchanges induced by X-irradiation

The X-ray-sensitive Chinese hamster ovary (CHO) mutant cell lines xrs 5 and xrs 6 were used to study the relation between X-ray-induced DNA lesions and biological effects. The frequencies of chromosomal aberrations and sister-chromatid exchanges (SCE) were determined in wild-type CHO-K1 as well as mutants xrs 5 and xrs 6 cells following X-irradiation under aerobic and anaerobic conditions. Furthermore, we used a newly developed immunochemical method (based on the binding of a monoclonal antibody to single-stranded DNA) to assay DNA single-strand breaks (SSBs) induced by gamma-rays in these CHO cells, after a repair time of up to 4 h. For all cell lines tested the frequency of X-ray-induced chromosomal aberrations was strongly increased after irradiation in air compared with hypoxic conditions. When compared to the wild-type line, the xrs mutants known to have a defect in repair of DNA double-strand breaks (DSBs) exhibited a markedly enhanced sensitivity to aerobic irradiation, and a high OER (oxygen enhancement ratio) of 2.8-3.5, compared with 1.8-2 in CHO-K1 cells. The induction of SCE by X-rays was relatively little affected in CHO-K1 irradiated in air compared with hypoxic conditions (OER = 0.8), and in xrs 5 (OER = 0.7). A dose-dependent increase in the frequency of SCEs was obtained in xrs 6 cells treated with X-rays in air, and a further increase by a factor of 2 was evident under hypoxic conditions (OER = 0.4). With the immunochemical assay of SSB following gamma-irradiation, no difference was found between wild-type and mutant strains in the number of SSBs induced. The observed rate of rejoining of SSBs was also the same for all cell lines studied.     

Cytogenetical characterization of Chinese hamster ovary X-ray-sensitive mutant cells, xrs 5 and xrs 6. IV. Study of chromosomal aberrations and sister-chromatid exchanges by restriction endonucleases and inhibitors of DNA topoisomerase II

Induction of chromosomal aberrations and sister-chromatid exchanges (SCEs) was studied in wild-type Chinese hamster ovary (CHO-K1) cells and its 2 X-ray-sensitive mutants xrs 5 and xrs 6 (known to be deficient in repair of DNA double-strand breaks (DSBs] by restriction endonucleases (REs) and inhibitors of DNA topoisomerase II known to induce DNA strand breaks. Five different types of REs, namely CfoI, EcoRI, HpaII (which induce cohesive DSBs), HaeIII and AluI (which induce blunt DSBs) were employed. REs that induce blunt-end DNA DSBs were found to be more efficient in inducing chromosomal aberrations than those inducing cohesive breaks. xrs 5 and xrs 6 mutants responded with higher sensitivity (50-100% increase in the frequency of aberrations per aberrant cell) to these REs than wild-type CHO-K1 cells. All these REs were also tested for their ability to induce SCEs. The frequency of SCEs increased in wild-type as well as mutant CHO cells, the induced frequency being about 2-fold higher in xrs mutants than in the wild-type cells. We also studied the effect of inhibitors of DNA topoisomerase II, namely 4'-(9-acridinylamino)methanesulfon-m-anisidide (m-AMSA) and etoposid (VP 16), at different stages of the cell cycle of these 3 types of cells. Both drugs increased the frequency of chromosomal aberrations in G2 cells. The mutants showed increased sensitivity to m-AMSA and VP 16, xrs 6 cells being 10- and 2-fold more sensitive than wild-type CHO-K1 cells respectively, and xrs 5 responding with 2-fold higher sensitivity than xrs 6 cells. G1 treatment of CHO cells with m-AMSA increased both chromosome- and chromatid-type aberrations, xrs mutants being about 3-fold more sensitive than CHO-K1 cells. The frequency of SCEs increased also after treatment of exponentially growing and S-phase CHO cells with m-AMSA and the higher sensitivity of xrs mutants (2-fold) was evident. The S-phase appeared to be a specific stage which is most prone for the induction of SCEs by m-AMSA. The results indicate that DNA DSBs induced by REs and inhibitors of DNA topoisomerase II correlate closely with induced chromosomal aberrations and SCEs in these cell lines, indicating that DSBs are responsible for the production of these 2 genetic endpoints.     

Mutations induced in Drosophila during space flight.

To examine the possible effects of space radiation on living organisms, fruit flies Drosophila melanogaster were loaded on the US Space Shuttle Endeavour, and after the flight we have analyzed two types of mutations, sex-linked recessive lethal mutations induced in male reproductive cells and somatic mutations which give rise to morphological changes in hairs growing on the surface of wing epidermal cells. Wild type strains and a radiation-sensitive strain mei-41 were used. The frequencies of sex-linked recessive lethal mutations in flight groups were 2 and 3 times higher for wild type Canton-S and mei-41 strains, respectively, than those in ground control groups. By contrast, the frequencies of wing-hair somatic mutations differed little between flight and control groups. The possibility that the space environment causes mutations in certain types of cells such as male reproductive cells, is discussed.