DNA repair and the genetic control of radiosensitivity in yeast

The effects of three rad genes on UV induced mutation rates in haploid and diploid Saccharomyces cells

Effects of the rad 2-20, rad 9-4, r1s, and the corresponding wild type RAD alleles in haploid and homozygous diploid Saccharomyces strains on UV induced mutation rates from adenine, lysine and histidine dependence to independence are reported. The UV induced mutation rates were similar for the RAD, r1s, and rad 9-4 haploids, whereas the rad 2-20 mutation causes a marked increase in the UV induced mutation rates. The diploid rad 2-20 strain also exhibits a marked increase in the UV induced mutation rates, whereas the rad 9-4 diploid has reduced mutation rates when compared to the wildtype. The UV induced mutation rates of haploid and diploid RAD strains are almost identical. For the rad 2-20 and rad 9-4 diploids, however, these rates are smaller than in the corresponding haploid strains. Differential effects of the rad genes on the ratio of locus to suppressor mutations were found. The implications of these findings on possible repair processes in yeasts are discussed.

Cellular radiation effects and hyperthermia cell cycle kinetics of radiation sensitive mutants of Saccharomyces cerevisiae after X-irradiation and hyperthermia

Radiosensitive mutants rad2, rad9, and rad51 of Saccharomyces cerevisiae were X-irradiated with 120 Gy or 60 Gy, heated at 50 degrees C for 30 min or treated with a combination of both and incubated in nutrient medium at 30 degrees C. Cell number, percentage of budding cells, and cell cycle progression were determined in 45-min intervals. Cell cycle kinetics were investigated by flow cytofluorometry. Hyperthermia leads mainly to a lengthening of G1, whereas X-rays arrest cells of the rad2 and rad9 mutant in G2 and the rad51--mutant additionaly in a state with DNA contents above G2. Cell division delay is influenced by oxygen in all strains but to a lesser extent in the rad2 mutant. The effect of the combined treatment appears to be merely additive in the rad2 and rad9 mutant while the rad51 mutant is sensitized to X-irradiation by hyperthermia. No selective action of hyperthermia on hypoxic cells was found.

Radioprotecting action of chemical compounds on gamma-irradiated yeast cells of various genotypes

The radioprotective efficiency of cysteamine and cysteine has been studied on haploid and diploid, Saccharomyces cerevisiae, wild-type and various X-ray repair deficient rad mutants. The correlation between the radioprotecting action of cysteamine and cell repair capacity was demonstrated for diploid yeasts; such a correlation was not expressed for wild-type and rad mutant haploid yeast cells. It was concluded that the radioprotective action may involve cellular recovery processes, which may be mediated by a recombination-like mechanism, for which the diploid state is required. Liquid holding recovery was shown not to participate in radioprotection, judged by the absence of the influence of cysteine on the delay of the first postradiation budding as well as by the additive action of cysteine and liquid holding recovery.

Studies on possible genetic effects of microwaves in procaryotic and eucaryotic cells

The biological effects of microwaves in the hyperfrequency range, 9,4 GHz, and 70-75 GHz were investigated in bacteria and yeast. At power densities below 60 mW/cm2 and SAR values not exceeding 28 mW/g no significant effects on survival of repair competent an deficient strains were observed in Escherichia coli and Saccharomyces cerevisiae. In addition, microwaves of 17 GHz did not induce mutations in E. coli B/r WP2 trp- uvr- above the spontaneous level, and the induction of nuclear reversions, cytoplasmic "petite" mutations and mitotic recombination as well as the efficiency of sporulation was not affected in yeast.

Genetic and physiological factors affecting repair and mutagenesis in yeast

Current views of DNA repair and mutagenesis in the yeast Saccharomyces cerevisiae are discussed in the light of recent data and with emphasis on the isolation and characterization of genetically well-defined mutations that affect DNA metabolism in general (including replication and recombination). Various "pathways" of repair are described, particularly in relation to their involvement in mutagenic mechanisms. In addition to genetic control, certain physiological factors such as "cell age," DNA replication, and the regulatory state of the mating-type locus are shown to also play a role in repair and mutagenesis.

Ultraviolet light induction of diphtheria toxin-resistant mutations in normal and DNA repair-deficient human and Chinese hamster fibroblasts

The role of unrepaired DNA lesions in the production of mutations is suspected of contributing to the initiation phase of carcinogenesis. Since the molecular basis of mutagenesis is not understood in eukaryotic cells, development of new genetic markers for quantitative in vitro measurement of mutations for mammalian cells is needed. Furthermore, mammalian cells, genetically deficient for various DNA repair enzymes, will be needed to study the role of unrepaired DNA lesions in mutagenesis. The results in this report relate to preliminary attempts (1) to characterize the diphtheria toxin resistance marker as a useful quantitative genetic marker in human cells and (2) to isolate and characterize various DNA repair-deficient Chinese hamster cells.

Ozone response in wild type and radiation-sensitive mutants of Saccharomyces cerevisiae

The effect of ozone exposure on Saccharomyces cerevisiae was studied. Factors such as ozone concentration, treatment time, media, initial cell concentration and growth phase were shown to influence ozone response in this organism. Logarithmic phase cells were much more sensitive than stationary phase cells to the lethal effect of ozone. The radiation-sensitive mutants rad3, rad6, rad51 and rad52 of S. cerevisiae were exposed, in water, to 50 ppm of ozone for 30 min. On comparing their survival curves, the rad51 and the rad52 mutants showed a greater sensitivity to ozone exposure than the wild type.

Abolition of the cyclic variations in radiosensitivity during meiosis in a sporulation mutant blocked in premeiotic DNA synthesis

The response to ultraviolet light (254 nm) of two sporulation mutants during the meiotic process was compared to that of a wild type diploid strain of Saccharomyces cerevisiae. The cyclic pattern for cell killing and rho- induction characteristic of diploid wild type cells persists in a strain able to perform the premeiotic DNA synthesis but which is blocked in the further steps of meiosis (spo8 DMS1). On the contrary, these fluctations are abolished in a derived mutant (spo8 dsm1) which is blocked in the premeiotic DNA synthesis. Under these conditions, the response to cell killing can be dissociated from the observed for rho- induction.

Isolation and characterization of yeast mutants auxotrophic for 2'-deoxythymidine 5'-monophosphate

Mutant strains of Saccharomyces cerevisiae auxotrophic for deoxythymidine monophosphate (dTMP) were isolated and characterized. Two distinct classes of auxotrophs were obtained. One class had a simple requirement for dTMP and was analogous to thymine-requiring bacteria. The second class required dTMP, adenine, histidine and methionine and this complex nutritional phenotype was due to defects in folate metabolism. The dTMP-dependent growth of respiratory-competent grande auxotrophs was found to be markedly affected by media composition and carbon source. In the absence of dTMP thymineless death occurred in both mutant classes.

Interactions between radiation-sensitive mutations in double-mutant haploids of Dictyostelium discoideum

Haploid strains of Dictyostelium discoideum carrying radiation-sensitive mutations in both the radA and radC genes are more sensitive to UV light irradiation than the "additive" sensitivity of the single-mutant haploids. This synergistic interaction indicates that the radA and radC gene products are involved in two different pathways of repair following UV-induced DNA damage. Double-mutant haploids bearing mutations in both the radA and radB genes have the same sensitivity as the radB single mutant indicating that the radA and radB gene products are involved in the same repair pathway.

Isolation of UV-sensitive clones from a haploid frog cell line

An isolation procedure has been developed which yielded five clones of haploid frog cells which are sensitive to ultraviolet light. This procedure employed a conventional mutagenesis, followed by time for phenotypic expression and then an enrichment for UV-sensitive mutants. The enrichment relies upon the uptake of bromodeoxyuridine (BrdU) by repairing cells following UV-induced damage, rendering repair-proficient cells differentially sensitive to photolysis by black light. The photolysis is potentiated by use of the bisbenzimidazole dye Hoechst 33258. The enriched population was screened for radiation-sensitive isolates resulting in 5 sensitives from 96 tested. No mutants were obtained from 300 isolates tested from a population which had not undergone enrichment.

Comparison of sensitivity and liquid holding recovery in rad mutants of Saccharomyces cerevisiae inactivated by UV and DEB

Twenty one UV-sensitive rad mutants were tested for their sensitivity towards DEB. All mutants were more sensitive to this treatment than the wild type. Seven mutants were classified as supersensitive to DEB (rad 1-1, 2,3, 6, 15 and 18-2), while only rad2 and rad3 can be classified as supersensitive to UV. For all mutants ability for liquid holding recovery (LHR) after UV and DEB was compared. Mutants rad 1-1, 3, 5, 6, 9 and 11 differ in their response to LH after the two treatments. Survival of rad1-1 and rad3 increases signficantly during LH after DEB but not after UV exposure. In contrast rad5, 6, 11, and 22 show marked LHR after UV but no increase of survival after DEB treatment.

Caffeine enhancement of radiation killing in different strains of Saccharomyces cerevisiae

Haploid and diploid wild type strains, and three classes of radiation-sensitive mutants of Saccharomyces cerevisiae were tested for enhancement of UV-inactivation by caffeine in growth medium. In addition, the sensitizing effect of caffeine was studied in a haploid and a diploid wild type strain after gamma-irradiation. The drug sensitized the UV-irradiated cells of all strains except those reported to be only slightly UV-sensitive but highly sensitive to ionizing radiation. After gamma-irradiation, no caffeine-enhancement of killing was observed in stationary phase cells of either the haploid or the diploid strain. However, log-phase cells of both strains were partially sensitized. The results of both sets of experiments suggested that caffeine interferes with a recombinational repair occurring in cells in S or G2 phase.

Spontaneous mutation by mutagenic repair of spontaneous lesions in DNA

Strains of yeast carrying mutations in many of the steps in pathways repairing radiation-induced damage to DNA have enhanced spontaneous mutation rates. Most strains isolated because they have enhanced spontaneous mutation carry mutations in DNA repair systems. This suggests that much spontaneous mutation arises by mutagenic repair of spontaneous lesions.