Four radiation sensitive mutants of Saccharomyces. Survival after UV- and x-ray-irradiation as well as UV-induced reversion rates from isoleucine-valine dependence to independence

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.

The effect of three rad genes on survival, inter- and intragenic mitotic recombination in Saccharomyces. I. UV irradiation without photoreactivation or liquid-holding post-treatment

The effect of UV irradiation on the survival, inter- and intragenic mitotic recombination of 3 diploid UV sensitive Saccharomyces mutants was studied and compared with the wild type RAD. These strains, homozygous for either the RAD, r1s rad 9-4, or rad 2-20 gene, have DRF values for survival of 1:1.6:3:20.6 respectively, at LD1. Their recombination behaviour is not correlated to their survival characteristics. The RAD, r1s, and rad 2-20 strains showed UV induced mitotic inter- and intragenic recombinants; the induction in the r1s diploid is ca. 100 times greater for both the inter- and intragenic recombinants than in the RAD strain. The rad 9-4 diploid produced no UV induced mitotic recombinants whatsoever, and is therefore considered to be a rec- mutation.

Antimutagenicity in yeast

In recent years there has been increasing interest in antimutagenesis, and studies have been done using both prokaryotic and eukaryotic systems. In eukaryotic systems the first studies were performed with different strains of Schizosaccharomyces pombe. In particular, caffeine and L-methionine were investigated. Different strains of Saccharomyces cerevisiae were employed in studies of a wide variety of compounds, including acridine, saccharin, salts, tumor promoters and co-carcinogens. Strain D7 was widely employed and antimutagenic activity of spermine, chlorophyllin, cobaltous chloride and fermented milk is reported.

The molecular genetics of the incision step in the DNA excision repair process

This review describes the evolution of research into the genetic basis of how different organisms use the process of excision repair to recognize and remove lesions from their cellular DNA. One particular aspect of excision repair, DNA incision, and how it is controlled at the genetic level in bacteriophage, bacteria, S. cerevisae, D. melanogaster, rodent cells and humans is examined. In phage T4, DNA is incised by a DNA glycosylase-AP endonuclease that is coded for by the denV gene. In E. coli, the products of three genes, uvrA, uvrB and uvrC, are required to form the UVRABC excinuclease that cleaves DNA and releases a fragment 12-13 nucleotides long containing the site of damage. In S. cerevisiae, genes complementing five mutants of the RAD3 epistasis group, rad1, rad2, rad3, rad4 and rad10 have been cloned and analyzed. Rodent cells sensitive to a variety of mutagenic agents and deficient in excision repair are being used in molecular studies to identify and clone human repair genes (e.g. ERCC1) capable of complementing mammalian repair defects. Most studies of the human system, however, have been done with cells isolated from patients suffering from the repair defective, cancer-prone disorder, xeroderma pigmentosum, and these cells are now beginning to be characterized at the molecular level. Studies such as these that provide a greater understanding of the genetic basis of DNA repair should also offer new insights into other cellular processes, including genetic recombination, differentiation, mutagenesis, carcinogenesis and aging.

Spontaneous mutagenesis: the roles of DNA repair, replication, and recombination

There appears to be no dearth of mechanisms to explain spontaneous mutagenesis. In the case of base substitutions, data for bacteriophage T4 and especially for E. coli and S. cerevisiae suggest important roles in spontaneous mutagenesis for the error-prone repair of DNA damage (to produce mutations) and for error-free repair of DNA damage (to avoid mutagenesis). Data from the very limited number of studies on the subject suggest that about 50% of the spontaneous base substitutions in E. coli, and perhaps 90% in S. cerevisiae are due to error-prone DNA repair. On the other hand, spontaneous frameshifts and deletions seem to result from mechanisms involving recombination and replication. Spontaneous insertions have been shown to be important in the strongly polar inactivation of certain loci, but it is less important at other loci. Perhaps with continued study, the term "spontaneous mutagenesis" will be replaced by more specific terms such as 5-methylcytosine deamination mutagenesis, fatty acid oxidation mutagenesis, phenylalanine mutagenesis, and imprecise-recombination mutagenesis. While most studies have concentrated on mutator mutations, the most conclusive data for the actual source of spontaneous mutations have come from the study of antimutator mutations. Further study in this area, perhaps along with an understanding of chemical antimutagens, should be invaluable in clarifying the bases of spontaneous mutagenesis.

Genetic control of budding-cell resistance in the diploid yeast Saccharomyces cerevisiae exposed to gamma-radiation

The gamma-radiation response of stationary and budding cells of wild-type diploid strains (RAD) and radiation-sensitive strains rad2, 6, 9, 18, 50-55, 57 and rec4 was studied. As compared with the wild-type strains, mutants generally showed enhanced sensitivity in both stages of the cell cycle. Budding-cell resistance was totally absent from rad50-55 strains. Mutants rad6, 9 and 18 showed some degree of budding-cell resistance. The response of rad2 and rec4 strains was identical with that of the corresponding wild-type strains. These results suggest that the pathway dependent upon the expression of RAD50-55 loci functions more efficiently in budding cells compared with the pathway dependent on RAD2 and RAD6, 9 and 18 loci. Recombination between sister chromatids appears to play an important role in budding-cell resistance, and this process is under the control of the RAD52 repair pathway. The relationship between the repair pathways associated with budding-cell resistance and post-irradiation cellular recovery (LHR) is discussed.

Mutagenesis induced by mono- and bi-functional alkylating agents in yeast mutants sensitive to photo-addition of furocoumarins (pso)

The inactivation and the induction of forward and reverse mutations by a mono- and a bifunctional nitrogen mustard in 3 pso mutants of Saccharomyces cerevisiae, initially selected for their sensitivity to psoralen photo-addition, were compared with that of the wild-type. The pso1-1 mutant was very sensitive to both alkylating agents, and the mutagenicity was abolished. This correlates with the defect in the error-prone repair capacity for lesions induced by psoralen photo-addition and radiations already observed for this mutant. Therefore it appears that the PSO1+ gene product acts on a spectrum of DNA lesions. The pso2-1 mutant was highly sensitive to the lethal effect of the bifunctional nitrogen mustard and was only slightly sensitive to the monofunctional one. For both agents a reduction in induced mutagenesis was seen. The same was true for mono- and bifunctional psoralen derivatives. The pso2-1 mutant having the same sensitivity as the wild-type to UV and ionizing radiations, it is suggested that the PSO2+ gene product is predominantly necessary for the repair of cross-links irrespective of their molecular nature. In contrast with psoralen photo-induced inactivation the pso3-1 mutant had the same sensitivity as the wild-type to alkylating agents. However, a reduction in induced mutagenesis was seen in both cases. This response was modulated according to dose and type of mutation. Consequently, it appeared that the PSO3+ gene product acts specifically on psoralen photo-induced sub-lethal lesions and on a fraction of premutagenic lesions independently of their structure.

Genetic analysis of gamma-ray mutagenesis in yeast. III. Double-mutant strains

Comparisons between the 60Co gamma-ray survival curves of diploid strains of the yeast Saccharomyces cerevisiae that are homozygous for two non-allelic radiation-sensitive mutations and the corresponding single-mutant diploids suggest that there are two main types of repair of ionizing radiation damage in this organism. The first, which is defined by the rad52 epistasis group, depends on the activities of the RAD50 through RAD57 genes and is responsible for repairing the larger amount of lethal damage. Previous work [22] shows that this type of repair is essentially error-free. The second, defined by the rad6 epistasis group, depends on the activities of the RAD6, RAD9, RAD18, REV1 and REV3 genes and repairs a smaller, though still substantial, amount of lethal damage. It is also responsible for induced mutagenesis [22, 23]. Data for survival and mutation induction after irradiation in air and partial anoxia show that oxygen-dependent damage can be repaired by either of these two pathways. They also show similar oxygen-enhancement ratios for survival and mutagenesis.

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.

The Role of Radiation (rad) Genes in Meiotic Recombination in Yeast

In yeast, the functions controlled by radiation-repair genes RAD6, RAD50, RAD52 and RAD57 are essential for normal meiosis; diploids with lesions in these genes either fail to sporulate (rad6) or sporulate but produce inviable spores (rad50, 52, 57). Since RAD genes may control aspects of DNA metabolism, we attempted to define more precisely the role of each gene in meiosis, especially with regard to possible roles in premeiotic DNA replication and recombination. We constructed diploids singly homozygous for each of the four rad mutations, heteroallelic at his1 and heterozygous for a recessive canavanine-resistance marker. Each strain was exposed to sporulation-inducing conditions and monitored for (1) completion of mitotic cell cycles, (2) cell viability, (3) utilization of acetate for mass increases, (4) premeiotic DNA synthesis, (5) intragenic recombination at his1, and (6) formation of viable haploid spores. Control strains heterozygous for the rad mutations completed mitosis, metabolized acetate, replicated their DNA, and showed typically high levels of gene conversion and viable-spore formation. The mutant diploids also completed mitosis, utilized acetate, and carried out premeiotic DNA replication. The mutants, however, showed little or no meiotic gene conversion. The rad50, 52 and 57 strains sporulated, but the spores were inviable. The rad6 strain did not sporulate. The rad50, 52 and 57 strains exhibited viability losses that coincided with the period of DNA synthesis, but not with later meiotic events; the rad6 strain did not lose viability. We propose that the normal functions specified by RAD50, 52 and 57 are not essential for either the initial or terminal steps in meiosis, but are required for successful recombination. The rad6 strain may be recombination-defective, or it may fail to progress past DNA replication in the overall sequence leading to formation and recovery of meiotic recombinants.

Three additional genes involved in pyrimidine dimer removal in Saccharomyces cerevisiae: RAD7, RAD14 and MMS19

The ability to remove ultraviolet (UV)-induced pyrimidine dimers from the nuclear DNA of yeast was examined in two radiation-sensitive (rad) mutants and one methyl methanesulfonate-sensitive (mms) mutant of the yeast Saccharomyces cerevisiae. The susceptibility of DNA from irradiated cells to nicking by an endonuclease activity prepared from crude extracts of Micrococcus luteus was used to measure the presence of dimers in DNA. The rad7, rad14 and mms19 mutants were found to be defective in their ability to remove UV-induced dimers from nuclear DNA. All three mutants belong to the same epistatic group as the other mutants involved in excision-repair. All three mutants show enhanced UV-induced mutations. The rad14 mutant also shows epistatic interactions with genes in the other two UV repair pathways.

Responses of radiation-sensitive mutants of Saccharomyces cerevisiae to lethal effects of bleomycin

Haploid and diploid strains of yeast containing genes conferring radiation-sensitivity were studied under growing and nongrowing experimental conditions for their relative sensitivities to growth-inhibitory effects of bleomycin (BM). The rad1, rad2, rad3, rad4, rad5 (and allelic rev2), rad7, rad10, rad11, rad 12, rad14, rad15, rad16 and rev3 strains exhibited responses similar to normal (Rad+) yeast strains. It is concluded from these findings that the excision-repair function deficient in several of these mutant strains is not important for repair of bleomycin-induced damages in yeast. The sensitive strains contained rad6, rad9, rad18, rad22, rad50, rad51, rad52, rad53, rad54, rad55, rad56, rad57 and rs1. Strains bearing rad8 or rad19 could not be classified unambiguously. With one exception, all rad mutants found very sensitive to BM were sensitive to X-rays, suggesting that some aspect of the repair of BM- and X-ray-induced damages in yeast may be similar. Sensitivities to BM and radiation co-segregated in pedigrees following meiosis, and several BM-resistant revertants isolated from two rad6 mutant strains sensitive to BM, X-rays and UV were cross-resistant to all three agents. These results confirm that the rad mutants were responsible for the cross-sensitivities in the original strains.

UV-induced mutation in bacteriophage T4

Two late gene am mutants of bacteriophage T4 that can be induced to revert by UV were crossed to a temperature-sensitive ligase mutant. In the double mutants, UV-induced reversion was eliminated at a semirestrictive temperature. When the single am mutants were irradiated and then allowed a single passage in a permissive host, the UV-induced reversion frequency was increased by 15- to 25-fold. This increased mutagenesis was also abolished by the presence of the ligase allele. When the UV-irradiated single am mutants multiply infected a permissive host, allowing multiplicity reactivation to occur, the induced reversion frequency was reduced similarly to the reduction in lethality. The mutagenesis that remained was again abolished by the presence of the ligase allele. It is concluded that UV induces mutations in phage T4 through the action of a pathway that includes polynucleotide ligase. The increase in mutation frequency after growth in a permissive host implies that mutagenesis can occur at more than one stage of the infection rather than only in an early stage before expression of the mutant genome. The process of multiplicity reactivation appears to be error-free since it overcomes lethal lesions without inducing new mutations.

Spontaneous mutability in UV-sensitive excision-defective strains of Saccharomyces

The genes RAD1, RAD2, RAD3 and RAD4 encode enzymes in the pathway leading to excision repair of UV-induced DNA damage in Saccharomyces cerevisiae. Four mutant alleles of these loci (rad1-1, rad2-2, rad3-12, and rad4-3) were studied for their effect on spontaneous reversion rate to lysine and histidine independence, by means of the 1000-compartment fluctuation test of von Borstel, Cain and Steinberg. Of these four excision-defective alleles, only rad3-12 was found to substantially increase the spontaneous reversion rate of the nonsense-suppressible lys1-1 allele, both through locus reversion as well as by forward mutation at one of eight suppressor loci. Similarly, only rad3-12 conferred a considerable increase in the reversion frequency of the missense his1-7 mutant. As the RAD3 gene product is believed to mediate the first step in the excision-repair pathway, it is assumed that spontaneous lesions in the rad3 strain are channelled into a mutagenic repair pathway, thus accounting for the enhanced spontaneous mutation rate.

Repair of pyrimidine dimers in radiation-sensitive mutants rad3, rad4, rad6 and rad9 of Saccharomyces cerevisiae

The ability to remove ultraviolet (UV)-induced pyrimidine dimers was examined in four radiation-sensitive mutants of Saccharomyces cerevisiae. The susceptibility of DNA from irradiated cells to nicking by either the T4 UV-endonuclease or an endonuclease activity found in crude extracts of Micrococcus luteus was used to measure the presence of dimers in DNA. The rad3 and rad4 mutants are shown to be defective in dimer excision whereas the rad6 and rad9 mutants are proficient in dimer excision.

Induction of mutations by chemicals and gamma rays in mutants of yeast refractory to UV-mutagenesis

Radiation-sensitive mutants of Schizosaccharomyces pombe, known to be refractory to UV-mutagenesis, were tested for mutability caused by treatments with chemicals and gamma rays. One such mutant (rad3) was studied over a wide range of UV doses to compare the kinetics of its mutational response to that of the wild type. All such comparisons were carried out using a forward mutation system. Data show that, unlike UV, the chemical mutagens as well as gamma rays produced mutations (although at reduced frequency), in the strains of S. pombe tested, indicating the existence of an additional mechanism(s) for chemical and gamma ray induced mutations. These observations are discussed as these relate to the pathways for repair of mutational damage in yeast.

Defective thymine dimer excision in radiation-sensitive mutants rad10 and rad16 of Saccharomyces cerevisiae

Two rad mutants of yeast, rad10 and rad16, are shown to be defective in the removal of UV-induced pyrimidine dimers since DNAs obtained from irradiated cells following a post-irradiation incubation in the dark still retain UV-endonuclease-sensitive sites. Both rad10 and rad16 mutants are in the same pathway of excision-repair as the rad1, rad2, rad3 and rad4 mutants.

The relation between repair of DNA and radiation and chemical mutagenesis in Saccharomyces cerevisiae

The effect of various genes involved in DNA repair functions on radiation and chemical mutagenesis in Escherichia coli is discussed and compared to similar studies done in yeast. Results of the effect of various genes conferring radiation-sensitivity on mutation induction in yeast are presented and related to current ideas of mutagenesis.

Effect of Genes Controlling Radiation Sensitivity on Chemically Induced Mutations in SACCHAROMYCES CEREVISIAE

The effect of 16 different genes (rad) conferring radiation sensitivity on chemically induced reversion in the yeast Saccharomyces cerevisiae was determined. The site of reversion used was a well-defined chain initiation mutant mapping in the structural gene coding for iso-1-cytochrome c. High doses of EMS and HNO(2) resulted in decreased reversion of cyc1-131 in rad6, rad9 and rad15 strains compared to the normal RAD+ strains. In addition, rad52 greatly decreased EMS reversion of cyc1-131 but had not effect on HNO( 2)-induced reversion; rad18, on the other hand, increased HNO( 2)-induced reversion but did not alter EMS-induced reversion. When NQO was used as the mutagen, every rad gene tested, except for rad14 , had an effect on reversion; rad6, rad9, rad15, rad17, rad18, rad22, rev1, rev2 and rev3 lowered NQO reversion while rad1, rad2, rad3, rad4, rad10, rad12 and rad16 increased it compared to the RAD+ strain. The effect of rad genes on chemical mutagenesis is discussed in terms of their effect on UV mutagenesis. It is concluded that although the nature of the repair pathways may differ for UV- and chemically-induced mutations in yeast, a functional repair system is required for the induction of mutation by the chemical agents NQO, EMS and HNO(2).

UV mutagenesis in radiation-sensitive strains of yeast

The yeast Saccharomyces cerevisiae appears to possess a single mutagenic or "error prony" pathway for the repair of UV damage; rev1, rev2, rev3 (Lemontt 1971a), rad6, rad8, rad9 and rad18 (Lawrence et al. 1974; present results). Strains carrying rad6 are the most sensitive to the lethal effects of UV light in this group and double mutants carrying rad6 and either rev1, rev3, rad9 or rad18 are no more sensitive than this single mutant strain, rev3 rad6 doubl- mutant diploids failed to show any UV-induced reversion of the normally highly reversion of the normally highly revertible ochre allele cycl-9, even though a total of more than 2.5 X 10(9) viable cells was examined, suggesting that strains of this kind are entirely UV-immutable; spontaneous revertants could be recovered, however.-The rad6 and rev3 gene products would appear to be necessary for all kinds of mutagenic events at all sites within the genome, but the products of the other genes that act in the "error-prone" pathway have a more restricted role and are involved in the production of only some kinds of mutations. It is suggested that such selectivity arises from the interaction of some repair enzymes with specific nucleotide sequences.

8-Methoxypsoralen plus 365 nm light effects and repair in yeast

Haploid wild-type and mutant cells of Saccharomyces carrying one of the single genes rad2-20 or rad9-4 and the double mutant rad2-20rad9-4 were tested for their response to a treatment with 8-methoxypsoralen plus 365 nm light using immediate and delayed plating techniques. The mutant defective in the excision of ultraviolet-induced pyrimidine dimers (rad2-20) as well as that presumably deficient in a recombinational repair system (rad9-4) are more sensitive than wild type cells. The double mutant (rad2-20rad9-4) demonstrates a higher sensitivity than each of the single mutants, indicating that at least two pathways are involved in the repair of the 8-methoxypsoralen plus 365 nm induced damages. In all cases survival curves have shoulders. The survival of wild type and rad9-4 cells is increased after dark holding whereas it remains constant for the rad2-20 mutant and for the double mutant. These results show that the induced damages are reparable. Respiratory deficient mutant (p-) were compared to the corresponding respiratory competent cells. It is shown that the respiratory function is required for the expression of the excision repair activity. The 8-methoxypsoralen plus 365 nm ligh treatment appears to be less effective than ultraviolet irradiation (254 nm) in the induction of the cytoplasmic 'petite' mutation at the same survival levels.

Lack of chemically induced mutation in repair-deficient mutants of yeast

Two genes, rad6 and rad9, that confer radiation sensitivity in the yeast Saccharomyces cerevisiae also greatly reduce the frequency of chemically-induced reversions of a tester mutant cyc1-131, which is a chain initiation mutant in the structural gene determining iso-1-cytochrome c. Mutations induced by ethyl methanesulfonate (EMS), diethyl sulfate (DES), methyl methanesulfonate (MMS), dimethyl sulfate (DMS), nitroquinoline oxide (NQO), nitrosoguanidine (NTG), nitrogen mustard (HN2), beta-propiolactone, and tritiated uridine, as well as mutations induced by ultraviolet light (UV) and ionizing radiation were greatly diminished in strains homozygous for either the rad6 or rad9 gene. Nitrous acid and nitrosoimidazolidone (NIL), on the other hand, were highly mutagenic in these repair-deficient mutants, and at low doses, these mutagens acted with about the same efficiency as in the normal RAD strain. At high doses of either nitrous acid or NIL, however, reversion frequencies were significantly reduced in the two rad mutants compared to normal strains. Although both rad mutants are immutable to about the same extent, the rad9 strains tend to be less sensitive to the lethal effect of chemical mutagens than rad6 strains. It is concluded that yeast requires a functional repair system for mutation induction by chemical agents.