The repair of double-strand breaks in chromosomal DNA of yeast

The Lowest Radiation Dose Having Molecular Changes in the Living Body

We herein attempted to identify the lowest radiation dose causing molecular changes in the living body. We investigated the effects of radiation in human cells, animals, and humans. DNA double-strand breaks (DSBs) formed in cells at γ- or X-ray irradiation doses between 1 mGy and 0.5 Gy; however, the extent of DSB formation differed depending on the cell species. The formation of micronuclei (MNs) and nucleoplasmic bridges (NPBs) was noted at radiation doses between 0.1 and 0.2 Gy. Stress-responsive genes were upregulated by lower radiation doses than those that induced DNA DSBs or MN and NPBs. These γ- or X-ray radiation doses ranged between approximately 10 and 50 mGy. In animals, chromosomal aberrations were detected between 50 mGy and 0.1 Gy of low linear energy transfer radiation, 0.1 Gy of metal ion beams, and 9 mGy of fast neutrons. In humans, DNA damage has been observed in children who underwent computed tomography scans with an estimated blood radiation dose as low as 0.15 mGy shortly after examination. The frequencies of chromosomal translocations were lower in residents of high background areas than in those of control areas. In humans, systemic adaptive responses may have been prominently expressed at these radiation doses.

Homologous recombination in budding yeast expressing the human RAD52 gene reveals a Rad51-independent mechanism of conservative double-strand break repair

RAD52 is a homologous recombination (HR) protein that is conserved from bacteriophage to humans. Simultaneously attenuating expression of both the RAD52 gene, and the HR and tumor suppressor gene, BRCA2, in human cells synergistically reduces HR – indicating that RAD52 and BRCA2 control independent mechanisms of HR. We have expressed the human RAD52 gene (HsRAD52) in budding yeast strains lacking the endogenous RAD52 gene and found that HsRAD52 supports repair of DNA double-strand breaks (DSB) by a mechanism of HR that conserves genome structure. Importantly, this mechanism of HR is independent of RAD51, which encodes the central strand exchange protein in yeast required for conservative HR. In contrast, BRCA2 exerts its effect on HR in human cells together with HsRAD51, potentially explaining the synergistic effect of attenuating the expression of both HsRAD52 and BRCA2. This suggests that multiple mechanisms of conservative DSB repair may contribute to tumor suppression in human cells.

Rad51 inhibits translocation formation by non-conservative homologous recombination in Saccharomyces cerevisiae

Chromosomal translocations are a primary biological response to ionizing radiation (IR) exposure, and are likely to result from the inappropriate repair of the DNA double-strand breaks (DSBs) that are created. An abundance of repetitive sequences in eukaryotic genomes provides ample opportunity for such breaks to be repaired by homologous recombination (HR) between non-allelic repeats. Interestingly, in the budding yeast, Saccharomyces cerevisiae the central strand exchange protein, Rad51 that is required for DSB repair by gene conversion between unlinked repeats that conserves genomic structure also suppresses translocation formation by several HR mechanisms. In particular, Rad51 suppresses translocation formation by single-strand annealing (SSA), perhaps the most efficient mechanism for translocation formation by HR in both yeast and mammalian cells. Further, the enhanced translocation formation that emerges in the absence of Rad51 displays a distinct pattern of genetic control, suggesting that this occurs by a separate mechanism. Since hypomorphic mutations in RAD51 in mammalian cells also reduce DSB repair by conservative gene conversion and stimulate non-conservative repair by SSA, this mechanism may also operate in humans and, perhaps contribute to the genome instability that propels the development of cancer.

Cohesin Is limiting for the suppression of DNA damage-induced recombination between homologous chromosomes

Double-strand break (DSB) repair through homologous recombination (HR) is an evolutionarily conserved process that is generally error-free. The risk to genome stability posed by nonallelic recombination or loss-of-heterozygosity could be reduced by confining HR to sister chromatids, thereby preventing recombination between homologous chromosomes. Here we show that the sister chromatid cohesion complex (cohesin) is a limiting factor in the control of DSB repair and genome stability and that it suppresses DNA damage-induced interactions between homologues. We developed a gene dosage system in tetraploid yeast to address limitations on various essential components in DSB repair and HR. Unlike RAD50 and RAD51, which play a direct role in HR, a 4-fold reduction in the number of essential MCD1 sister chromatid cohesion subunit genes affected survival of gamma-irradiated G(2)/M cells. The decreased survival reflected a reduction in DSB repair. Importantly, HR between homologous chromosomes was strongly increased by ionizing radiation in G(2)/M cells with a single copy of MCD1 or SMC3 even at radiation doses where survival was high and DSB repair was efficient. The increased recombination also extended to nonlethal doses of UV, which did not induce DSBs. The DNA damage-induced recombinants in G(2)/M cells included crossovers. Thus, the cohesin complex has a dual role in protecting chromosome integrity: it promotes DSB repair and recombination between sister chromatids, and it suppresses damage-induced recombination between homologues. The effects of limited amounts of Mcd1and Smc3 indicate that small changes in cohesin levels may increase the risk of genome instability, which may lead to genetic diseases and cancer.

The Saccharomyces cerevisiae RAD9, RAD17 and RAD24 genes are required for suppression of mutagenic post-replicative repair during chronic DNA damage

In Saccharomyces cerevisiae, a DNA damage checkpoint in the S-phase is responsible for delaying DNA replication in response to genotoxic stress. This pathway is partially regulated by the checkpoint proteins Rad9, Rad17 and Rad24. Here, we describe a novel hypermutable phenotype for rad9Delta, rad17Delta and rad24Delta cells in response to a chronic 0.01% dose of the DNA alkylating agent MMS. We report that this hypermutability results from DNA damage introduction during the S-phase and is dependent on a functional translesion synthesis pathway. In addition, we performed a genetic screen for interactions with rad9Delta that confer sensitivity to 0.01% MMS. We report and quantify 25 genetic interactions with rad9Delta, many of which involve the post-replication repair machinery. From these data, we conclude that defects in S-phase checkpoint regulation lead to increased reliance on mutagenic translesion synthesis, and we describe a novel role for members of the S-phase DNA damage checkpoint in suppressing mutagenic post-replicative repair in response to sublethal MMS treatment.

DNA damage-inducible and RAD52-independent repair of DNA double-strand breaks in Saccharomyces cerevisiae

Chromosomal repair was studied in stationary-phase Saccharomyces cerevisiae, including rad52/rad52 mutant strains deficient in repairing double-strand breaks (DSBs) by homologous recombination. Mutant strains suffered more chromosomal fragmentation than RAD52/RAD52 strains after treatments with cobalt-60 gamma irradiation or radiomimetic bleomycin, except after high bleomycin doses when chromosomes from rad52/rad52 strains contained fewer DSBs than chromosomes from RAD52/RAD52 strains. DNAs from both genotypes exhibited quick rejoining following gamma irradiation and sedimentation in isokinetic alkaline sucrose gradients, but only chromosomes from RAD52/RAD52 strains exhibited slower rejoining (10 min to 4 hr in growth medium). Chromosomal DSBs introduced by gamma irradiation and bleomycin were analyzed after pulsed-field gel electrophoresis. After equitoxic damage by both DNA-damaging agents, chromosomes in rad52/rad52 cells were reconstructed under nongrowth conditions [liquid holding (LH)]. Up to 100% of DSBs were eliminated and survival increased in RAD52/RAD52 and rad52/rad52 strains. After low doses, chromosomes were sometimes degraded and reconstructed during LH. Chromosomal reconstruction in rad52/rad52 strains was dose dependent after gamma irradiation, but greater after high, rather than low, bleomycin doses with or without LH. These results suggest that a threshold of DSBs is the requisite signal for DNA-damage-inducible repair, and that nonhomologous end-joining repair or another repair function is a dominant mechanism in S. cerevisiae when homologous recombination is impaired.

A double-strand break repair component is essential for S phase completion in fission yeast cell cycling

Fission yeast rad22(+), a homologue of budding yeast RAD52, encodes a double-strand break repair component, which is dispensable for proliferation. We, however, have recently obtained a cell division cycle mutant with a temperature-sensitive allele of rad22(+), designated rad22-H6, which resulted from a point mutation in the conserved coding sequence leading to one amino acid alteration. We have subsequently isolated rad22(+) and its novel homologue rti1(+) as multicopy suppressors of this mutant. rti1(+) suppresses all the defects of cells lacking rad22(+). Mating type switch-inactive heterothallic cells lacking either rad22(+) or rti1(+) are viable, but those lacking both genes are inviable and arrest proliferation with a cell division cycle phenotype. At the nonpermissive temperature, a synchronous culture of rad22-H6 cells performs DNA synthesis without delay and arrests with chromosomes seemingly intact and replication completed and with a high level of tyrosine-phosphorylated Cdc2. However, rad22-H6 cells show a typical S phase arrest phenotype if combined with the rad1-1 checkpoint mutation. rad22(+) genetically interacts with rad11(+), which encodes the large subunit of replication protein A. Deletion of rad22(+)/rti1(+) or the presence of rad22-H6 mutation decreases the restriction temperature of rad11-A1 cells by 4-6 degrees C and leads to cell cycle arrest with chromosomes incompletely replicated. Thus, in fission yeast a double-strand break repair component is required for a certain step of chromosome replication unlinked to repair, partly via interacting with replication protein A.

A unique pathway of double-strand break repair operates in tandemly repeated genes

The RAD52 gene product of the yeast Saccharomyces cerevisiae is required for most spontaneous recombination and almost all double-strand break (DSB) repair. In contrast to recombination elsewhere in the genome, recombination in the ribosomal DNA (rDNA) array is RAD52 independent. To determine the fate of a DSB in the rDNA gene array, a cut site for the HO endonuclease was inserted into the rDNA in a strain containing an inducible HO gene. DSBs were efficiently repaired at this site, even in the absence of the RAD52 gene product. Efficient RAD52-independent DSB repair was also observed at another tandem gene array, CUP1, consisting of 18 repeat units. However, in a smaller CUP1 array, consisting of only three units, most DSBs (ca. 80%) were not repaired and resulted in cell death. All RAD52-independent DSB repair events examined resulted in the loss of one or more repeat units. We propose a model for DSB repair in repeated sequences involving the generation of single-stranded tails followed by reannealing.

A unique pathway of double-strand break repair operates in tandemly repeated genes

The RAD52 gene product of the yeast Saccharomyces cerevisiae is required for most spontaneous recombination and almost all double-strand break (DSB) repair. In contrast to recombination elsewhere in the genome, recombination in the ribosomal DNA (rDNA) array is RAD52 independent. To determine the fate of a DSB in the rDNA gene array, a cut site for the HO endonuclease was inserted into the rDNA in a strain containing an inducible HO gene. DSBs were efficiently repaired at this site, even in the absence of the RAD52 gene product. Efficient RAD52-independent DSB repair was also observed at another tandem gene array, CUP1, consisting of 18 repeat units. However, in a smaller CUP1 array, consisting of only three units, most DSBs (ca. 80%) were not repaired and resulted in cell death. All RAD52-independent DSB repair events examined resulted in the loss of one or more repeat units. We propose a model for DSB repair in repeated sequences involving the generation of single-stranded tails followed by reannealing.

The PSO4 gene is responsible for an error-prone recombinational DNA repair pathway in Saccharomyces cerevisiae

The induction of mitotic gene conversion and crossing-over in Saccharomyces cerevisiae diploid cells homozygous for the pso4-1 mutation was examined in comparison to the corresponding wild-type strain. The pso4-1 mutant strain was found to be completely blocked in mitotic recombination induced by photoaddition of mono- and bifunctional psoralen derivatives as well as by mono- (HN1) and bifunctional (HN2) nitrogen mustards or 254 nm UV radiation in both stationary and exponential phases of growth. Concerning the lethal effect, diploids homozygous for the pso4-1 mutation are more sensitive to all agents tested in any growth phase. However, this effect is more pronounced in the G2 phase of the cell cycle. These results imply that the ploidy effect and the resistance of budding cells are under the control of the PSO4 gene. On the other hand, the pso4-1 mutant is mutationally defective for all agents used. Therefore, the pso4-1 mutant has a generalized block in both recombination and mutation ability. This indicates that the PSO4 gene is involved in an error-prone repair pathway which relies on a recombinational mechanism, strongly suggesting an analogy between the pso4-1 mutation and the RecA or LexA mutation of Escherichia coli.

Relationship of histidine sensitivity to DNA damage and stress induced responses in mutagen sensitive mutants of Neurospora crassa

Previous work in other laboratories has shown that several mutagen sensitive mutants of Neurospora crassa are extremely sensitive to low levels of histidine in the culture medium. We have shown that wild type Neurospora accumulates nicks or breaks in the DNA in the presence of histidine. The number of nicks accumulating in histidine sensitive mutants is found to increase in relation to their sensitivity to histidine. Although these nicks can be repaired by both wild type and histidine sensitive mutants when histidine is removed from the medium, a steady state number of nicks exists as long as histidine is present. We suggest that the presence of these nicks or breaks induces an increase in recombination in these possibly recombination defective mutants and that this is the source of the high level of histidine sensitivity. We speculate on the mechanisms by which histidine induces this DNA damage. This report also shows that several polypeptides are induced by the wild type organism in the presence of histidine. Some of these polypeptides are also induced during other stress situations, such as heat shock and DNA damage due to ultraviolet irradiation. Two of the histidine induced proteins cannot be induced by any of the histidine sensitive mutants.

Possible occurrence of DNA double-strand breaks during repair of u.v.-induced damage in yeast

The yeast mutant rad54-3, which is temperature conditional for dsb rejoining, is sensitive to u.v. light when held at the restrictive temperature following exposure. We propose that this is attributable to the enzymatic formation of dsb in DNA containing u.v. lesions and a subsequent lack of dsb repair in this mutant.

Fast neutron r.b.e. for lethality and genotoxicity in a wild-type and a repair-deficient strain of yeast

The relative biological effectiveness (r.b.e.) of cyclotron-produced fast neutrons (11 MeV) in relation to 60Co gamma-rays, was studied in a wild-type and a DNA repair-deficient yeast strain for cell killing and genotoxicity. In the wild-type (D7) strain the r.b.e. varied from 2.7 to 4.1 for lethality, 2.8 to 7.1 for reverse mutation and 3.5 to 7.8 for mitotic gene conversion. At different survival levels, the repair deficient strain (D7 rad 52/rad 52) generally showed a lower r.b.e. for both cell killing and genotoxicity (25.2 to 37.2 per cent reduction for the cell death and 24.8 to 70.6 per cent for mutation and gene conversion) compared to the wild type. Except at very low dose levels, the r.b.e. values for cell killing and genotoxicity were similar within a given strain. At similar survival levels, neutrons were no more genotoxic than gamma-rays.

Chemically induced aneuploidy in mammalian cells: mechanisms and biological significance in cancer

A growing body of evidence from human and animal cancer cytogenetics indicates that aneuploidy is an important chromosome change in carcinogenesis. Aneuploidy may be associated with a primary event of carcinogenesis in some cancers and a later change in other tumors. Evidence from in vitro cell transformation studies supports the idea that aneuploidy has a direct effect on the conversion of a normal cell to a preneoplastic or malignant cell. Induction of an aneuploid state in a preneoplastic or neoplastic cell could have any of the following four biological effects: a change in gene dosage, a change in gene balance, expression of a recessive mutation, or a change in genetic instability (which could secondarily lead to neoplasia). To understand the role of aneuploidy in carcinogenesis, cellular and molecular studies coupled with the cytogenetic studies will be required. There are a number of possible mechanisms by which chemicals might induce aneuploidy, including effects on microtubules, damage to essential elements for chromosome function (ie, centromeres, origins of replication, and telomeres), reduction in chromosome condensation or pairing, induction of chromosome interchanges, unresolved recombination structures, increased chromosome stickiness, damage to centrioles, impairment of chromosome alignment, ionic alterations during mitosis, damage to the nuclear membrane, and a physical disruption of chromosome segregation. Therefore, a number of different targets exist for chemically induced aneuploidy. Because the ability of certain chemicals to induce aneuploidy differs between mammalian cells and lower eukaryotic cells, it is important to study the mechanisms of aneuploidy induction in mammalian cells and to use mammalian cells in assays for potential aneuploidogens (chemicals that induce aneuploidy). Despite the wide use of mammalian cells for studying chemically induced mutagenesis and chromosome breakage, aneuploidy studies with mammalian cells are limited. The lack of a genetic assay with mammalian cells for aneuploidy is a serious limitation in these studies.

Cell cycle-dependent repair of double-strand DNA breaks in a gamma-ray-sensitive Chinese hamster cell

A Chinese hamster cell mutant has been isolated which is extremely sensitive to killing by gamma-irradiation in the G1 and early S phases of the cell cycle (LD50 of 20 vs. 250 rads for parent), but which has nearly normal resistance in late S. The mutant cell is able to repair single-stranded DNA breaks introduced by gamma-radiation. However, in comparison to its parental cell, the mutant is deficient in the repair of double-stranded DNA breaks produced by gamma-irradiation during the sensitive G1-early S period, while in the resistant late S period, the repair is nearly the same for both cell types. This correlation between gamma-ray sensitivity and repair strongly suggests that an inability to repair double-strand DNA breaks in G1 is the basis for the hypersensitivity of the mutant to killing by gamma-rays in this phase of the cell cycle. It also provides direct evidence in mammalian cells that the ability to repair double-strand DNA breaks induced by ionizing radiation is an important biochemical function in cell survival and supports the hypothesis that unrepaired double-strand breaks are a major lethal lesion in mammalian cells. A plausible explanation for the appearance of the cell cycle phenotype of the mutant is that in normal cells there are at least two pathways for the repair of double-strand breaks, one of which functions primarily in late S phase, and the other, either throughout the cell cycle or only in the G1 and early S phases.

DNA polymerases, deoxyribonucleases, and recombination during meiosis in Saccharomyces cerevisiae

We utilized strains of Saccharomyces cerevisiae that exhibit high efficiency of synchrony of meiosis to examine several aspects of meiosis including sporulation, recombination, DNA synthesis, DNA polymerase I and II, and Mg2+-dependent alkaline DNases. The kinetics of commitment to intragenic recombination and sporulation are similar. The synthesis of DNA, as measured directly with diphenylamine, appears to precede the commitment to recombination. Both DNA polymerase I and II activities and total DNA-synthesizing activity in crude extracts increase two- to threefold before the beginning of meiotic DNA synthesis. Increases of 10- to 20-fold over mitotic levels are found for Mg2+-dependent alkaline DNase activity in crude extracts before and during the commitment to meiotic intragenic recombination. Of particular interest is the comparable increase in a nuclease under the control of the RAD52 gene; this enzyme has been identified by the use of antibody raised against a similar enzyme from Neurospora crassa. Since the RAD52 gene is essential for meiotic recombination, the nuclease is implicated in the high levels of recombination observed during meiosis. The effects observed in this report are meiosis specific since they are not observed in an alpha alpha strain.

DNA polymerases, deoxyribonucleases, and recombination during meiosis in Saccharomyces cerevisiae

We utilized strains of Saccharomyces cerevisiae that exhibit high efficiency of synchrony of meiosis to examine several aspects of meiosis including sporulation, recombination, DNA synthesis, DNA polymerase I and II, and Mg2+-dependent alkaline DNases. The kinetics of commitment to intragenic recombination and sporulation are similar. The synthesis of DNA, as measured directly with diphenylamine, appears to precede the commitment to recombination. Both DNA polymerase I and II activities and total DNA-synthesizing activity in crude extracts increase two- to threefold before the beginning of meiotic DNA synthesis. Increases of 10- to 20-fold over mitotic levels are found for Mg2+-dependent alkaline DNase activity in crude extracts before and during the commitment to meiotic intragenic recombination. Of particular interest is the comparable increase in a nuclease under the control of the RAD52 gene; this enzyme has been identified by the use of antibody raised against a similar enzyme from Neurospora crassa. Since the RAD52 gene is essential for meiotic recombination, the nuclease is implicated in the high levels of recombination observed during meiosis. The effects observed in this report are meiosis specific since they are not observed in an alpha alpha strain.

Effect of caffeine on ozone-sensitivity in Saccharomyces cerevisiae

The addition of 0.1% caffeine to the plating medium markedly reduced the ozone-survival of the wild-type and the rad1 and rad6 mutants of Saccharomyces cerevisiae, whereas no effect was observed in the rad52 mutant. Since, in S. cerevisiae, caffeine has been reported to interfere with the recombinational repair pathway under the control of the RAD52 gene, these results support previous observations suggesting that this pathway is involved in the repair of ozone-induced DNA damage.

Repair of gamma-ray induced DNA strand breaks in the radiation-sensitive mutant rad18-2 of Saccharomyces cerevisiae

The repair of gamma-ray induced DNA single and double-strand breaks was looked at in wild type and rad18-2 strains of the yeast Saccharomyces cerevisiae using sucrose gradient centrifugation. It was found that rad18-2 diploid cells could repair single and double-strand breaks induced by gamma-rays. It was also found that rad18-2 cells experienced a breakup of their DNA during post-irradiation incubation to a size smaller than seen in cells just receiving irradiation. This breakup of DNA in rad18-2 cells is not degradation due to cell death since wild type cells irradiated to similar low survival levels do not show this breakup of DNA with 8 h incubation. The breakup of DNA in rad18-2 cells is not due to replication gaps being formed by synthesis on a damaged template since treatment of rad18-2 a mating type cells with alpha factor, to prevent initiation of DNA synthesis, does not prevent breakup of the DNA.

Characterization of postreplication repair in Saccharomyces cerevisiae and effects of rad6, rad18, rev3 and rad52 mutations

Postreplication repair of nuclear DNA was examined in an excision defective haploid strain of yeast lacking mitochondrial DNA (rad1 rho 0). The size of the DNA synthesized in cells exposed to various fluences of ultraviolet light (UV) corresponds approximately to the average interdimer distance in the parental DNA. Upon further incubation of cells following exposure to 2.5 J/m2, the DNA increases in size; by 4 h, it corresponds to DNA from uniformly labeled cells. The alkaline sucrose sedimentation pattern of DNA pulse labeled at various times after UV irradiation, for up to 4 h, does not change substantially, indicating that dimers continue to block DNA replication. A significant amount of postreplication repair requires de novo protein synthesis, as determined by its inhibition by cycloheximide. The rad6 mutant does not carry out postreplication repair, the rad18 and rad52 mutants show great inhibition while the rev3 mutation does not affect postreplication repair. Both recombinational and nonrecombinational repair mechanisms may function in postreplication repair and most of postreplication repair is error free.

a/alpha-specific effect on the mms3 mutation on ultraviolet mutagenesis in Saccharomyces cerevisiae

A new gene involved in error-prone repair of ultraviolet (UV) damage has been identified in Saccharomyces cerevisiae by the mms3-1 mutation. UV-induced reversion is reduced in diploids that are homozygous for mms3-1, only if they are also heterozygous (MATa/MAT alpha) at the mating type locus. The mms3-1 mutation has no effect on UV-induced reversion either in haploids or MATa/MATa or MAT alpha/MAT alpha diploids. The mutation confers sensitivity to UV and methyl methane sulfonate in both haploids and diploids. Even though mutation induction by UV is restored to wild-type levels in MATa/MATa mms3-1/mms3-1 or MAT alpha/MAT alpha mms3-1/mms3-1 diploids, such strains still retain sensitivity to the lethal effects of UV. Survival after UV irradiation in mms3-1 rad double mutant combinations indicates that mms3-1 is epistatic to rad6-1 whereas non-epistatic interactions are observed with rad3 and rad52 mutants. When present in the homozygous state in MATa/MAT alpha his1-1/his1-315 heteroallelic diploids, mms3-1 was found to lower UV-induced mitotic recombination.

Measurement of the kinetics of DNA double strand break repair in Ehrlich ascites tumour cells using the unwinding method

Two main components of DNA strand break repair have been found using the unwinding method. The first has a time constant (t37) of some minutes and the second, much slower component, a time constant of several hours. The time constant for the slower component of repair was found to vary with the conditions of incubation and to depend on the quality of the radiation. The t37 values obtained for slow repair under various conditions after X-irradiation and after alpha-irradiation were found to be close to those for repair of double strand breaks as measured by velocity sedimentation. Values for initial breaks, obtained by extrapolation of slow repair data back to time zero, were also close to those obtained for double strand breaks. We therefore propose that the unwinding method can be a useful technique for monitoring the repair of double strand breaks.

Characterization of the mutator mutation mut5-1

The mutator mutation mut5-1 has been characterized with respect to a range of parameters which have been used to describe DNA repair mutants of yeast. No marked effect of the mutation on UV-mutability at lower doses was apparent. Diploids homozygous for the mutation are deficient in UV-induced recombination between the alleles his1-1 and hist1-315, mutation being sufficient to account for all the UV-induced histidine prototrophs. Complementation and mapping studies indicate that mut5-1 is allelic to rad51-1, supporting the conclusion of Hastings et al. (1976) that a mutator may increase spontaneous mutation by modifying repair parameters. Both mut5-1 homozygous and heterozygous diploids give rise to spontaneous or UV-induced segregants which appear to be the products of nondisjunction events. The levels of parameiotic recombination (see Sherman and Roman, 1963; Esposito and Esposito, 1974), sporulation and spore viability observed in mut5-1/mut5-1 diploids indicate that the function encoded by RAD51 is required at 2 times during meiosis. An essential role of the function encoded by RAD51 in mitotic and meiotic recombination is indicated.

The DNA repair capability of cdc9, the Saccharomyces cerevisiae mutant defective in DNA ligase

The cell cycle mutant, cdc9, in the yeast Saccharomyces cerevisiae is defective in DNA ligase be deficient in the repair of DNA damaged by methyl methane sulphonate. On the other hand survival of cdc9 after irradiation by gamma-rays is little different from that of the wild-type, even after a period of stress at the restrictive temperature. The mutant cdc9 is not allelic with any known rad or mms mutants.

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.

Mitochondrial genetic damage induced in yeast by a photoactivated furocoumarin in combination with ethidium bromide or ultraviolet light

Ethidium bromide (EB) and ultraviolet light (UV) in combination are known to produce a synergistic induction of "petite" mutants in yeast. Two other agents were combined with EB, 3-Carbethoxypsoralene (3 CPs) activated by 365 nm light or gamma rays. EB in combination with 3 CPs also resulted in an enhanced production of "petite" mutants. After the photoaddition of 3 CPs in exponential phase cells, recovery of the "petite" mutation during dark liquid holding was inhibited by the presence of EB producing an enhanced number of "petite" mutants. The behavior of mitochondrial antibiotic resistance markers after individual and combined treatments with EB and 3 CPs indicates a random loss of markers after EB and a preferential loss of a certain region for the 3 CPs photoaddition. The combination of the two agents leads to an additivity of total drug marker losses rather than a synergistic loss. The combination of EB with gamma rays produced no enhancement in "petite" induction. A combination of UV and 3 CPs showed a synergistic interaction for "petite" induction. These results indicate that the three agents, EB, UV and 3 CPs photoaddition may share a common repair step for mitochondrial lesions.

The repair of double-strand breaks in the nuclear DNA of Saccharomyces cerevisiae and its genetic control

With the use of neutral sucrose sedimentation techniques, the size of unirradiated nuclear DNA and the repair of double-strand breaks induced in it by ionizing radiation have been determined in both wild-type and homozygous rad52 diploids of the yeast Saccharomyces cerevisiae. The number average molecular weight of unirradiated DNA in these experiments is 3.0 X 10(8)+/-0.3 Daltons. Double-strand breaks are induced with a frequency of 0.58 X 10(-10) per Daltonkrad in the range of 25 to 100 krad. Since repair at low doses is observed in wild-type but not homozygous rad52 strains, the corresponding rad52 gene product is concluded to have a role in the repair process. Cycloheximide was also observed to inhibit repair to a limited extent indicating a requirement for protein synthesis. Based on the sensitivity of various mutants and the induction frequency of double-strand breaks, it is concluded that there are 1 to 2 double-strand breaks per lethal event in diploid cells incapable of repairing these breaks.