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

Simultaneous induction of multiple mutations by N-methyl-N'-nitro-N-nitrosoguanidine in the yeast Saccharomyces cerevisiae

Contrary to what happens in bacteria, mutations induced by nitrosoguanidine in yeast are not accompanied by an excess of mutations in nearby genes. We have investigated nitrosoguanidine mutagenesis in three regions of the yeast genome: the contiguous DNA segments HIS4A, HIS4B and HIS4C, located on chromosome III; ADE1 and CDC15 separated by about 3 map units on chromosome I; and CAN1, some 50 map units away from the centromere on chromosome V. Revertants at HIS4C never suffered mutations at HIS4A or HIS4B. Reversion at CDC15 did not affect the frequency of mutation at ADE1. No tsm mutations, leading to thermonsensitivity, were found in the immediate vicinity of the locus CAN1 after selecting for canavanine resistant mutants. However, as expected from nitrosoguanidine mutagenesis of replication points and the fixed pattern of chromosome replication, the induced tsm mutations seem not to map randomly over the yeast genome; in fact, two out of the three groups of such tsm mutations studied are located in the same chromosome arm as CAN1, indicating that these two regions are replicated at the same time as CAN1. Replication synchrony is less than perfect, since the tsm mutations of each group affect many different genes.

MRE11 and RAD50, but not NBS1, are essential for gene targeting in the moss Physcomitrella patens

The moss Physcomitrella patens is unique among plant models for the high frequency with which targeted transgene insertion occurs via homologous recombination. Transgene integration is believed to utilize existing machinery for the detection and repair of DNA double-strand breaks (DSBs). We undertook targeted knockout of the Physcomitrella genes encoding components of the principal sensor of DNA DSBs, the MRN complex. Loss of function of PpMRE11 or PpRAD50 strongly and specifically inhibited gene targeting, whilst rates of untargeted transgene integration were relatively unaffected. In contrast, disruption of the PpNBS1 gene retained the wild-type capacity to integrate transforming DNA efficiently at homologous loci. Analysis of the kinetics of DNA-DSB repair in wild-type and mutant plants by single-nucleus agarose gel electrophoresis revealed that bleomycin-induced fragmentation of genomic DNA was repaired at approximately equal rates in each genotype, although both the Ppmre11 and Pprad50 mutants exhibited severely restricted growth and development and enhanced sensitivity to UV-B and bleomycin-induced DNA damage, compared with wild-type and Ppnbs1 plants. This implies that while extensive DNA repair can occur in the absence of a functional MRN complex; this is unsupervised in nature and results in the accumulation of deleterious mutations incompatible with normal growth and development.

RAD51 loss of function abolishes gene targeting and de-represses illegitimate integration in the moss Physcomitrella patens

Gene targeting (GT) is a major tool for basic and applied research during which the transforming DNA, which shares sequence homology with a chromosomal target, integrates at the corresponding locus by homologous recombination (HR). In eukaryotes, GT recruits enzymes from the HR-mediated double strand break repair pathway. Different mechanisms of HR have been described which depend on the Rad52 epistasis group of genes, but which specific mechanism is used by the cell for GT remains unclear. In Saccharomyces cerevisiae, the RAD52 protein is essential for GT, and the RAD51 protein plays a minor role. In filamentous fungi and animal cells, however, GT depends on RAD51 and is weakly affected by suppression of RAD52. Genetic evidence also indicates that the non-homologous end-joining pathway of DSB repair has a negative impact on GT efficiencies, but how the balance between these two pathways is controlled is poorly understood. Here, we have examined the role of RAD51 in the only plant that exhibits high GT frequencies, the model bryophyte Physcomitrella patens. Our results show that the two RAD51 proteins have partially redundant functions in the maintenance of genome integrity and resistance to ionizing radiation. Furthermore, we demonstrate that loss of function of the two RAD51 proteins completely abolishes GT and strongly increases illegitimate integration rates in this moss. These findings demonstrate for the first time in plant the critical role of RAD51 in controlling the balance between targeted and random integration events observed upon transgenesis, and confirm that P. patens is a particularly interesting tool for studying GT in higher eukaryotes.

The genetic activity of N6-hydroxyadenine and 2-amino-N6-hydroxyadenine in Escherichia coli, Salmonella typhimurium and Saccharomyces cerevisiae

The genetic activity of 2-amino-N6-hydroxyadenine or 2-amino-N-hydroxylaminopurine (AHA) and N6-hydroxyadenine or 6-N-hydroxylaminopurine (HAP) was studied in S. typhimurium, E. coli and Saccharomyces cerevisiae strains. AHA was a more potent mutagen for bacteria and a less potent mutagen for yeast than HAP. The mutagenic activity of analogs was not influenced by excision, mutagenic or double-strand DNA repair mutations. On the other hand, the uvrBdel mutation has a drastic effect on the mutagenicity and toxicity of both analogs in the Salmonella strains studied. HAP was a very potent mutagen in yeast with a low capability of inducing mitotic recombination contrary to common mutagens, possessed unique intergenic specificity and was able to induce mutations in diploids at rather high frequency.

Identification of proteins whose synthesis in Saccharomyces cerevisiae is induced by DNA damage and heat shock

Protein synthesis in Saccharomyces cerevisiae after exposure to ultraviolet light (UV) was examined by two-dimensional gel electrophoresis of pulse-labelled proteins. The synthesis of 12 distinct proteins was induced by treatment with UV doses of 10-200 J/m2. The induced proteins differed in the minimum dose necessary for induction, the maximum dose at which induction still occurred and the constitutive level present in unirradiated cells. A chemical mutagen, 4-nitroquinoline-1-oxide, induced synthesis of the same proteins. Induction after UV treatment was observed in seven different yeast strains, including three mutants deficient in DNA repair. Synthesis of five of the proteins was also induced by brief heat shock treatment. These five proteins may be members of a family of proteins whose synthesis is regulated by two different pathways responding to different types of stress.

Allelism between pso1-1 and rev3-1 mutants and between pso2-1 and snm1 mutants in Saccharomyces cerevisiae

In the yeast Saccharomyces cerevisiae, allelism between the pso1-1 and the rev3-1 mutants on the one hand and the pso2-1 and snm1 mutants on the other, is demonstrated by the comparison of phenotypes, complementation tests and meiotic segregation analysis.

Mitochondrial mutagenesis in yeast: mutagenic specificity of EMS and the effects of RAD9 and REV3 gene products

EMS is capable of inducing point mutations in mitochondrial genomes of yeast. It induces efficiently the mitochondrial suppressor mutation of the mitochondrial ochre mutation oxi 1-V25. The base changes leading to the suppression effect have not been identified. AT----GC base substitutions in mitochondrial genomes are inefficiently induced by EMS. The RAD9 and REV3 gene products participate in EMS mutagenesis in nuclear, as well as mitochondrial genomes of yeast.

Specific Saccharomyces cerevisiae genes are expressed in response to DNA-damaging agents

When exposed to DNA-damaging agents, the yeast Saccharomyces cerevisiae induces the expression of at least six specific genes. We have previously identified one damage inducible (DIN) gene as a gene fusion (din-lacZ fusion) whose expression increases in response to DNA-damaging treatments. We describe here the identification of five additional DIN genes as din-lacZ fusions and the responses of all six DIN genes to DNA-damaging agents. Northern blot analyses of the transcripts of two of the DIN genes show that their levels increase after exposure to DNA-damaging agents. Five of the din-lacZ fusions are induced in S. cerevisiae cells exposed to UV light, gamma rays, methotrexate, or alkylating agents. One of the din-lacZ fusions is induced by either UV or methotrexate but not by the other agents. This finding suggests that there are sets of DIN genes that are regulated differently.

Specific Saccharomyces cerevisiae genes are expressed in response to DNA-damaging agents

When exposed to DNA-damaging agents, the yeast Saccharomyces cerevisiae induces the expression of at least six specific genes. We have previously identified one damage inducible (DIN) gene as a gene fusion (din-lacZ fusion) whose expression increases in response to DNA-damaging treatments. We describe here the identification of five additional DIN genes as din-lacZ fusions and the responses of all six DIN genes to DNA-damaging agents. Northern blot analyses of the transcripts of two of the DIN genes show that their levels increase after exposure to DNA-damaging agents. Five of the din-lacZ fusions are induced in S. cerevisiae cells exposed to UV light, gamma rays, methotrexate, or alkylating agents. One of the din-lacZ fusions is induced by either UV or methotrexate but not by the other agents. This finding suggests that there are sets of DIN genes that are regulated differently.

Repair of double-strand DNA breaks in Drosophila

This study describes the repair kinetics of DNA double-strand breaks in primary and established cell cultures of Drosophila melanogaster. Double-strand breaks, induced by X-irradiation, were monitored by neutral elution. In primary cell cultures 50% of the double-strand breaks induced by 10 kR of X-rays are repaired within 45 min and 80% of the breaks are repaired within 2-3 h. Repair kinetics in established cell cultures are similar; 50% of the induced breaks are repaired within 20 min and 88% within 3 h. Mutants deficient in other types of DNA repair were also assayed for their capacity to repair double-strand breaks.

Mutagenesis at the ad-3A and ad-3B loci in haploid UV-sensitive strains of Neurospora crassa. V. Comparison of dose--response curves of single- and double-mutant strains with wild-type

The interactions of mutant alleles that individually confer radiation sensitivity in Neurospora crassa are being studied with regard to their effects on radiation-induced inactivation and forward-mutation induction at the ad-3 loci. This paper reports attempts to construct 3 double-mutant strains containing the following pair-wise combinations of repair-deficient mutants: upr-1,uvs-2; uvs-2,uvs-6; and uvs-3,uvs-6. The double-mutant strain with the 2 excision-repair-deficient mutants upr-1 and uvs-2 shows increased sensitivity to X-ray-induced mutagenesis and inactivation, relative to that shown by either of the parental single-mutant strains. This double mutant is no more sensitive than the parental single-mutant strains to either UV mutagenesis or inactivation. The combination of the uvs-2 and uvs-6 double-mutant strain is considerably more sensitive to both UV and X-ray inactivation than either the uvs-2 or uvs-6 strain, but it shows no greater sensitivity than the parental strains to ad-3 mutation induction by either agent. The combination of the uvs-3 and uvs-6 alleles is inviable. Tetrad analysis and microscopical examination of ascospores shows that ascospores of presumptive genotype uvs-3, uvs-6 do not grow beyond the formation of a few hyphal threads. The lethal and mutagenic effects of UV and X-irradiation in these double-mutant strains are interpreted in terms of the repair systems in Neurospora and other microorganisms.

Repair of interstrand cross-links in DNA of Saccharomyces cerevisiae requires two systems for DNA repair: the RAD3 system and the RAD51 system

We have studied the role of the excision-repair system and the recombination-repair system in the removal of cross-links and monoadducts caused by furocoumarins plus 360 nm radiation in yeast DNA by neutral and alkaline sucrose gradients and by a fluorometric procedure which detects cross-linked DNA molecules. We found that the excision-repair system, represented by the rad3 mutations, is required both for the removal of monoadducts, causing single-strand break formation, and for the removal of cross-links, causing double-strand break formation. The recombination-repair system, represented by the rad51 mutation, is necessary for double-strand break repair following cross-link removal, but it has no role in the repair of monoadducts. It can be concluded, that at least some of the same enzymes are used in yeast for both the excision of pyrimidine dimers and the excision of cross-links or monoadducts caused by furocoumarins plus light. The RAD3 and RAD51 repair systems, which act independently in the repair of UV-induced lesions, are part of a single system for the repair of cross-links.

Mutagenesis at the ad-3A and ad-3B loci in haploid UV-sensitive strains of Neurospora crassa. IV. Comparison of dose-response curves for MNNG, 4NQO and ICR-170 induced inactivation and mutation-induction

The genetic effects of MNNG, 4NQO and ICR-170 have been compared on 5 different UV-sensitive strains and a standard wild-type strain of Neurospora crassa with regard to inactivation and the induction of forward-mutations at the ad-3A and ad-3B loci. Whereas all UV-sensitive strains (upr-1, uvs-2, uvs-3, uvs-5 and uvs-6) are more sensitive to inactivation by MNNG and ICR-170 than wild-type, only uvs-5 shows survival comparable to wild-type after 4NQO treatment, all other strains are more sensitive to 4NQO. In contrast to the effects on inactivation, a wide variety of effects were found for the induction of ad-3A and ad-3B mutations: higher forward-mutation frequencies than were found in wild-type were obtained after treatment with MNNG or 4NQO for upr-1 and uvs-2, no significant increase over the spontaneous mutation frequency was found with uvs-3 after MNNG, 4NQO or ICR-170 treatment; mutation frequencies comparable to that found in wild-type were obtained with uvs-6 after MNNG, 4NQO or ICR-170 treatment and with upr-1 after ICR-170 treatment. Lower forward-mutation frequencies than were found in wild-type were obtained with uvs-2 after ICR-170 treatment and with uvs-5 after MNNG, 4NQO or ICR-170 treatment. These data clearly show that the process of forward-mutation at the ad-3A and ad-3B loci is under genetic control by mutations at other loci (e.g. upr-1, uvs-2, uvs-3, uvs-5 and uvs-6) and that the effect is markedly mutagen-dependent.

Mutagen-sensitive mutants in Neurospora

Initial work on the fungus Neurospora crassa has shown that a least two DNA-repair systems exist in this eukaryote: excision repair and a mutation-prone repair. The evidence suggests that there is also a third repair system. Recently, new mutagen-sensitive strains have been isolated in several laboratories, but they are not yet fully characterized. A hunt for cytoplasmically inherited UV sensitivity has failed to turn up any such mutants among 25 new UV-sensitive isolates.

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.

Role of cellular systems in modifying the response to chemical mutagens

Neocarzinostatin (NCS) produces apurinic/apyrimidinic (AP) sites in DNA which are repaired by the AP excision repair system. Survival after NCS treatment is not determined exclusively by this repair system, presumably because of the production of other, lethal, lesions. MNNG also produces multiple lesions which may be handled by cells in different ways. In E. coli, MNNG treatment results in rapid induction of a system which removes O6-methylguanine. Inhibition of this induction with chloramphenicol results in a large increase in mutation frequency. Induction of an enzyme which removes O6-methylguanine probably accounts for the enrichment of mutations near DNA growing points. MNNG also induces multiple closely linked mutations. The production of multiple mutations but not of single-site mutations is blocked in rec A and uvr E strains. The exact nucleotide site at which DNA synthesis is blocked in vitro by reaction with mutagens can be observed in a phi X174 system in which the nucleotide sequence is known. DNA polymerase I catalyzed synthesis is blocked one nucleotide before the reacted base on the template strand. In contrast, with some damaged templates, AMV reverse transcriptase can insert a base at the level of the reacted nucleotide on the template.

Endonuclease alpha from Saccharomyces cerevisiae shows increased activity on ultraviolet irradiated native DNA

Endonuclease alpha isolated from the nucleus of the yeast Saccharomyces cerevisiae is a DNA endonuclease which has been shown to act preferentially on denatured T7 DNA. The purified enzyme is more active with UV-irradiated native T7 DNA than with unirradiated substrate. The relation between damage, measured by pyrimidine dimer concentration, and excess endonuclease activity is most readily explained by local denaturation caused by presence of pyrimidine dimers. When three radiation sensitive mutants of yeast were tested for the level of endonuclease alpha present, none were found lacking the enzyme. However, nuclei of strain rad 1-1, a mutant that may be defective in heteroduplex repair as well as excision repair, were found to contain reduced levels of the endonuclease. The enzyme isolated from this strain had less than one half the specific activity of similar preparations from wild type yeast.

Genetic activity of chemicals in yeast: DNA alterations and mutations induced by alkylating anti-cancer agents

The simple eukaryotic organism baker's yeast allows demonstration of primary DNA lesions in parallel with measurement of mutagenicity and lethality after treatment with alkylating chemicals. Several anti-cancer drugs formed cross-linked DNA molecules and were genetically active. The mutagenicity and lethality of these drugs varied substantially and were dependent on the function of some processes of DNA dark-repair.