Isolation and characterization of three mutants with increased sensitivity to photoactivated 3-carbethoxypsoralen in Saccharomyces cerevisiae

Antioxidant activity of diphenyl diselenide prevents the genotoxicity of several mutagens in Chinese hamster V79 cells

Diphenyl diselenide (DPDS) is an electrophilic reagent used in the synthesis of a variety of pharmacologically active organic selenium compounds. Studies have shown its antioxidant, hepatoprotective, neuroprotective, anti-inflammatory, and antinociceptive effects. We recently showed the antioxidant effect of DPDS in V79 cells, and established the beneficial and toxic doses of this compound in this cell line. Here, we report the antigenotoxic and antimutagenic properties of DPDS, investigated by using a permanent lung fibroblast cell line derived from Chinese hamsters. We determined the cytotoxicity by clonal survival assay, and evaluated DNA damage in response to several mutagens by comet assay and micronucleus test in binucleated cells. In the clonal survival assay, at concentrations ranging from 1.62 to 12.5microM, DPDS was not cytotoxic, while at concentrations up to 25microM, it significantly decreased survival. The treatment with this organoselenium compound at non-cytotoxic dose range increased cell survival after challenge with hydrogen peroxide, methyl-methanesulphonate, and UVC radiation, but did not protect against 8-methoxypsoralen plus UVA-induced cytotoxicity. In addition, the treatment prevented induced DNA damage, as verified in the comet assay. The mutagenic effect of these genotoxins, as measured by the micronucleus test, similarly attenuated or prevented cytotoxicity and DNA damage. Treatment with DPDS also decreased lipid peroxidation levels after exposure to hydrogen peroxide MMS, and UVC radiation, and increased glutathione peroxidase activity in the extracts. Our results clearly demonstrate that DPDS at low concentrations presents antimutagenic properties, which are most probably due to its antioxidant properties.

Specific transcriptional responses induced by 8-methoxypsoralen and UVA in yeast

Treatment of eukaryotic cells with 8-methoxypsoralen plus UVA irradiation (8-MOP/UVA) induces pyrimidine monoadducts and interstrand crosslinks and initiates a cascade of events leading to cytotoxic, mutagenic and carcinogenic responses. Transcriptional activation plays an important part in these responses. Our previous study in Saccharomyces cerevisiae showed that the repair of these lesions involves the transient formation of DNA double-strand breaks and the enhanced expression of landmark DNA damage response genes such as RAD51, RNR2 and DUN1, as well as the Mec1/Rad53 kinase signaling cascade. We have now used DNA microarrays to examine genome-wide transcriptional changes produced after induction of 8-MOP/UVA photolesions. We found that 128 genes were strongly induced and 29 genes strongly repressed. Modifications in gene expression concern numerous biological processes. Compared to other genotoxic treatments, c. 42% of the response genes were specific to 8-MOP/UVA treatment. In addition to common DNA damage response genes and genes induced by environmental stresses, a large fraction of 8-MOP/UVA response genes correspond to membrane-related functions.

DNA interstrand cross-link repair in Saccharomyces cerevisiae

DNA interstrand cross-links (ICL) present a formidable challenge to the cellular DNA repair apparatus. For Escherichia coli, a pathway which combines nucleotide excision repair (NER) and homologous recombination repair (HRR) to eliminate ICL has been characterized in detail, both genetically and biochemically. Mechanisms of ICL repair in eukaryotes have proved more difficult to define, primarily as a result of the fact that several pathways appear compete for ICL repair intermediates, and also because these competing activities are regulated in the cell cycle. The budding yeast Saccharomyces cerevisiae has proven a powerful tool for dissecting ICL repair. Important roles for NER, HRR and postreplication/translesion synthesis pathways have all been identified. Here we review, with reference to similarities and differences in higher eukaryotes, what has been discovered to date concerning ICL repair in this simple eukaryote.

Role of PSO genes in repair of DNA damage of Saccharomyces cerevisiae

Photoactivated psoralens used in treatment of skin diseases like Psoriasis and Vitiligo cause DNA damage, the repair of which may lead to mutations and thus to higher risk to have skin cancer. The simple eukaryote Saccharomyces cerevisiae was chosen to investigate the cells' genetic endowment with repair mechanisms for this type of DNA damage and to study the genetic consequences of such repair. Genetic studies on yeast mutants sensitive to photoactivated psoralens, named pso mutants, showed their allocation to 10 distinct loci. Cloning and molecular characterization allowed their grouping into three functional classes: (I) the largest group comprises seven PSO genes that are either generally or specifically involved in error-prone DNA repair and thus affect induced mutability and recombination; (II) one PSO gene that represents error-free excision repair, and (III) two PSO genes encoding proteins not influencing DNA repair but physiological processes unrelated to nucleic acid metabolism. Of the seven DNA repair genes involved in induced mutagenesis three PSO loci [PSO1/REV3, PSO8/RAD6, PSO9/MEC3] were allelic to already known repair genes, whereas three, PSO2/SNM1, PSO3/RNR4, and PSO4/PRP19 represent new genes involved in DNA repair and nucleic acid metabolism in S. cerevisiae. Gene PSO2 encodes a protein indispensable for repair of interstrand cross-link (ICL) that are produced in DNA by a variety of bi- and polyfunctional mutagens and that appears to be important for a likewise repair function in humans as well. In silico analysis predicts a putative endonucleolytic activity for Pso2p/Snm1p in removing hairpins generated as repair intermediates. The absence of induced mutation in pso3/rnr4 mutants indicates an important role of this subunit of ribonucleotide reductase (RNR) in regulation of translesion polymerase zeta in error-prone repair. Prp19p/Pso4p influences efficiency of DNA repair via splicing of pre-mRNAs of intron-containing repair genes but also may function in the stability of the nuclear scaffold that might influence DNA repair capacity. The seventh gene, PSO10 which controls an unknown step in induced mutagenesis is not yet cloned. Two genes, PSO6/ERG3 and PSO7/COX11, are responsible for structural elements of the membrane and for a functional respiratory chain (RC), respectively, and their function thus indirectly influences sensitivity to photoactivated psoralens.

Interaction of the yeast Pso5/Rad16 and Sgs1 proteins: influences on DNA repair and aging

The interaction trap method was used to isolate putative binding partners of Rad16/Pso5, a protein responsible for repair of silent DNA. One of the interactors found was Sgs1, a DNA helicase influencing the life span of Saccharomyces cerevisiae, with homology to the human BLM, WRN and RECQL4 proteins. Using the same fusion proteins from the two-hybrid screening, we show evidence that both proteins also interact in vitro. We tested isogenic strains, containing mutant alleles of the two genes in single and double mutant combination, for phenotypic similarity. Life span in sgs1Delta single and sgs1Delta rad16Delta double mutants is about 40% of that of WT, and the rad16/pso5Delta single mutant also had its life span reduced to 75%. Sensitivity to different mutagens, whose lesions are poorly repaired in rad16/pso5Delta mutants, was tested in sgs1Delta mutants. The sgs1Delta conferred sensitivity to MMS, H2O2 and was moderately sensitive to UV(254nm) (UVC) and 4-NQO. An epistatic interaction between rad16 and sgs1 mutations after UVC, 4-NQO and H2O2 was observed. Moreover, we found that in a top3 background, functional Sgs1p and Rad16p apparently channel MMS, 4-NQO and H2O2 induced lesions into aberrant DNA repair. Our results demonstrate that Sgs1 is not only involved in genome stability, somatic recombination and aging, but is also implicated, together with Rad16/Pso5, in the repair of specific DNA damage.

Oxidative stress responses of the yeast Saccharomyces cerevisiae

All aerobically growing organisms suffer exposure to oxidative stress, caused by partially reduced forms of molecular oxygen, known as reactive oxygen species (ROS). These are highly reactive and capable of damaging cellular constituents such as DNA, lipids and proteins. Consequently, cells from many different organisms have evolved mechanisms to protect their components against ROS. This review concentrates on the oxidant defence systems of the budding yeast Saccharomyces cerevisiae, which appears to have a number of inducible adaptive stress responses to oxidants, such as H2O2, superoxide anion and lipid peroxidation products. The oxidative stress responses appear to be regulated, at least in part, at the level of transcription and there is considerable overlap between them and many diverse stress responses, allowing the yeast cell to integrate its response towards environmental stress.

Role of PSO genes in the repair of photoinduced interstrand cross-links and photooxidative damage in the DNA of the yeast Saccharomyces cerevisiae

Recent progress in elucidating the molecular structure of the PSSO genes PSO2 to PSO7 is presented. Their role in DNA repair and mutagenesis is discussed in the light of the putative proteins encoded in the respective ORFs and with the knowledge of recent progress in biological and biochemical experimentation. The role of the RecA protein in some steps of DNA repair in Saccharomyces cerevisiae is presented and discussed.

Gene PSO5 of Saccharomyces cerevisiae, involved in repair of oxidative DNA damage, is allelic to RAD16

The pos5-1 mutation renders Saccharomyces cerevisiae cells sensitive to DNA-damaging agents. We have isolated plasmids from a S. cerevisiae genomic library capable of restoring wild-type levels of 254-nm ultraviolet light sensitivity of the pso5-1 mutant. DNA sequence analysis revealed that the complementing activity resides in RAD16, a gene involved in excision repair. Tetrad analysis showed that PSO5, like RAD16, is tightly linked to LYS2 on chromosome II. Moreover, allelism between the pso5-1 and rad16 mutants was demonstrated by the comparison of mutagen sensitivity phenotypes, complementation tests, and by meiotic analysis. The cloned RAD16 gene was capable of restoring wild-type resistance of the pso5-1 mutant to H2O2 and photoactivated 3-carbethoxypsoralen, both treatments generating oxidative stress-related DNA damage. This indicates that RAD16/PSO5 might also participate in the repair of oxidative base damage.