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The Mre11/Rad50/Xrs2 (MRX) complex in Saccharomyces cerevisiae has well-characterized functions in DNA double-strand break processing, checkpoint activation, telomere length maintenance and meiosis. In this study, we demonstrate an involvement of the complex in the base excision repair (BER) pathway. We studied the repair of methyl-methanesulfonate-induced heat-labile sites in chromosomal DNA in vivo and the in vitro BER capacity for the repair of uracil- and 8-oxoG-containing oligonucleotides in MRX-deficient cells. Both approaches show a clear BER deficiency for the xrs2 mutant as compared to wildtype cells. The in vitro analyses revealed that both subpathways, long-patch and short-patch BER, are affected and that all components of the MRX complex are similarly important for the new function in BER. The investigation of the epistatic relationship of XRS2 to other BER genes suggests a role of the MRX complex downstream of the AP-lyases Ntg1 and Ntg2. Analysis of individual steps in BER showed that base recognition and strand incision are not affected by the MRX complex. Reduced gap-filling activity and the missing effect of aphidicoline treatment, an inhibitor for polymerases, on the BER efficiency indicate an involvement of the MRX complex in providing efficient polymerase activity.

NBS1 fulfills important functions for the maintenance of genomic stability and cellular survival. Mutations in the NBS1 (Nijmegen Breakage Syndrome 1) gene are responsible for the Nijmegen breakage syndrome (NBS) in humans. The symptoms of this disease and the phenotypes of NBS1-defective cells, especially their enhanced radiosensitivity, can be explained by an impaired DNA double-strand break-induced signaling and a disturbed repair of these DNA lesions. We now provide evidence that NBS1 is also important for cellular survival after oxidative or alkylating stress where it is required for the proper initiation of base excision repair (BER). NBS1 downregulated cells show reduced activation of poly-(adenosine diphosphate-ribose)-polymerase-1 (PARP1) following genotoxic treatment with H(2)O(2) or methyl methanesulfonate, indicating impaired processing of damaged bases by BER as PARP1 activity is stimulated by the single-strand breaks intermediately generated during this repair pathway. Furthermore, extracts of these cells have a decreased capacity for the in vitro repair of a double-stranded oligonucleotide containing either uracil or 8-oxo-7,8-dihydroguanine to trigger BER. Our data presented here highlight for the first time a functional role for NBS1 in DNA maintenance by the BER pathway.

Inactivation of the Saccharomyces cerevisiae gene YKU70 (HDF1), which encodes one subunit of the Ku heterodimer, confers a DNA double-strand break repair defect, shortening of and structural alterations in the telomeres, and a severe growth defect at 37°. To elucidate the basis of the temperature sensitivity, we analyzed subclones derived from rare yku70 mutant cells that formed a colony when plated at elevated temperature. In all these temperature-resistant subclones, but not in cell populations shifted to 37°, we observed substantial amplification and redistribution of subtelomeric Y′ element DNA. Amplification of Y′ elements and adjacent telomeric sequences has been described as an alternative pathway for chromosome end stabilization that is used by postsenescence survivors of mutants deficient for the telomerase pathway. Our data suggest that the combination of Ku deficiency and elevated temperature induces a potentially lethal alteration of telomere structure or function. Both in yku70 mutants and in wild type, incubation at 37° results in a slight reduction of the mean length of terminal restriction fragments, but not in a significant loss of telomeric (C1-3A/TG1-3)n sequences. We propose that the absence of Ku, which is known to bind to telomeres, affects the telomeric chromatin so that its chromosome end-defining function is lost at 37°.

Radiation-induced chromosome aberrations, particularly exchange-type aberrations, are thought to result from misrepair of DNA double-strand breaks. The relationship between individual pathways of break repair and aberration formation is not clear. By electrophoretic karyotyping of single-cell clones derived from irradiated cells, we have analyzed the induction of stable aberrations in haploid yeast cells mutated for the RAD52 gene, the RAD54 gene, the HDF1(=YKU70) gene, or combinations thereof. We found low and comparable frequencies of aberrational events in wildtype and hdf1 mutants, and assume that in these strains most of the survivors descended from cells that were in G2 phase during irradiation and therefore able to repair breaks by homologous recombination between sister chromatids. In the rad52 and the rad54 strains, enhanced formation of aberrations, mostly exchange-type aberrations, was detected, demonstrating the misrepair activity of a rejoining mechanism other than homologous recombination. No aberration was found in the rad52 hdf1 double mutant, and the frequency in the rad54 hdf1 mutant was very low. Hence, misrepair resulting in exchange-type aberrations depends largely on the presence of Hdf1, a component of the nonhomologous end-joining pathway in yeast.

We have applied fermenter techniques to analyse the variations of glutathione (GSH) content in cultures of the diploid strain D7 of Saccharomyces cerevisiae. Choosing various experimental conditions of controlled batch and fed-batch fermentation we give evidence that the GSH levels of the yeast cultures depend on growth phase, the carbon source supply and the carbon source metabolism in an unexpectedly complex manner. Additionally, we analysed yeast cells with low GSH levels which were obtained either by depleting GSH with chloroacetophenone (CN) chemically or by using a GSH-deficient diploid strain (gsh1/gsh1). In order to study the relevance of the factors influencing the GSH concentration for genotoxicity testing in yeast we have used N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) which is activated by GSH. We show that in cells which are GSH proficient the extent of genotoxicity of MNNG correlates well with the GSH levels in the cells. Conditions of high GSH content (stationary phase of growth) corresponds with high genotoxic activity of MNNG, whereas conditions of low GSH content as logarithmic growth, glucose repression, GSH deficiency caused by the gsh1 mutation and GSH depletion by CN treatment correspond with a very moderate genotoxic effect of MNNG. These findings emphasize the necessity to use metabolically highly standardized cells for genotoxicity testing, since the carbon source catabolism, the concentration of glucose, growth rate and possibly other parameters influence the metabolization of xenobiotic agents in yeast.