Further characterization of the role of Pso2 in the repair of DNA interstrand cross-link-associated double-strand breaks in Saccharomyces cerevisiae.

DNA interstrand cross-links (ICL) are thought to be one of the most lethal forms of DNA damage. Therefore, they present a colossal challenge for the DNA damage response and repair pathways. In Saccharomyces cerevisiae, ICL repair utilizes factors from all of the three major repair groups: nucleotide excision repair (RAD3 epistasis group), post-replication repair (RAD6 epistasis group) and recombinational repair (RAD52 epistasis group). Moreover, there are additional factors significantly influencing the repair of ICL in this organism. These have been designated PSO1-10 based on the psoralen sensitive phenotype of the corresponding mutants. Phenotype of the pso2 mutant suggests that Pso2 is not involved in incision step of ICL repair, but it rather functions in some downstream event such as processing of DNA ends created during generation of ICL-associated double-strand breaks (DSB). In order to address the question whether function of Pso2 in the repair of ICL-associated DSB could be mediated through protein-protein interactions, we have conducted a comprehensive two-hybrid screen examining a possibility of interaction of Pso2 with Yku70, Yku80, Nej1, Lif1, Dnl4, Rad50, Mre11, Xrs2, Rad51, Rad52, Rad54, Rad55, Rad57, Rad59 and Rdh54. Here we show that Pso2 associates with none of the above DSB repair proteins, suggesting that this protein very likely does not act in the repair of ICL-associated DSB via crosstalk with DSB repair machinery. Instead, its function in this process seems to be rather individual.

Disruption of the RAD51 gene sensitizes S. cerevisiae cells to the toxic and mutagenic effects of hydrogen peroxide.

The RAD51 gene was disrupted in three different parental wild-type strains to yield three rad51 null strains with different genetic background. The rad51 mutation sensitizes yeast cells to the toxic and mutagenic effects of H2O2, suggesting that Rad51-mediated repair, similarly to that of RecA-mediated, is relevant to the repair of oxidative damage in S. cerevisiae. Moreover, pulsed-field gel electrophoresis analysis demonstrated that increased sensitivity of the rad51 mutant to H2O2 is accompanied by its decreased ability to repair double-strand breaks induced by this agent. Our results show that ScRad51 protects yeast cells from H2O2-induced DNA double-strand breakage.

Effect of expression of the Escherichia coli nth gene in Saccharomyces cerevisiae on the toxicity of ionizing radiation and hydrogen peroxide.

PURPOSE: To examine the contribution of endonuclease III (Nth)-repairable lesions to the cytotoxicity of ionizing radiation (IR) and hydrogen peroxide (H2O2) in the yeast Saccharomyces cerevisiae. MATERIALS AND METHODS: A selectable expression vector containing the E. coli nth gene was transformed into two different wild-type strains (7799-4B and YNN-27) as well as one rad52 mutant strain (C5-6). Nth expression was verified by Western analysis. Colony-forming assay was used to determine the sensitivity to IR and H2O2 in both stationary and exponentially growing cells. RESULTS: The pADHnth-transformed wild-type (77994B) strain was considerably more resistant than vector-only transformants to the toxic effects of IR, in both stationary and exponential growth phases, although this was not the case in another wild-type strain (YNN-27). In contrast, there were no significant effects of nth expression on the sensitivity of the wild-type cells to H2O2. Moreover, nth expression caused no effects on the H2O2 sensitivity in the rad52 mutant cells, but it led to a slight increase in sensitivity in these cells following IR, particularly at the highest dose levels used. CONCLUSIONS: Whilst other damage-processing systems may play a role, DNA lesions that are substrates for Nth can also make a contribution to the toxic effects of IR in certain wild-type yeast. Hence, DNA double-strand breaks should not be considered the sole lethal lesions following IR exposure.

Increased DNA double strand breakage is responsible for sensitivity of the pso3-1 mutant of Saccharomyces cerevisiae to hydrogen peroxide.

Escherichia coli endonuclease III (endo III) is the key repair enzyme essential for removal of oxidized pyrimidines and abasic sites. Although two homologues of endo III, Ntgl and Ntg2, were found in Saccharomyces cerevisiae, they do not significantly contribute to repair of oxidative DNA damage in vivo. This suggests that an additional activity(ies) or a regulatory pathway(s) involved in cellular response to oxidative DNA damage may exist in yeast. The pso3-1 mutant of S. cerevisiae was previously shown to be specifically sensitive to toxic effects of hydrogen peroxide (H2O2) and paraquat. Here, we show that increased DNA double strand breakage is very likely the basis of sensitivity of the pso3-1 mutant cells to H2O2. Our results, thus, indicate an involvement of the Pso3 protein in protection of yeast cells from oxidative stress presumably through its ability to prevent DNA double strand breakage. Furthermore, complementation of the repair defects of the pso3-1 mutant cells by E. coli endo III has been examined. It has been found that expression of the nth gene in the pso3-1 mutant cells recovers survival, decreases mutability and protects yeast genomic DNA from breakage following H2O2 treatment. This might suggest some degree of functional similarity between Pso3 and Nth.

MNNG-induced [corrected] RecBCD dependent DNA degradation in recA13 mutant cells is not the basis of their hypersensitivity to this agent.

We have examined the hypersensitivity of Escherichia coli recA13 mutant cells to killing by N-methyl-N'-nitro-N-nitro-soguanidine (MNNG) and have shown out that despite MNNG-induced adaptation they remained vastly more sensitive to the cytotoxic effect of this agent than wild type cells. Because this might have been a consequence of a different extent of induction of the adaptive response in the recA13 background, we have measured O6-alkylguanine-DNA alkyltransferase (ATase) activity in extracts of adapted and non-adapted recA13 mutant and wild type cells. Adaptation increased ATase levels by 28- and 34-fold in wild type and recA13 mutant cells, respectively. Thus, the adaptive response was no less inducible in recA13 mutant cells than in wild type cells. This indicates that the extreme sensitivity of recA13 cells to MNNG is not caused by an inability to repair the principal toxic lesions induced in DNA. Low doses of MNNG caused substantial degradation of cellular DNA in recA13 mutant cells but not in the wild type cells. This DNA degradation is shown to be the RecBCD-enzyme dependent. Since recA13 recB21 double mutants were even more sensitive to MNNG than recA single mutants, DNA degradation appears not to be the cause of the MNNG-hypersensitivity in recA13 cells.