To study some mechanisms underlying the stress responses in eukaryotic cells, we investigated the effect of heat shock (HS) on the induction of DNA double strand breaks as well as on potentially lethal and mutagenic events induced by the radiomimetic antibiotic bleomycin (BLM) in Saccharomyces cerevisiae. Haploid wild-type yeast cells in the logarithmic phase of growth were exposed to different concentrations of BLM (0-30 microg/ml, 1.5 h) without and with a previous HS (38 degrees C, 1 h). Immediately after treatments, survival as well as mutation frequency were determined, and quantitative analysis of chromosomal DNA by laser densitometry were performed both immediately after treatments and after incubation of cells during different time intervals in liquid nutrient medium free of BLM. Our results indicate that HS induces resistance to potentially lethal and mutagenic effects of BLM. Quantitative analysis of chromosomal DNA performed immediately after treatments showed the same DNA fragmentation, either upon BLM as single agent or preceded by HS. However, HS pretreated cells incubated during 4 h in liquid nutrient medium free of BLM repaired DNA double strand breaks more efficiently as compared to non-pretreated cells. On this basis, we propose that the observed HS-induced resistance to BLM depends on a regulatory network acting after DNA-induced damage, which includes genes involved in DNA repair, HS response and DNA metabolism.
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