A putativeHsp100 gene was cloned from the fungusPleurotus sajor-caju. mRNA expression studies demonstrated that this gene (designatedPsHsp100) is highly induced by high temperature, induced less strongly by exposure to ethanol, and not induced by drought or salinity. Heat shock induction is detectable at 37‡C and reaches a maximum level at 42‡C.PsHsp100 mRNA levels sharply increased within 15 min of exposure to high temperature, and reached a maximum expression level at 2 h that was maintained for several hours. These results indicate that PsHsp100 could work at an early step in thermotolerance. To examine its function,PsHsp100 was transformed into a temperature-sensitivehsp104 deletion mutantSaccharomyces cerevisiae strain to test the hypothesis that PsHSP100 is an protein that functions in thermotolerance. Overexpression of PsHSP100 complemented the thermotolerance defect of thehsp104 mutant yeast, allowing them survive even at 50‡C for 4 h. These results indicate that PsHSP100 protein is functional as an HSP100 in yeast and could play an important role in thermotolerance inP. sajor-caju.