IS THERMOTOLERANCE OF YEAST DEPENDENT ON TREHALOSE ACCUMULATION

During heat stress, trehalose concentration increases in yeast cells i n parallel to thermotolerance. This parallelism suggested that trehalo se mediated thermotolerance. We show in this work that, under certain conditions, trehalose accumulation and increase in thermotolerance do not go in parallel. A mutant deficient in the trehalose-degrading neut ral trehalase shows, after shift from 40 degrees C to 30 degrees C, lo w thermotolerance in spite of a high trehalose concentration. When glu cose is added to stationary yeast cells with high trehalose concentrat ion and high thermotolerance, trehalose concentration decreases while thermotolerance remains high. A mutant deficient in ubiquitin-conjugat ing genes, ubc4ubc5, shows during exponential growth a low trehalose c oncentration, but a high thermotolerance, in contrast to wild-type cel ls. Because the ubc4ubc5 mutant synthesizes heat-shock proteins consti tutively, it is proposed that, under these conditions, accumulation of heat-shock proteins, and not trehalase, mediates thermotolerance.