Alterations in cellular lipids may be responsible for the transient nature of the yeast heat shock response

The stress-sensing systems leading to the cellular heat shock response (HSR) and the mechanism responsible for the desensitizing of this response in stress-acclimated cells are largely unknown. Here it is demonstrated that there is a close correlation between a 3 degrees C increase in the temperature required for maximal activation of a heat-shock (HS)-inducible gene in Saccharomyces cerevisiae and an increase in the percentage of cellular unsaturated fatty acids when cells are subjected to extended periods of growth at 37 degrees C. The latter occurs with the same kinetics as HS gene down-regulation during a prolonged HS and is reversed by reacclimation to growth at 25 degrees C. The transient nature of the HS may therefore be due to a lipid-mediated decrease in cellular heat sensitivity. Further evidence that unsaturated fatty acids desensitize cells to heat, with a resultant down-regulation of the HSR, is provided by demonstrating a 9 degrees C increase in the temperature required for maximal induction of this HS-inducible gene in cells containing high levels of unsaturated fatty acids assimilated during anaerobic growth at 25 degrees C.