Activation of heat shock transcription factor in yeast is not influenced by the levels of expression of heat shock proteins

Heat shock transcription factor (HSF) transiently induces the expression of a universally conserved set of proteins, the heat shock proteins (Hsps), when cells are exposed to elevated temperatures as well as to a wide range of other environmental stresses. The tight control of heat shock gene expression has prompted a model, according to which HSF activity and 'free' heat shock protein levels are tied up in a regulatory loop. Other data have indicated that HSF senses stress directly. Here, we report that yeast cells in which the basal expression levels of Hsps have been significantly increased exhibit improved thermotolerance but display no detectable difference in the temperature required for transient activation of HSF. In a separate experiment, overexpression of SSA2, a member of the Hsp70 family and a prominent candidate for the feedback regulation of HSF, did not inhibit the heat shock response. Our findings challenge the dogma that relief of the suppression of HSF activity by Hsps can account for the acute heat shock response.