Heat-induced unresponsiveness of heat shock gene expression is regulated at the transcriptional level.

The induction kinetics of the heat shock proteins hsp68, hsp70 and hsp84 were studied. Studies on hsp mRNA levels and protein synthetic rates, with or without the presence of actinomycin D, showed that regulation took place at the transcriptional level. Hsp mRNA induction was followed by a transient state of unresponsiveness. At the time point where the induced hsp mRNAs were decreasing again, hsp68, hsp70 and hsp84 mRNA could not be induced by a second, identical, heat shock. Hsp68 mRNA could be induced again 12-16 h after the first heat shock. Apparently, this state really seems to be a state of reduced sensitivity, since a higher heat dose could partially overcome this unresponsiveness.