Transcriptional derepression of the Saccharomyces cerevisiae HSP26 gene during heat shock.

hsp26, the small heat shock protein of Saccharomyces cerevisiae, accumulates in response to heat and other types of stress. It also accumulates during the normal course of development, as cells enter stationary phase growth or begin to sporulate (S. Kurtz, J. Rossi, L. Petko, and S. Lindquist, Science 231:1154-1157, 1986). Analysis of deletion and insertion mutations demonstrated that transcriptional control plays a critical role in regulating HSP26 expression. The HSP26 promoter was found to be complex and appears to contain repressing elements as well as activating elements. Several upstream deletion mutations resulted in strong constitutive expression of HSP26. Furthermore, upstream sequences from the HSP26 gene repressed the constitutive expression of a heterologous heat shock gene. We propose that basal repression and heat-induced depression of transcription play major roles in regulating the expression of HSP26. None of the recombinant constructs that we analyzed separated cis-regulatory sequences responsible for heat shock regulation from those responsible for developmental regulation of HSP26. Depression of HSP26 transcription may be the general mechanism of HSP26 induction in yeast cells. This regulatory scheme is very different from that described for the regulation of most other heat shock genes.