A conserved heptapeptide restrains the activity of the yeast heat shock transcription factor

In yeast, expression of heat shock genes is regulated by a factor (HSF) which binds constitutively to DNA, but activates transcription efficiently only after heat shock. We have compared the HSFs from Saccharomyces cerevisiae and Kluyveromyces lactis. Both factors contain an activation domain whose activity is masked at low temperature, but the amino acid sequences of these activators are unrelated. Masking requires the evolutionarily conserved DNA binding and oligomerization domains, as well as a short conserved element close to the activator. Although this element contains potential phosphorylation sites, they are not required for induction. We suggest that the conserved element binds either to the structural core of the protein or to another polypeptide, holding the activator in an inactive configuration, and that high temperatures disrupt this interaction. Our results emphasize the importance of global protein structure in the regulation of transcription factor activity.