The carboxyl-terminal transactivation domain of heat shock factor 1 is negatively regulated and stress responsive.

We have characterized a stress-responsive transcriptional activation domain of mouse heat shock factor 1 (HSF1) by using chimeric GAL4-HSF1 fusion proteins. Fusion of the GAL4 DNA-binding domain to residues 124 to 503 of HSF1 results in a chimeric factor that binds DNA yet lacks any transcriptional activity. Transactivation is acquired upon exposure to heat shock or by deletion of a negative regulatory domain including part of the DNA-binding-domain-proximal leucine zippers. Analysis of a collection of GAL4-HSF1 deletion mutants revealed the minimal region for the constitutive transcriptional activator to map within the extreme carboxyl-terminal 108 amino acids, corresponding to a region rich in acidic and hydrophobic residues. Loss of residues 395 to 425 or 451 to 503, which are located at either end of this activation domain, severely diminished activity, indicating that the entire domain is required for transactivation. The minimal activation domain of HSF1 also confers enhanced transcriptional response to heat shock or cadmium treatment. These results demonstrate that the transcriptional activation domain of HSF1 is negatively regulated and that the signal for stress induction is mediated by interactions between the amino-terminal negative regulator and the carboxyl-terminal transcriptional activation domain.