THE CARBOXYL-TERMINAL TRANSACTIVATION DOMAIN OF HEAT-SHOCK FACTOR-1 IS NEGATIVELY REGULATED AND STRESS-RESPONSIVE

We have characterized a stress-responsive transcriptional activation d omain of mouse heat shock factor 1 (HSP1) 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 exposur e to heat shock or by deletion of a negative regulatory domain includi ng part of the DNA-binding domain-proximal leucine zippers. Analysis o f a collection of GAL4-HSF1 deletion mutants revealed the minimal regi on for the constitutive transcriptional activator to map within the ex treme carboxyl-terminal 108 amino acids, corresponding to a region ric h in acidic and hydrophobic residues. Loss of residues 395 to 425 or 4 51 to 503, which are located at either end of this activation domain, severely diminished activity, indicating that the entire domain is req uired for transactivation. The minimal activation domain of HSF1 also confers enhanced transcriptional response to heat shock or cadmium tre atment. 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 negat ive regulator and the carboxyl terminal transcriptional activation dom ain.