The heat shock transcription factor (HSF) is constitutively expressed in Drosophila cells as an inactive monomer. Upon heat shock HSF undergoes trimerization and acquires high affinity DNA binding ability, leading to specific interaction with its cognate elements in heat shock promoters. Here we show that the transactivation function of HSF is conferred by the extreme C-terminal region of the protein. Deletion analysis of HSF fragments fused to the GAL4 DNA-binding domain demonstrates that transactivation is dependent on HSF residues 610-691. This domain is located beyond the C-terminal heptad repeat (leucine zipper 4) whose presence or integrity is dispensable for transactivation. The transactivation domain is functional in the absence of heat shock and can be replaced by the extreme C-terminal region of human HSF1. The Drosophila and human HSF transactivation domains are both rich in hydrophobic and acidic residues and may be structurally conserved, despite limited sequence identity.
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