Arabidopsis heat shock factor is constitutively active in Drosophila and human cells

Heat shock factors (HSF) are the transcriptional activators of the heat shock response. The conversion of constitutively expressed HSF to a form that can bind DNA requires the trimerization of the protein, involving leucine zipper interactions as shown for yeast, Drosophila, chicken and human HSFs. Like other metazoan HSFs, the endogenous Arabidopsis HSF displays heat shock-inducible DNA-binding activity in gel retardation assays. The heat shock-inducible binding of a recombinant Arabidopsis HSF (ATHSF1) expressed in Arabidopsis plants suggests that ATHSF1 is the major heat shock factor regulating the heat stress response. However, on transient expression in Drosophila and human cells, ATHSF1 fails to exhibit proper regulation, as demonstrated by constitutive binding to DNA, and by constitutive expression of a chloramphenicol acetyltransferase (CAT) reporter gene under the control of the Drosophila hsp70 promoter. These results suggest that the regulation of ATHSF1 is normally dependent on a specific factor that inhibits the DNA-binding and transcriptional activities under non-heat shock conditions.