HEAT-STRESS TRANSCRIPTION FACTORS FROM TOMATO CAN FUNCTIONALLY REPLACE HSF1 IN THE YEAST SACCHAROMYCES-CEREVISIAE

The fact that yeast HSF1 is essential for survival under nonstress con ditions can be used to test heterologous Hsfs for the ability to subst itute for the endogenous protein. Our results demonstrate that like Hs f of Drosophila, tomato Hsfs A1 and A2 can functionally replace the co rresponding yeast protein, but Hsf B1 cannot. In addition to survival at 28 degrees C, we checked the transformed yeast strains for temperat ure sensitivity of growth, induced thermotolerance and activator funct ion using two different lacZ reporter constructs. Tests with full-leng th Hsfs were supplemented by assays using mutant Hsfs lacking parts of their C-terminal activator region or oligomerization domain, or conta ining amino acid substitutions in the DNA-binding domain. Remarkably, results with the yeast system are basically similar to those obtained by the analysis of the same Hsfs as transcriptional activators in a to bacco protoplast assay. Most surprising is the failure of HsfB1 to sub stitute for the yeast Hsf. The defect can be overcome by addition to H sfB1 of a short C-terminal peptide motif from HsfA2 (34 amino acid res idues), which represents a type of minimal activator necessary for int eraction with the yeast transcription apparatus. Deletion of the oligo merization domain (HR-A/B) does not interfere with Hsf function for su rvival or growth at higher temperatures. But monomeric Hsf has a marke dly reduced affinity for DNA, as shown by lacZ reporter and band-shift assays.