Plants contain a novel multi-member class of heat shock factors without transcriptional activator potential

Based on phylogeny of DNA-binding domains and the organization of hydrophob ic repeats, two families of heat shock transcription factors (HSFs) exist i n plants. Class A HSFs are involved in the activation of the heat shock res ponse, but the role of class B HSFs is not clear. When transcriptional acti vities of full-length HSFs were monitored in tobacco protoplasts, no class B HSFs from soybean or Arabidopsis showed activity under control or heat st ress conditions. Additional assays confirmed the finding that the class B H SFs lacked the capacity to activate transcription. Fusion of a heterologous activation domain from human HSF1 (AD2) to the C-terminus of GmHSFB1-34 ga ve no evidence of synergistic enhancement of AD2 activity, which would be e xpected if weak activation domains were present. Furthermore, activity of A tHSFB1-4 (class B) was not rescued by coexpression with AtHSFA4-21 (class A ) indicating that the class A HSF was not able to provide a missing functio n required for class B activity. The transcriptional activation potential o f Arabidopsis AtHSFA4-21 was mapped primarily to a 39 amino acid fragment i n the C-terminus enriched in bulky hydrophobic and acidic residues. Deletio n mutagenesis of the C-terminal activator regions of tomato and Arabidopsis HSFs indicated that these plant HSFs lack heat-inducible regulatory region s analogous to those of mammalian HSF1. These findings suggest that heat sh ock regulation in plants may differ from metazoans by partitioning negative and positive functional domains onto separate HSF proteins. Class A HSFs a re primarily responsible for stress-inducible activation of heat shock gene s whereas some of the inert class B HSFs may be specialized for repression, or down-regulation, of the heat shock response.