Plant heat shock transcription factors: positive and negative aspects of regulation

Six heat shock transcription factors (HSFs) have been isolated and characterized from soybean and two from Arabidopsis (Czarnecka-Verner et al. 1995; Barros, Czarnecka-Verner and Gurley, unpublished). Based on a phylogeny analysis of the DNA binding domains and organization of oligomerization domains, they have been assigned to classes A2 and B of the plant HSF family (Nover et al. 1996). In vivo studies of full length HSFs were conducted in transient expression systems using a GUS reporter driven by a heat shock element (HSE) located upstream from the minimal 35S CaMV promoter. Neither soybean nor Arabidopsis HSF class B members were able to function as transcriptional activators under control or heat stress conditions. Conversely, class A HSFs from tomato and Arabidopsis have the capacity to activate transcription. Co-expression studies of activator HSFs from tomato and Arabidopsis, and inert HSFs from soybean and Arabidopsis demonstrated that the inert HSFs were able to trans-attenuate the transcriptional activity of activator HSFs. We suggest that heat shock regulation in plants may differ from metazoan systems by partitioning negative and positive functional domains onto separate HSF proteins. In plants two classes of HSFs exist: class A members, i.e. HSF activators, and a novel class B (inert HSFs) which is largely specialized for repression, or attenuation, of the heat shock response.