Abstract
In plants, a family of more than 20 heat stress transcription factors (Hsf) controls the expression of heat stress (hs) genes. There is increasing evidence for the functional diversification between individual members of the Hsf family fulfilling distinct roles in response to various environmental stress conditions and developmental signals. In response to hs, accumulation of both heat stress proteins (Hsp) and Hsfs is induced. In tomato, the physical interaction between the constitutively expressed HsfA1 and the hs-inducible HsfA2 results in synergistic transcriptional activation (superactivation) of hs gene expression. Here, we show that the interaction is strikingly specific and not observed with other class A Hsfs. Hetero-oligomerization of the two-component Hsfs is preferred to homo-oligomerization, and each Hsf in the HsfA1/HsfA2 hetero-oligomeric complex has its characteristic contribution to its function as superactivator. Distinct regions of the oligomerization domain are responsible for specific homo- and hetero-oligomeric interactions leading to the formation of hexameric complexes. The results are summarized in a model of assembly and function of HsfA1/A2 superactivator complexes in hs gene regulation.
Highlights
We conclude that HsfA1 and HsfA2 form a pair of interacting heat stress transcription factors (Hsf), which collaborate synergistically as a type of superactivator of hs gene expression
Three consecutive phases of gene expression control can be discriminated: (i) Rapid reprogramming of gene expression at the onset of stress connected with a block of housekeeping gene expression, (ii) induction and maintenance of stress gene expression at high levels ensuring the necessary accumulation of chaperones and other protective proteins, and (iii) after return to normal conditions, efficient recovery of housekeeping gene expression is a prerequisite to allow resumption of normal plant development
Formation of HsfA1/A2 hetero-oligomeric complexes leads to synergistic effects on the activation of expression of Hsf-dependent genes, which is usually 3–5-fold higher than the additive activities of the individual partners (Fig. 2 and supplemental Fig. S1)
Summary
Synergistic Gene Expression by HsfA1 and HsfA2—To investigate the specificity of interactions of tomato class A Hsfs, we performed transient reporter assays using an Hsf-dependent hsp17-promoter::GUS reporter construct in tobacco mesophyll protoplasts [22]. In combination with the corresponding partner Hsf, these hybrid constructs allow analysis of the contribution of individual domains to the synergistic activity, by formation of HsfA1/A2 complexes, which carry one domain changed to a homo-oligomeric constellation while all other parts are identical to wild type HsfA1/A2 complexes.
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