Tomato Heat Stress Transcription Factor HsfB1 Represents a Novel Type of General Transcription Coactivator with a Histone-Like Motif Interacting with the Plant CREB Binding Protein Ortholog HAC1[W]

Abstract

In contrast with the class A heat stress transcription factors (HSFs) of plants, a considerable number of HSFs assigned to classes B and C have no evident function as transcription activators on their own. However, in the following article, we provide evidence that tomato (Lycopersicon peruvianum) HsfB1 represents a novel type of coactivator cooperating with class A HSFs (e.g., with tomato HsfA1). Provided the appropriate promoter architecture, the two HSFs assemble into an enhanceosome-like complex, resulting in strong synergistic activation of reporter gene expression. Moreover, HsfB1 also cooperates in a similar manner with other activators, for example, with the ASF1/2 enhancer binding proteins of the 35S promoter of Cauliflower mosaic virus or with yet unidentified activators controlling housekeeping gene expression. By these effects, HsfB1 may help to maintain and/or restore expression of certain viral or housekeeping genes during ongoing heat stress. The coactivator function of HsfB1 depends on a histone-like motif in its C-terminal domain with an indispensable Lys residue in the center (GRGKMMK). This motif is required for recruitment of the plant CREB binding protein (CBP) ortholog HAC1. HsfA1, HsfB1, and HAC1/CBP form ternary complexes in vitro and in vivo with markedly enhanced efficiency in promoter recognition and transcription activation in plant and mammalian (COS7) cells. Using small interfering RNA–mediated knock down of HAC1 expression in Arabidopsis thaliana mesophyll protoplasts, the crucial role for the coactivator function of HsfB1 was confirmed.

Role of Heat Stress Transcription Factor HsfA5 as Specific Repressor of HsfA4

“…functional DNA binding and oligomerization domains as well as the C-terminal activator domain with AHA motifs, are basically similar with those defined for HsfA1 and HsfA2 (18,22). Therefore, we wanted to know whether the repressor effect of HsfA5 could also act on HsfA1, provided the OD regions are compatible for interaction.…”

“…Expression constructs for GST pull-down bait proteins were based on plasmid pGEX-4T-1 (Amersham Biosciences, Freiburg, Germany) and generated by in-frame fusions of DNA fragments encoding C-terminal parts of tomato Hsfs A4b (amino acids residues 112-393) and A5 (amino acids residues 110 -478), respectively. Full-length HsfA1 and 3HA-HsfA5 encoding sequences were cloned in pJC vectors (22).…”

“…Detailed investigations on the structural and functional diversification so far are restricted to Hsfs A1, A2, A3, A9, and B1 (10,11,(13)(14)(15)(16)(17)(18)(19)(20)(21)(22)(23)(24). 3 An interesting new aspect of functional diversification of Hsfs was reported when a mutant of rice (spl7) with spontaneous necrotic lesions in mature leaves was identified as a point mutation with a Trp Ͼ Cys substitution in the N-terminal part of the DNA binding domain of HsfA4d.…”

HsfB2b-mediated repression ofPRR7directs abiotic stress responses of the circadian clock

“…We used a construct encoding a GFP C-terminal tag under the control of the UBIQUITIN10 (UBQ10) promoter (proUBQ10:Hsf-GFP), which has intermediate strength compared with the popular 35S constitutive promoter. The 35S sequence was reported to contain potential HSE elements, and the overexpression of HsfB2b can lead to cell death (20,24).…”

HsfA2 Controls the Activity of Developmentally and Stress-Regulated Heat Stress Protection Mechanisms in Tomato Male Reproductive Tissues

“…In Arabidopsis, the master regulator function is shared among HsfA1a, HsfA1b, and HsfA1d as concluded from thermotolerance assays in single and multiple gene knockout mutants (Liu et al, 2011;Yoshida et al, 2011). In addition, Arabidopsis HsfB1 acts as transcriptional repressor during the attenuation phase of HSR, while tomato HsfB1 possesses both coactivator and repressor functions (Bharti et al, 2004;Ikeda et al, 2011).…”