Dissection of the wheat transcription factor HBP-1a(17) reveals a modular structure for the activation domain

Abstract

The wheat bZIP protein HBP-1a(17) is a putative transcription factor regulating histone gene expression. To delineate the functional domain(s) of this factor, we made a series of effector constructs expressing fusion proteins, in which various portions of HBP-1a(17) are fused to the DNA-binding domain of the yeast transcriptional activator GAL4, in plant cells. When the β-glucuronidase (GUS) reporter gene, driven by the wheat histone H3 core promoter harboring the GAL4-binding sequence, was co-transfected with such effector genes into tobacco protoplasts, several portions of HBP-1a(17) influenced reporter gene expression. The N-terminal one-third of HBP-1a(17), termed the P region (residues 1–118) due to its Pro content, did not activate the reporter gene, in contrast to the corresponding Pro-rich region of Arabidopsis GBF1 (residues 1–110), which functions as an activation domain. When the P region was divided into two, however, both its N-terminal (1–56; termed NP) and C-terminal (58–118; termed PC) halves were able to enhance expression of the reporter gene. When the NP region was further divided into NP(5–30) and NP(30–56), both regions still retained activating ability. These results suggest that the P region of HBP-1a(17) is composed of several modules each having activating function, and modification and/or conformational changes of the P region might influence its function.