Plant class B HSFs inhibit transcription and exhibit affinity for TFIIB and TBP

Plant heat shock transcription factors (HSFs) are capable of transcriptional activation (class A HSFs) or both, activation and repression (class B HSFs). However, the details of mechanism still remain unclear. It is likely, that the regulation occurs through interactions of HSFs with general transcription factors (GTFs), as has been described for numerous other transcription factors. Here, we show that class A HSFs may activate transcription through direct contacts with TATA-binding protein (TBP). Class A HSFs can also interact weakly with TFIIB. Conversely, class B HSFs inhibit promoter activity through an active mechanism of repression that involves the C-terminal regulatory region (CTR) of class B HSFs. Deletion analysis revealed two sites in the CTR of soybean GmHSFB1 potentially involved in protein-protein interactions with GTFs: one is the repressor domain (RD) located in the N-terminal half of the CTR, and the other is a TFIIB binding domain (BD) that shows affinity for TFIIB and is located C-terminally from the RD. A Gal4 DNA binding domain-RD fusion repressed activity of LexA-activators, while Gal4-BD proteins synergistically activated strong and weak transcriptional activators. In vitro binding studies were consistent with this pattern of activity since the BD region alone interacted strongly with TFIIB, and the presence of RD had an inhibitory effect on TFIIB binding and transcriptional activation.