In this study, a genetic screen for the intragenic suppressors of the gain-of-function mutant bzr1-1D reveals an important function of an evolutionarily conserved ERF-associated amphiphilic repression (EAR) motif in BZR1 regulation of target gene expression. However, it remains unclear how BZR1 represses gene expression, and specifically how its gene repression and activation activities are mediated by interactions with general transcriptional regulators. In addition, recent studies have suggested that myeloblastosis transcription factor like2 (MYBL2) and homeodomain-leucine zipper protein1 (HAT1) interact with BZR2/BES1 and repress several BR target genes 20, 21. Several other transcription factors and histone-modifying enzymes have been reported to interact with BZR2/BES1 and participate in BZR2/BES1 activation of gene expression 17, 18, 19. The BZR1-PIF4 and BZR2-BIM1 heterodimers bind to the E-box motif in the promoters of target genes and activate gene expression 13, 16. BZR2/BES1 also interacts with BES1-interacting Myc-like1 (BIM1). BZR1 and its homologous BZR2/BES1 interact with the bHLH factor phytochrome interacting factor4 (PIF4) and BZR1 shares with PIF4 over 50% of their target genes in the genome. Indeed the genome-wide data showed that BR response element is enriched in the promoters of BR-repressed gene, whereas the E-box motif (CANNTG), a binding site for bHLH factors, is more enriched in the promoters of BR-activated genes 15. Apparently, BZR1 transcriptional activity is partly determined by the context of promoter cis-elements and trans-factors. Previous genome-wide identification of direct binding sites of BZR1 coupled with transcriptome profiling showed that BZR1 binds to promoters of both BR-repressed and -induced genes 5, 15. SRDX RECOGNITION DOMAIN ACTIVATORSimilar to many plant transcription factors, BZR1 functions as a transcriptional repressor for some promoters but an activator for others. BZR2 (also named BES1), the close homologue of BZR1, is regulated through similar mechanisms 6, 12, 13, 14. Dephosphorylated BZR1 enters the nucleus and binds to target promoters to mediate BR-responsive gene expression 3. 11), resulting in dephosphorylation of BZR1 catalysed by protein phospatase 2A (PP2A) 12. BR binding to BRI1 triggers a signalling cascade that leads to inactivation of BIN2 (ref. The phosphorylated BZR1 is unable to bind DNA and also is retained in the cytosol due to binding to the 14-3-3 proteins 9, 10. In the absence of BR, a glycogen synthase kinase 3 (GSK3)-like kinase named BIN2 interacts with BZR1 via a docking motif at the C terminus of BZR1 and phosphorylates BZR1 (refs 6, 7, 8). Both subcellular localization and DNA-binding activity of BZR1 are tightly regulated by phosphorylation. The mechanisms that specify BZR1’s function in transcription activation and repression are not understood.īZR1 is a plant-specific transcription factor with a DNA-binding domain that recognizes BR response element (CGTG(T/C)G) 5. BZR1 activates and represses different target genes to programme genome expression and cell growth. BRI1 signalling activates the members of the brassinazole resistant1 (BZR1) family of transcription factors through a phosphorylation-mediated signal transduction pathway 3, 4. Unlike animal steroid hormones, which directly bind to nuclear receptor transcription factors, the plant steroid hormone brassinosteroid (BR) binds to a transmembrane receptor kinase, brassinosteroid insensitive1 (BRI1) 1, which has a leucine-rich repeat extracellular domain similar to the Toll receptors in metazoans 2. Steroids are important hormones for regulation of gene expression and development in both animals and plants. Our study demonstrates key roles of the EAR motif and TPL in BR regulation of gene expression and plant growth. BR repression of gene expression also requires histone deacetylases that interact with TPL. A triple tpl mutant ( tpl tpr1 tpr4) shows reduced BR sensitivity and suppresses the gain-of-function bzr1-1D mutant phenotype. The EAR motif in BZR1 mediates recruitment of TPL to BZR1-repressed promoters. Specific deletion or mutation of an evolutionarily conserved ERF-associated amphiphilic repression (EAR) motif at the carboxy terminus abolishes BZR1’s abilities to regulate gene expression and cell elongation, but these defects are rescued by TPL fusion to the EAR motif-mutated BZR1. Here we show that BZR1 represses target genes by recruiting the Groucho/TUP1-like transcriptional corepressor TOPLESS (TPL). The mechanisms that determine BZR1’s transcriptional activities remain largely unknown. Brassinosteroid (BR) regulates plant development by activating the transcription factor brassinazole resistant 1 (BZR1), which activates and represses different target genes to switch cellular programmes.
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