A New Class of Transcription Factors Mediates Brassinosteroid-Regulated Gene Expression in Arabidopsis

Abstract

Brassinosteroids (BRs) signal through a plasma membrane-localized receptor kinase to regulate plant growth and development. We showed previously that a novel protein, BES1, accumulates in the nucleus in response to BRs, where it plays a role in BR-regulated gene expression; however, the mechanism by which BES1 regulates gene expression is unknown. In this study, we dissect BES1 subdomains and establish that BES1 is a transcription factor that binds to and activates BR target gene promoters both in vitro and in vivo. BES1 interacts with a basic helix-loop-helix protein, BIM1, to synergistically bind to E box (CANNTG) sequences present in many BR-induced promoters. Loss-of-function and gain-of-function mutants of BIM1 and its close family members display BR response phenotypes. Thus, BES1 defines a new class of plant-specific transcription factors that cooperate with transcription factors such as BIM1 to regulate BR-induced genes.

The brassinosteroid signal transduction pathway

“…Downstream components of this cascade include the BIN2 kinase, the BSU1 phosphatase, and the transcription factors BZR1 [42,43] and its homolog BES1/BZR2 [44,45]. BZR1 and BES1/BZR2 are substrates of the BIN2 kinase [44,46] and BSU1 phosphatase [47], and phosphorylated BZR1 is degraded by the proteasome [46].…”

“…BR induces dephosphorylation and accumulation of BZR1 and BES1/BZR2, possibly by inhibiting the BIN2 kinase or activating the BSU1 phosphatase. BZR1 and BES1 bind to the promoters of BR regulated genes to activate or repress gene expression [45,46]. With direct interactions demonstrated for BR, BRI1 and BAK1, and among BIN2, BZR1/BES1 and promoter DNA, the only gap in the BR signal transduction pathway is between BRI1 or BAK1 and BIN2 or BSU1.…”

“…The biochemical functions of BZR1 and BES1 have been illustrated recently [43,45]. Through extensive protein-DNA interaction studies, including gel shift competition assays, DNA footprint, binding site selection from random sequences, and chromatin immunoprecipitation, He et al demonstrated unequivocally that BZR1 specifically binds to CGTG(T/C)G sequence, which was named BR response element (BRRE) [43].…”

“…BES1 has also been shown to bind to DNA [45]. In this case, BES1 was shown to interact with three homologous basic-helix-loop-helix (bHLH) transcription factors (BIM1-3) in yeast two-hybrid assays [45].…”

“…In this case, BES1 was shown to interact with three homologous basic-helix-loop-helix (bHLH) transcription factors (BIM1-3) in yeast two-hybrid assays [45]. Gel shift assays demonstrated that BES1 could bind to the E-box (CANNTG) elements in the promoter of SAUR-AC1, a gene showing early induction by both BR and auxin [56,57].…”