BRZ-INSENSITIVE-LONG 1 (BIL1)/BRASSINAZOLE-RESISTANT 1 (BZR1) and its homologues are plant-specific transcription factors that convert the signalling of the phytohormones brassinosteroids (BRs) to transcriptional responses, thus controlling various physiological processes in plants. Although BIL1/BZR1 upregulates some BR-responsive genes and downregulates others, the molecular mechanism underlying the dual roles of BIL1/BZR1 is still poorly understood. Here we show that BR-responsive transcriptional repression by BIL1/BZR1 requires the tight binding of BIL1/BZR1 alone to the 10 bp elements of DNA fragments containing the known 6 bp core-binding motifs at the centre. Furthermore, biochemical and structural evidence demonstrates that the selectivity for two nucleobases flanking the core motifs is realized by the DNA shape readout of BIL1/BZR1 without direct recognition of the nucleobases. These results elucidate the molecular and structural basis of transcriptional repression by BIL1/BZR1 and contribute to further understanding of the dual roles of BIL1/BZR1 in BR-responsive gene regulation.

BRZ-INSENSITIVE-LONG 1 (BIL1)/BRASSINAZOLE-RESISTANT 1 (BZR1) 及其同源物是植物特异性转录因子，可将植物激素油菜素类固醇 (BR) 的信号转导为转录反应，从而控制植物的各种生理过程。尽管 BIL1/BZR1 上调一些 BR 反应基因并下调其他基因，但对 BIL1/BZR1 双重作用的分子机制仍知之甚少。在这里，我们表明 BIL1/BZR1 的 BR 反应性转录抑制需要 BIL1/BZR1 单独与中心含有已知 6 bp 核心结合基序的 DNA 片段的 10 bp 元素紧密结合。此外，生化和结构证据表明，对核心基序两侧的两个核碱基的选择性是通过 BIL1/BZR1 的 DNA 形状读出实现的，而无需直接识别核碱基。这些结果阐明了 BIL1/BZR1 转录抑制的分子和结构基础，有助于进一步了解 BIL1/BZR1 在 BR 反应基因调控中的双重作用。