Direct repression of KNOX loci by the ASYMMETRIC LEAVES1 complex of Arabidopsis
Direct repression of KNOX loci by the ASYMMETRIC LEAVES1 complex of Arabidopsis
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DOI：
10.1105/tpc.107.056127
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发表时间：
2008-01-01
期刊：
影响因子：
11.6
通讯作者：
Timmermans, Marja C. P.
中科院分区：
文献类型：
--
作者：
Guo, Mengjuan;Thomas, Julie;Timmermans, Marja C. P.
类结蛋白1同源框（KNOX）基因促进干细胞活性，并且在形成确定的侧生器官时必须被抑制。在器官发生过程中稳定的KNOX基因沉默已知涉及预测的DNA结合蛋白不对称叶1（AS1）和AS2以及染色质重塑因子HIRA。然而，沉默的机制尚不清楚。在此，我们表明AS1和AS2形成一种阻遏复合物，它直接结合到存在于KNOX基因短柄（BP）和KNAT2启动子两个位点的调控基序CWGTTD和KMKTTGAHW上。这两个结合位点具有非冗余作用，并且在这些位点上AS1 - AS2复合物之间的相互作用是抑制BP所必需的。启动子缺失分析进一步表明，叶片中BP表达所需的增强子元件位于AS1 - AS2复合物结合位点之间。我们提出AS1 - AS2复合物相互作用在KNOX启动子中形成一个环，并且可能通过招募HIRA，形成一种抑制性染色质状态，在器官发生过程中阻断增强子活性。我们提出的AS1 - AS2介导的KNOX基因沉默模型在概念上类似于绝缘子的作用。这种调控机制可能在单子叶和双子叶谱系的单叶物种中是保守的，并且是复叶进化的一个潜在关键决定因素。
KNOTTED1-like homeobox (KNOX) genes promote stem cell activity and must be repressed to form determinate lateral organs. Stable KNOX gene silencing during organogenesis is known to involve the predicted DNA binding proteins ASYMMETRIC LEAVES1 (AS1) and AS2 as well as the chromatin-remodeling factor HIRA. However, the mechanism of silencing is unknown. Here, we show that AS1 and AS2 form a repressor complex that binds directly to the regulatory motifs CWGTTD and KMKTTGAHW present at two sites in the promoters of the KNOX genes BREVIPEDICELLUS (BP) and KNAT2. The two binding sites act nonredundantly, and interaction between AS1-AS2 complexes at these sites is required to repress BP. Promoter deletion analysis further indicates that enhancer elements required for BP expression in the leaf are located between the AS1-AS2 complex binding sites. We propose that AS1-AS2 complexes interact to create a loop in the KNOX promoter and, likely through recruitment of HIRA, form a repressive chromatin state that blocks enhancer activity during organogenesis. Our model for AS1-AS2-mediated KNOX gene silencing is conceptually similar to the action of an insulator. This regulatory mechanism may be conserved in simple leafed species of monocot and dicot lineages and constitutes a potential key determinant in the evolution of compound leaves.