Zhou Yue, Hartwig Benjamin, James Geo Velikkakam, Schneeberger Korbinian, Turck Franziska
Department Plant Developmental Biology, Max Planck Institute for Plant Breeding Research, 50829 Köln, Germany.
Department Plant Developmental Biology, Max Planck Institute for Plant Breeding Research, 50829 Köln, Germany
Plant Cell. 2016 Jan;28(1):87-101. doi: 10.1105/tpc.15.00787. Epub 2015 Dec 31.
In multicellular organisms, Polycomb Repressive Complex 1 (PRC1) and PRC2 repress target genes through histone modification and chromatin compaction. Arabidopsis thaliana mutants strongly compromised in the pathway cannot develop differentiated organs. LIKE HETEROCHROMATIN PROTEIN1 (LHP1) is so far the only known plant PRC1 component that directly binds to H3K27me3, the histone modification set by PRC2, and also associates genome-wide with trimethylation of lysine 27 of histone H3 (H3K27me3). Surprisingly, lhp1 mutants show relatively mild phenotypic alterations. To explain this paradox, we screened for genetic enhancers of lhp1 mutants to identify novel components repressing target genes together with, or in parallel to, LHP1. Two enhancing mutations were mapped to TELOMERE REPEAT BINDING PROTEIN1 (TRB1) and its paralog TRB3. We show that TRB1 binds to thousands of genomic sites containing telobox or related cis-elements with a significant increase of sites and strength of binding in the lhp1 background. Furthermore, in combination with lhp1, but not alone, trb1 mutants show increased transcription of LHP1 targets, such as floral meristem identity genes, which are more likely to be bound by TRB1 in the lhp1 background. By contrast, expression of a subset of LHP1-independent TRB1 target genes, many involved in primary metabolism, is decreased in the absence of TRB1 alone. Thus, TRB1 is a bivalent transcriptional modulator that maintains downregulation of Polycomb Group (PcG) target genes in lhp1 mutants, while it sustains high expression of targets that are regulated independently of PcG.
在多细胞生物中，多梳抑制复合体1（PRC1）和PRC2通过组蛋白修饰和染色质压缩来抑制靶基因。在该途径中严重受损的拟南芥突变体无法发育出分化的器官。类异染色质蛋白1（LHP1）是目前已知的唯一一种直接与PRC2所设定的组蛋白修饰H3K27me3结合的植物PRC1组分，并且在全基因组范围内也与组蛋白H3赖氨酸27的三甲基化（H3K27me3）相关联。令人惊讶的是，lhp1突变体表现出相对温和的表型改变。为了解释这一矛盾现象，我们筛选了lhp1突变体的遗传增强子，以鉴定与LHP1共同或平行抑制靶基因的新组分。两个增强突变被定位到端粒重复结合蛋白1（TRB1）及其旁系同源物TRB3。我们发现TRB1与数千个含有端粒盒或相关顺式元件的基因组位点结合，在lhp1背景下结合位点和结合强度显著增加。此外，与lhp1联合（而非单独）时，trb1突变体显示出LHP1靶标的转录增加，如参与花分生组织特征决定的基因，在lhp1背景下这些基因更有可能被TRB1结合。相比之下，在单独缺失TRB1时，一部分不依赖LHP1的TRB1靶基因（许多参与初级代谢）的表达降低。因此，TRB1是一种双价转录调节因子，在lhp1突变体中维持多梳蛋白家族（PcG）靶基因的下调，同时维持独立于PcG调控的靶标的高表达。
