Aichinger Ernst, Villar Corina B R, Farrona Sara, Reyes José C, Hennig Lars, Köhler Claudia
Department of Biology and Zurich-Basel Plant Science Center, Swiss Federal Institute of Technology, ETH Centre, Zurich, Switzerland.
PLoS Genet. 2009 Aug;5(8):e1000605. doi: 10.1371/journal.pgen.1000605. Epub 2009 Aug 14.
Dynamic regulation of chromatin structure is of fundamental importance for modulating genomic activities in higher eukaryotes. The opposing activities of Polycomb group (PcG) and trithorax group (trxG) proteins are part of a chromatin-based cellular memory system ensuring the correct expression of specific transcriptional programs at defined developmental stages. The default silencing activity of PcG proteins is counteracted by trxG proteins that activate PcG target genes and prevent PcG mediated silencing activities. Therefore, the timely expression and regulation of PcG proteins and counteracting trxG proteins is likely to be of fundamental importance for establishing cell identity. Here, we report that the chromodomain/helicase/DNA-binding domain CHD3 proteins PICKLE (PKL) and PICKLE RELATED2 (PKR2) have trxG-like functions in plants and are required for the expression of many genes that are repressed by PcG proteins. The pkl mutant could partly suppress the leaf and flower phenotype of the PcG mutant curly leaf, supporting the idea that CHD3 proteins and PcG proteins antagonistically determine cell identity in plants. The direct targets of PKL in roots include the PcG genes SWINGER and EMBRYONIC FLOWER2 that encode subunits of Polycomb repressive complexes responsible for trimethylating histone H3 at lysine 27 (H3K27me3). Similar to mutants lacking PcG proteins, lack of PKL and PKR2 caused reduced H3K27me3 levels and, therefore, increased expression of a set of PcG protein target genes in roots. Thus, PKL and PKR2 are directly required for activation of PcG protein target genes and in roots are also indirectly required for repression of PcG protein target genes. Reduced PcG protein activity can lead to cell de-differentiation and callus-like tissue formation in pkl pkr2 mutants. Thus, in contrast to mammals, where PcG proteins are required to maintain pluripotency and to prevent cell differentiation, in plants PcG proteins are required to promote cell differentiation by suppressing embryonic development.
染色质结构的动态调控对于调节高等真核生物的基因组活动至关重要。多梳蛋白组（PcG）和三胸蛋白组（trxG）蛋白的相反活性是基于染色质的细胞记忆系统的一部分，可确保特定转录程序在特定发育阶段的正确表达。PcG蛋白的默认沉默活性被激活PcG靶基因并阻止PcG介导的沉默活性的trxG蛋白抵消。因此，PcG蛋白和起抵消作用的trxG蛋白的及时表达和调控对于建立细胞特性可能至关重要。在此，我们报道染色质结构域/解旋酶/DNA结合结构域CHD3蛋白腌黄瓜（PKL）和腌黄瓜相关蛋白2（PKR2）在植物中具有类似trxG的功能，并且是许多被PcG蛋白抑制的基因表达所必需的。pkl突变体可以部分抑制PcG突变体卷叶的叶和花表型，支持CHD3蛋白和PcG蛋白在植物中拮抗决定细胞特性的观点。PKL在根中的直接靶标包括PcG基因SWINGER和胚胎花2，它们编码负责组蛋白H3赖氨酸27三甲基化（H3K27me3）的多梳抑制复合物的亚基。与缺乏PcG蛋白的突变体类似，缺乏PKL和PKR2会导致H3K27me3水平降低，因此，根中一组PcG蛋白靶基因的表达增加。因此，PKL和PKR2是激活PcG蛋白靶基因直接所需的，在根中也是抑制PcG蛋白靶基因间接所需的。PcG蛋白活性降低会导致pkl pkr2突变体中的细胞去分化和愈伤组织样组织形成。因此，与哺乳动物中PcG蛋白需要维持多能性并防止细胞分化相反，在植物中PcG蛋白需要通过抑制胚胎发育来促进细胞分化。
