Collinge Margaret A, Spillane Charles, Köhler Claudia, Gheyselinck Jacqueline, Grossniklaus Ueli
Institute of Plant Biology, University of Zürich, 8008 Zürich, Switzerland.
Plant Cell. 2004 Apr;16(4):1035-46. doi: 10.1105/tpc.019059. Epub 2004 Mar 12.
The eukaryotic origin recognition complex (ORC) is made up of six subunits and functions in nuclear DNA replication, chromatin structure, and gene silencing in both fungi and metazoans. We demonstrate that disruption of a plant ORC subunit homolog, AtORC2 of Arabidopsis (Arabidopsis thaliana), causes a zygotic lethal mutant phenotype (orc2). Seeds of orc2 abort early, typically producing embryos with up to eight cells. Nuclear division in the endosperm is arrested at an earlier developmental stage: only approximately four nuclei are detected in orc2 endosperm. The endosperm nuclei in orc2 are dramatically enlarged, a phenotype that is most similar to class B titan mutants, which include mutants in structural maintenance of chromosomes (SMC) cohesins. The highest levels of ORC2 gene expression were found in preglobular embryos, coinciding with the stage at which homozygous orc2 mutant seeds arrest. The homologs of the other five Arabidopsis ORC subunits are also expressed at this developmental stage. The orc2 mutant phenotype is partly suppressed by a mutation in the Polycomb group gene MEDEA. In double mutants between orc2 and medea (mea), orc2 homozygotes arrest later with a phenotype intermediate between those of mea and orc2 single mutants. Either alterations in chromatin structure or the release of cell cycle checkpoints by the mea mutation may allow more cell and nuclear divisions to occur in orc2 homozygous seeds.
真核生物起源识别复合物（ORC）由六个亚基组成，在真菌和后生动物的核DNA复制、染色质结构和基因沉默中发挥作用。我们证明，破坏植物ORC亚基同源物拟南芥（Arabidopsis thaliana）的AtORC2会导致合子致死突变体表型（orc2）。orc2的种子早期败育，通常产生最多含八个细胞的胚胎。胚乳中的核分裂在更早的发育阶段就停止了：在orc2胚乳中仅检测到大约四个核。orc2中的胚乳核显著增大，这种表型与B类泰坦突变体最为相似，其中包括染色体结构维持（SMC）黏连蛋白的突变体。在球形胚前期胚胎中发现了最高水平的ORC2基因表达，这与纯合orc2突变体种子停滞发育的阶段一致。拟南芥其他五个ORC亚基的同源物在这个发育阶段也有表达。orc2突变体表型部分被多梳蛋白家族基因MEA的突变所抑制。在orc2和MEA（mea）的双突变体中，orc2纯合子的停滞发育时间较晚，其表型介于mea和orc2单突变体之间。MEA突变导致的染色质结构改变或细胞周期检查点的释放，可能使orc2纯合子种子发生更多的细胞和核分裂。
