Li Fei, Goto Derek B, Zaratiegui Mikel, Tang Xie, Martienssen Rob, Cande W Zacheus
Department of Molecular and Cell Biology, University of California, Berkeley, Berkeley, California 94720, USA.
Curr Biol. 2005 Aug 23;15(16):1448-57. doi: 10.1016/j.cub.2005.07.021.
Chromosomal behavior during mitosis and meiosis depends in part on heterochromatic modifications such as histone H3 lysine-9 methylation (H3K9me). In fission yeast, the Heterochromatin Protein 1 homolog Swi6 recognizes H3K9me, silences transcription, and retains cohesin at pericentromeric repeats. Heterochromatin formation also depends on processing of transcripts derived from centromeric repeats by the RNAi machinery. The DDB1 homolog, Rik1, and histone methyltransferase, Clr4, act in a complex to promote H3K9me. However, the mechanism underlying this interaction is poorly understood.
Using a cytological screen, we have identified two novel genes, dos1(+) and dos2(+), which are required for localization of Swi6. Deletion of either of these genes results in mitotic and meiotic chromosome missegregation, defects in mitotic centromeric cohesion and meiotic telomere clustering, and loss of heterochromatic silencing. Dos1 is predominantly located in the nucleus in a Dos2-dependent manner and directly interacts with Rik1. Each of these genes is required for the association of H3K9me with centromeric repeats, as well as for the production of small interfering RNAs.
Dos1 and Dos2 are required for the formation of heterochromatin in fission yeast. We hypothesize that the physical interaction between Dos1 and Rik1 represents a role in regulating activity of the Rik1/Clr4 complex. Dos2 contributes to this role by regulating Dos1 localization. Our findings suggest a mechanism for recruitment of Clr4 in the RNAi-dependent heterochromatin pathway, in which Dos1 and Dos2 are essential.
有丝分裂和减数分裂过程中的染色体行为部分取决于异染色质修饰，如组蛋白H3赖氨酸-9甲基化（H3K9me）。在裂殖酵母中，异染色质蛋白1同源物Swi6识别H3K9me，使转录沉默，并在着丝粒重复序列处保留黏连蛋白。异染色质的形成还取决于RNAi机制对来自着丝粒重复序列的转录本的加工。DDB1同源物Rik1和组蛋白甲基转移酶Clr4以复合物形式发挥作用来促进H3K9me。然而，这种相互作用的潜在机制尚不清楚。
通过细胞学筛选，我们鉴定出两个新基因dos1(+)和dos2(+)，它们是Swi6定位所必需的。缺失这两个基因中的任何一个都会导致有丝分裂和减数分裂染色体错分离、有丝分裂着丝粒黏连缺陷和减数分裂端粒聚集缺陷，以及异染色质沉默丧失。Dos1主要以Dos2依赖的方式定位于细胞核中，并直接与Rik1相互作用。这些基因中的每一个都是H3K9me与着丝粒重复序列结合以及产生小干扰RNA所必需的。
Dos1和Dos2是裂殖酵母中异染色质形成所必需的。我们假设Dos1和Rik1之间的物理相互作用代表了在调节Rik1/Clr4复合物活性中的作用。Dos2通过调节Dos1定位来促成这一作用。我们的发现提示了一种在RNAi依赖的异染色质途径中招募Clr4的机制，其中Dos1和Dos2是必不可少的。
