Laboratory of Bioorganic Chemistry, Department of Chemistry, Faculty of Science, Hokkaido University, Sapporo, Japan.
PLoS Genet. 2013;9(8):e1003677. doi: 10.1371/journal.pgen.1003677. Epub 2013 Aug 15.
Heterochromatin at the pericentromeric repeats in fission yeast is assembled and spread by an RNAi-dependent mechanism, which is coupled with the transcription of non-coding RNA from the repeats by RNA polymerase II. In addition, Rrp6, a component of the nuclear exosome, also contributes to heterochromatin assembly and is coupled with non-coding RNA transcription. The multi-subunit complex Mediator, which directs initiation of RNA polymerase II-dependent transcription, has recently been suggested to function after initiation in processes such as elongation of transcription and splicing. However, the role of Mediator in the regulation of chromatin structure is not well understood. We investigated the role of Mediator in pericentromeric heterochromatin formation and found that deletion of specific subunits of the head domain of Mediator compromised heterochromatin structure. The Mediator head domain was required for Rrp6-dependent heterochromatin nucleation at the pericentromere and for RNAi-dependent spreading of heterochromatin into the neighboring region. In the latter process, Mediator appeared to contribute to efficient processing of siRNA from transcribed non-coding RNA, which was required for efficient spreading of heterochromatin. Furthermore, the head domain directed efficient transcription in heterochromatin. These results reveal a pivotal role for Mediator in multiple steps of transcription-coupled formation of pericentromeric heterochromatin. This observation further extends the role of Mediator to co-transcriptional chromatin regulation.
着丝粒重复序列的异染色质通过 RNAi 依赖性机制组装和扩散，该机制与 RNA 聚合酶 II 从重复序列转录非编码 RNA 相关。此外，核 exosome 的一个组成部分 Rrp6 也有助于异染色质组装，并与非编码 RNA 转录相关。多亚基复合物 Mediator 指导 RNA 聚合酶 II 依赖性转录的起始，最近被认为在转录延伸和剪接等过程中在起始后发挥作用。然而，Mediator 在染色质结构调控中的作用尚不清楚。我们研究了 Mediator 在着丝粒异染色质形成中的作用，发现 Mediator 头部特定亚基的缺失会破坏异染色质结构。Mediator 头部对于 Rrp6 依赖的着丝粒异染色质核形成以及 RNAi 依赖性异染色质在邻近区域的扩散是必需的。在后一过程中，Mediator 似乎有助于从转录的非编码 RNA 中有效加工 siRNA，这对于异染色质的有效扩散是必需的。此外，头部指导异染色质中的高效转录。这些结果揭示了 Mediator 在转录偶联的着丝粒异染色质形成的多个步骤中起着关键作用。这一观察结果进一步将 Mediator 的作用扩展到了共转录的染色质调控。
