A conserved function of corepressors is to nucleate assembly of the preinitiation complex

The plant corepressor TPL is recruited to diverse chromatin contexts, yet
its mechanism of repression remains unclear. Previously, we have leveraged
the fact that TPL retains its function in a synthetic transcriptional
circuit in the yeast model Saccharomyces cerevisiae to localize repressive
function to two distinct domains. Here, we employed two unbiased whole
genome approaches to map the physical and genetic interactions of TPL at a
repressed locus. We identified SPT4, SPT5 and SPT6 as necessary for
repression with the SPT4 subunit acting as a bridge connecting TPL to SPT5
and SPT6. We also discovered the association of multiple additional
constituents of the transcriptional preinitiation complex at TPL-repressed
promoters, specifically those involved in early transcription initiation
events. These findings were validated in yeast and plants through multiple
assays, including a novel method to analyze conditional loss of function
of essential genes in plants. Our findings support a model where TPL
nucleates preassembly of the transcription activation machinery to
facilitate rapid onset of transcription once repression is relieved.

植物共阻遏蛋白TPL可被招募至多种染色质环境中，但其介导阻遏的分子机制仍未明确。此前，我们借助TPL在模式酵母酿酒酵母（Saccharomyces cerevisiae）的合成转录回路中仍可保留功能这一特性，将其阻遏功能定位至两个互不相同的结构域。本研究采用两种无偏全基因组研究策略，对TPL在受阻遏基因座上的物理互作与遗传互作进行了图谱解析。我们鉴定出SPT4、SPT5与SPT6是TPL介导阻遏过程所必需的因子，其中SPT4亚基可作为桥梁连接TPL与SPT5及SPT6。我们还发现，转录前起始复合物（transcriptional preinitiation complex）的多种其他组分可富集于TPL介导阻遏的启动子区域，特别是参与转录早期起始事件的组分。我们通过多种实验手段在酵母与植物中验证了上述研究结果，其中还开发了一种可用于分析植物中必需基因条件性功能丧失的全新实验方法。我们的研究结果支持如下模型：TPL可介导转录激活机器的预组装，从而在阻遏解除后快速启动转录进程。