Connection of core and and tail Mediator modules restrains transcription from TFIID-dependent promoters [ChEC-seq]

The Mediator coactivator complex is divided into four modules: head, middle, tail, and kinase. Deletion of the architectural subunit Med16 separates core Mediator (cMed), comprising the head, middle, and scaffold (Med14), from the tail. However, the direct global effects of tail/cMed disconnection are unclear. We find that rapid depletion of Med16 downregulates genes that require the SAGA complex for full expression, consistent with their reported tail dependence, but also moderately overactivates TFIID-dependent genes in a manner partly dependent on the separated tail, which remains associated with upstream activating sequences. Suppression of TBP dynamics via removal of the Mot1 ATPase partially restores normal transcriptional activity to Med16-depleted cells, suggesting that cMed/tail separation results in an imbalance in the levels PIC formation at SAGA-requiring and TFIID-dependent genes. We suggest that the preferential regulation of SAGA-requiring genes by tailed Mediator helps maintain a proper balance of transcription between these genes and those more dependent on TFIID. ChEC-seq of Mediator subunits in Med16 deletion or depletion strains.中介体辅激活因子复合物（Mediator coactivator complex）可分为四个模块：头部模块、中部模块、尾部模块与激酶模块。删除结构亚基Med16后，包含头部、中部及支架亚基Med14的核心中介体（core Mediator，cMed）会与尾部模块分离。然而，尾部/核心中介体分离所产生的直接全局效应仍不明确。我们发现，快速耗竭Med16会下调那些依赖SAGA复合物（SAGA complex）以实现完全表达的基因，这与此前报道的尾部依赖性特征相符；同时还会适度过度激活依赖TFIID的基因，该过程部分依赖于仍结合在上游激活序列（upstream activating sequences）中的分离尾部模块。通过移除Mot1 ATP酶（Mot1 ATPase）抑制TBP的动态过程，可部分恢复Med16耗竭细胞的正常转录活性，这表明核心中介体/尾部分离会导致依赖SAGA复合物的基因与依赖TFIID的基因在预起始复合物（pre-initiation complex，PIC）形成水平上出现失衡。我们提出，带有尾部模块的中介体对依赖SAGA复合物基因的优先调控，有助于维持这类基因与更依赖TFIID的基因之间的转录平衡。本研究包含对Med16缺失或耗竭菌株中介体亚基的ChEC-seq实验。