Transcription initiation constitutes a major checkpoint in gene regulation across all living organisms. Control of chromatin function is tightly linked to this checkpoint, which is best illustrated by the SAGA coactivator. This evolutionary conserved complex of 18–20 subunits was first discovered as a Gcn5p-containing histone acetyltransferase, but it also integrates a histone H2B deubiquitinase. The SAGA subunits are organized in a modular fashion around its central core. Strikingly, this central module of SAGA shares a number of proteins with the central core of the basal transcription factor TFIID. In this review I will compare the SAGA and TFIID complexes with respect to their shared subunits, structural organization, enzymatic activities and chromatin binding. I will place a special emphasis on the ancestry of SAGA and TFIID subunits, which suggests that these complexes evolved to control the activity of TBP (TATA-binding protein) in directing the assembly of transcription initiation complexes.

转录起始构成了所有活生物体基因调控的主要检查点。染色质功能的控制与该检查点紧密相连，SAGA辅助活化剂可以最好地说明这一点。这种具有18-20个亚基的进化保守复合物最初被发现为含有Gcn5p的组蛋白乙酰转移酶，但它也整合了组蛋白H2B去泛素酶。SAGA子单元围绕其中心核心以模块化方式组织。令人惊讶的是，SAGA的这一中央模块与基础转录因子TFIID的中央核心共享许多蛋白质。在这篇综述中，我将比较SAGA和TFIID复合物的共享亚基，结构组织，酶活性和染色质结合。我将特别强调SAGA和TFIID亚基的起源，