Sprouse Rebekka O, Wells Melissa N, Auble David T
Department of Biochemistry and Molecular Genetics, University of Virginia Health System, Charlottesville, Virginia 22908, USA.
J Biol Chem. 2009 Feb 13;284(7):4525-35. doi: 10.1074/jbc.M808951200. Epub 2008 Dec 21.
Mot1 is an essential TATA-binding protein (TBP)-associated factor and Snf2/Swi2 ATPase that both represses and activates transcription. Biochemical and structural results support a model in which ATP binding and hydrolysis induce a conformational change in Mot1 that drives local translocation along DNA, thus removing TBP. Although this activity explains transcriptional repression, it does not as easily explain Mot1-mediated transcriptional activation, and several different models have been proposed to explain how Mot1 activates transcription. To better understand the function of Mot1 in yeast cells in vivo, particularly with regard to gene activation, TBP mutants were identified that bypass the requirement for Mot1 in vivo. Although TBP has been extensively mutated and analyzed previously, this screen uncovered two novel TBP variants that are unique in their ability to bypass the requirement for Mot1. Surprisingly, in vitro analyses reveal that rather than having acquired an improved biochemical activity, one of the TBPs was defective for interaction with polymerase II preinitiation complex (PIC) components and other regulators of TBP function. The other mutant was defective for DNA binding in vitro yet was still recruited to chromatin in vivo. These results suggest that Mot1-mediated dissociation of TBP (or TBP-containing complexes) from chromatin can explain the Mot1 activation mechanism at some promoters. The results also suggest that PICs can be dynamically unstable and that appropriate PIC instability is critical for the regulation of transcription in vivo.
Mot1是一种必需的TATA结合蛋白（TBP）相关因子和Snf2/Swi2 ATP酶，它既能抑制转录也能激活转录。生化和结构研究结果支持一种模型，即ATP结合和水解会诱导Mot1发生构象变化，从而驱动其沿DNA进行局部移位，进而移除TBP。虽然这种活性解释了转录抑制现象，但它并不能轻易地解释Mot1介导的转录激活，因此人们提出了几种不同的模型来解释Mot1如何激活转录。为了更好地理解Mot1在酵母细胞体内的功能，特别是其在基因激活方面的功能，研究人员鉴定出了一些能在体内绕过对Mot1需求的TBP突变体。虽然TBP此前已被广泛地进行了突变和分析，但该筛选发现了两种新型的TBP变体，它们在绕过对Mot1需求的能力方面具有独特性。令人惊讶的是，体外分析表明，其中一种TBP并非获得了改善的生化活性，而是在与聚合酶II预起始复合物（PIC）组分以及TBP功能的其他调节因子相互作用方面存在缺陷。另一种突变体在体外与DNA结合方面存在缺陷，但在体内仍能被募集到染色质上。这些结果表明，Mot1介导的TBP（或含TBP的复合物）从染色质上解离，可以解释Mot1在某些启动子处的激活机制。这些结果还表明，PIC可能是动态不稳定的，并且适当的PIC不稳定性对于体内转录调控至关重要。
