REGULATION OF EXPRESSION OF MHC CLASS I GENES

MHC I类基因转录的调控是由基础启动子和上游调控元件的协调活动介导的，以达到组织特异性水平的表达，并根据细胞外信号进一步动态调节。实验室最近的研究重点是确定基础启动子的关键序列组织，转录启动子转录所需的转录起始复合物，以及调节基础启动子活性的上游元件。I类基础启动子由三个元件组成：一个TATAA盒、一个引发子（Inr）和一个新的S-box元件。TATAA和Inr元素的相对用法在不同的细胞类型中有所不同，但在任何测试的细胞类型中都不是绝对需要的元素。两个位点，3个TATAA盒子和5个Inr盒子，似乎确实是基础启动子活性所必需的。这些元素在体内的功能正在一系列转基因小鼠构建中进行检测。基础I类启动子的转录——无论是通过TATAA还是Inr元件——取决于TFIID复合体的一个组分TAFII250的功能。此外，我们已经证明HIV Tat蛋白与TAFII250结合，抑制其HAT活性并抑制I类转录。利用酵母双杂交系统，我们发现了一系列与TAFII250相似的新型细胞蛋白。我们推测这些因子有助于I类转录的正常细胞调节；他们的职能正在调查中。对TAFII250的依赖可以通过强上游增强子元件和转录因子来克服。这一观察结果使我们提出转录起始复合物的性质可以由组织特异性和动态控制的转录因子调节。为了支持这一假设，我们发现细胞类型特异性增强体有助于调节I类转录。特别是干扰素诱导的MHC II类表达调节剂CIITA，是一种有效的I类表达共激活剂。CIITA的共激活需要干扰素反应元件和CRE；相比之下，CBP、PCAF和p300都不影响I类转录。多种上游元件参与MHC I类基因的调控。一类基因的组织特异性表达是由一系列上游调控元件决定的。远端E盒元件作为细胞类型特异性增强子：它在神经母细胞瘤系中活跃，但在HeLa上皮细胞系中相对不活跃。bZIP转录因子USF1和USF2结合并激活I类转录。相反，在自然发生的变异中，USF2dE4是I类转录的显性负调控因子。该变体的相对水平决定了I类启动子的USF激活水平，因此有人提出，这可能代表了一种微调各种组织中I类表达的机制。

Regulation of MHC class I gene transcription is mediated by the coordinate activities of the basal promoter and upstream regulatory elements, to achieve tissue-specific levels of expression which are further dynamically modulated in response to extracellular signals. The recent research focus of the laboratory has been to define the critical sequence organization of the basal promoter, the transcription initiation complexes that are required for transcription of the basal promoter, and upstream elements that modulate basal promoter activity. The class I basal promoter consists of three elements: a TATAA box, an initiator (Inr), and a novel S-box element. The relative usage of TATAA and Inr elements varies among different cell types, but neither element is absolutely required in any cell type tested. Two sites, 3 of the TATAA box and 5 of the Inr, do appear to be necessary for basal promoter activity. The function of these elements in vivo is being examined in a series of transgenic mouse constructs. Transcription of the basal class I promoter -whether through the TATAA or Inr elements - depends on a functional TAFII250, a component of the TFIID complex. Furthermore, we have demonstrated that the HIV Tat protein binds to TAFII250, inhibiting its HAT activity and repressing class I transcription. Using a yeast two-hybrid system, we have identified a series of novel cellular proteins that similarly interact with TAFII250. We speculate that these factors contribute to the normal cellular regulation of class I transcription; their functions are under investigation. Dependence on TAFII250 can be overcome by strong upstream enhancer elements and transcription factors. This observation has led us to propose that the nature of the transcription initiation complex can be modulated by tissue-specific and dynamically controlled transcription factors. In support of this hypothesis, we find that cell type specific enhanceosomes contribute to the regulation of class I transcription. In particular, the interferon inducible regulator of MHC class II expression, CIITA, is a potent co-activator of class I expression. Coactivation by CIITA requires both the interferon response element and the CRE; in contrast, neither CBP, PCAF nor p300 affect class I transcription.A variety of upstream elements contribute to the regulation of MHC class I genes. Tissue specific expression of class I genes is determined by a series of upstream regulatory elements. A distal E box element acts as a cell type-specific enhancer: it is active in a neuroblastoma line but relatively inactive in the HeLa epithelial cell line. The bZIP transcription factors, USF1 and USF2, bind and activate class I transcription. In contrast, whereas the naturally occurring variant, USF2dE4 is a dominant negative regulator of class I transcription. The relative levels of the variant determine the level of USF activation of the class I promoter, leading to the proposal that this may represent one mechanism to fine-tune class I expression in various tissues.