REGULATION OF EXPRESSION OF MHC CLASS I GENES

MHC I类基因转录的调节由基础启动子和上游调节元件的协调活性介导，以实现组织特异性表达水平，其响应于细胞外信号而进一步动态调节。该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，基础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：TATAA盒、起始子（Inr）和新的S盒元件。TATAA和Inr元素的相对使用在不同的细胞类型之间变化，但在任何测试的细胞类型中都不是绝对需要的。两个位点，TATAA盒的3个和Inr的5个，似乎是基础启动子活性所必需的。这些元件在体内的功能正在一系列转基因小鼠构建体中进行研究。基础I类启动子的转录-无论是通过TATAA或Inr元件-取决于功能性TAFII 250，TFIID复合物的组分。此外，我们已经证明，HIV达特蛋白结合TAFII 250，抑制其HAT活性和抑制I类转录。使用酵母双杂交系统，我们已经确定了一系列新的细胞蛋白，类似地与TAFII 250相互作用。我们推测，这些因素有助于I类转录的正常细胞调节，其功能正在调查中。对TAFII 250的依赖性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通过组织特异性和动态控制的转录因子来调节。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。特别地，干扰素诱导的MHC II类表达调节剂CIITA是I类表达的有效共激活剂。CIITA的共激活需要干扰素应答元件和CRE，而CBP、PCAF和p300均不影响I类基因的转录，多种上游元件参与调节I类基因。I类基因的组织特异性表达由一系列上游调控元件决定。远端E盒元件充当细胞类型特异性增强子：其在神经母细胞瘤系中是活跃的，但在HeLa上皮细胞系中相对不活跃。bZIP转录因子USF 1和USF 2结合并激活I类转录。相比之下，而天然存在的变体，USF 2dE 4是I类转录的显性负调控因子。变体的相对水平决定了I类启动子的USF激活水平，从而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.