Regulation of Expression of MHC Class I Genes

MHC I 类基因表达的调节

• 批准号: 6433149
• 负责人: DINAH SINGER
• 金额: --
• 依托单位: DIVISION OF BASIC SCIENCES - NCI
• 依托单位国家: 美国
• 资助国家: 美国
• 起止时间: 至
• 项目状态: 未结题
• 来源: https://reporter.nih.gov/project-details/6433149
• 关键词: MHC class I antigen; gene expression; gene induction /repression; genetic enhancer element; genetic promoter element; genetic regulation; histocompatibility gene; immunogenetics; neurons; nucleic acid sequence; transcription factor

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 which consists of the TATA binding protein (TBP) associated with a series of TBP-associated factors (TAFs), that together participate in the assembly of the transcription preinitiation complex. TAFII250 is required for the transcription of a subset of genes, including those invovled in cell cycle and MHC class I. The tsBN462 cell line contains a temperature sensitive mutation of TAFII250, and differentially arrests transcription of many, but not all genes, at the restrictive temperature. We have found that the core promoter of MHC class I gene requires TAFII250. This dependence can be overcome by select upstream regulatory elements but not by core promoter elements. Thus, the coactivator CIITA rescues the basal promoter from the TAFII250 requirement, whereas introduction of a canonical TATAA box does not. Similarly, the SV40 core promoter requires TAFII250, but the presence of the 72bp enhancer overcomes this requirement. Further, the SV40 72 bp enhancer when placed upstream of the core class I promoter renders it independents of TAFII250. These data suggest that the assembly of transcription initiation complexes is dynamic and can be modulated by specific 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.TAFII250 is known to have acetyl transferase (AT) activity. We have shown AT activity is necessary for transcription of MHC class I genes: inhibition of the AT activity represses transcription. To identify potential cellular factors that might regulate the AT activity of TAFII250, a yeast two-hybrid library was screened with a TAFII250 segment (848-1279 aa) that spanned both the AT domain (848-1120) and the RAP74 binding domain (1120-1279). The TFIID component, TAFII55, was isolated and found to interact with the RAP74-binding domain. Surprisingly, TAFII55 binding to TAFII250 inhibits its AT activity. Furthermore, addition of recombinant TAFII55 to in vitro transcription assays inhibits TAFII250-dependent transcription. Thus, TAFII55 regulates TAFII250 function by modulating its AT activity.

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框的3'和Inr的5'，似乎确实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还是Inr元件--都依赖于功能性TAFII 250，其是TFIID复合物的一个组分，由与一系列TBP相关因子（TAF）相关的TATA结合蛋白（TBP）组成，它们共同参与转录前起始复合物的组装。 TAFII 250是一组基因转录所必需的，包括那些参与细胞周期和MHC I类的基因。tsBN 462细胞系含有TAFII 250的温度敏感性突变，并且在限制性温度下差异性地阻止许多但不是所有基因的转录。 我们发现MHC I类基因的核心启动子需要TAFII 250。这种依赖性可以通过选择上游调控元件而不是通过核心启动子元件来克服。因此，共激活因子CIITA从TAFII 250要求中拯救了基础启动子，而引入典型的TATAA盒则没有。类似地，SV 40核心启动子需要TAFII 250，但72 bp增强子的存在克服了这一要求。此外，当置于核心I类启动子上游时，SV 40 72 bp增强子使其不依赖于TAFII 250。这些数据表明，转录起始复合物的组装是动态的，可以通过特定的转录因子进行调节。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;相反，CBP、PCAF和p300都不影响I类转录。已知TAFII 250具有乙酰转移酶（AT）活性。我们已经表明AT活性对于MHC I类基因的转录是必需的：AT活性的抑制抑制转录。为了鉴定可能调节TAFII 250的AT活性的潜在细胞因子，用跨越AT结构域（848 -1120）和RAP 74结合结构域（1120-1279）的TAFII 250区段（848-1279 aa）筛选酵母双杂交文库。TFIID组分TAFII 55被分离并发现与RAP 74结合结构域相互作用。令人惊讶的是，TAFII 55与TAFII 250的结合抑制了其AT活性。此外，在体外转录测定中加入重组TAFII 55抑制TAFII 250依赖性转录。因此，TAFII 55通过调节其AT活性来调节TAFII 250功能。