NONSENSE SURVEILLANCE OF REARRANGING GENE TRANSCRIPTS

T细胞受体（TCR）和免疫球蛋白（IG）基因的独特特征 它们在淋巴细胞发育过程中会发生重排。 一 这些程序化突变事件的结果是， 三分之二重排的TCR和IG基因在框外， 包含提前终止密码子（PTC）。 研究已经证明 PTC导致TCR和IG mRNA水平显著下调。 这一过程被称为无义密码子介导的下调（NMD）， 已经显示出抑制有害的截短蛋白质的表达， 因此，它可能是由一种监视机制介导的， 细胞从显性负突变蛋白。有趣的是，TCR和IG 转录物被NMD机制下调的程度比 是其他已知的哺乳动物转录本，这表明， 经常获得PTC的基因重排已经进化出了机制， 来扩大NMD的反应。 因为无意义密码子 被细胞质翻译机制识别，它是 预计NMD将发生在细胞质中。出乎意料的是， 一系列证据表明NMD发生在淋巴细胞核中， 细胞，包括一个内含子是需要下游的一个事实， 无义密码子参与下调反应。 进一步证据 细胞核的一个作用是发现无义密码子增加 切除的选择性剪接TCR转录物的水平 PTC的缺点 本申请的具体目的是：（1） 解释无义密码子对核相关基因的矛盾作用 事件 (2)为了评估无义密码子是否涉及翻译- 类似的机制（尽管通常假设密码子诱导的事件 需要翻译，最近的几行证据表明， 可能不是这样）。(3)为了确定导致 TCR和IG mRNA对PTC应答的稳健下调。 (4)到 阐明携带PTC的TCR和IG mRNA是否被 一种不同于作用于其他哺乳动物的共同机制 成绩单 研究NMD将对我们的 了解免疫系统。 阐明无义密码子 调节核事件可能会改变基因表达的主流观点 及其在真核细胞中的区室化。

A unique feature of T-cell receptor (TCR) and immunoglobulin (Ig) genes is that they undergo rearrangements during lymphocyte development. A consequence of these programmed mutational events is that approximately two thirds of the rearranged TCR and Ig genes are out of frame, and thus contain premature termination codons (PTCs). Studies have demonstrated that PTCs cause TCR and Ig mRNA levels to be dramatically downregulated. This process, termed nonsense codon-mediated downregulation (NMD), has been shown to depress of expression of deleterious truncated proteins, and thus it may be mediated by a surveillance mechanism that protects cells from dominant negative-mutant proteins. Interestingly, TCR and Ig transcripts are more strongly downregulated by the NMD mechanism than are other known mammalian transcripts, suggesting the possibility that rearranging genes which frequently acquire PTCs have evolved mechanisms to amplify the NMD response. Because nonsense codons are only known to be recognized by the cytoplasmic translational machinery, it was expected that NMD would occur in the cytoplasm. Unexpectedly, several lines of evidence suggest that NMD occurs in the nucleus of lymphoid cells, including the fact that an intron is required downstream of a nonsense codon to engage the downregulatory response. Further evidence for a role of the nucleus is the finding that nonsense codons increase the levels of alternatively spliced TCR transcripts that have excised the offending PTC. The specific aims of this application are: (1) To explain the paradoxical effects of nonsense codons on nuclear-associated events. (2) To assess whether nonsense codon involves a translation- like mechanism (although a codon-induced event would normally be assumed to require translation, several lines of recent evidence suggest this might not be the case). (3) To identify the factors responsible for the robust downregulation of TCR and Ig mRNAs in response to PTCs. (4) To elucidate whether TCR and Ig mRNAs harboring PTCs are downregulated by a common mechanism that differs from that which acts on other mammalian transcripts. The study of NMD will contribute significantly to our understanding of the immune system. Elucidation of how nonsense codons regulate nuclear events may alter prevailing views of gene expression and its compartmentalization in eukaryotic cells.