NONSENSE SURVEILLANCE OF REARRANGING GENE TRANSCRIPTS

T细胞受体和免疫球蛋白基因的一个独特特征 它们在淋巴细胞发育过程中会经历重排。一个 这些程序化突变事件的后果是大约 三分之二重排的TCR和Ig基因是框架外的，因此 含有提前终止密码子(PTCs)。研究表明， PTCs导致TCR和Ig mRNA水平显著下调。 这一过程被称为无意义密码子介导的下调(NMD)，具有 被证明可以抑制有害的截短蛋白的表达， 因此，它可能是由一种监督机制来调节的，这种机制保护 来自显性负突变蛋白的细胞。有趣的是，TCR和Ig 转录产物受到NMD机制的下调比 是其他已知的哺乳动物转录产物，这表明有可能 重排经常获得PTC的基因已经进化了机制 放大国家导弹防御系统的反应。因为无意义的密码子只有 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，有几个 一系列证据表明，NMD发生在淋巴核。 细胞，包括基因下游需要内含子的事实 胡说八道的密码子引发了下调监管的反应。进一步的证据 对于原子核的一个作用是发现无意义的密码子增加 已切除的选择性剪接TCR转录本的水平 令人不快的PTC。本申请的具体目的是：(1) 解释无意义密码子对核相关基因的矛盾影响 事件。(2)评估无意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 为了需要翻译，最近的几行证据表明， 情况可能并非如此)。(3)找出造成上述情况的因素 TCR和Ig mRNAs的强劲下调，以应对PTCs。(4)至 阐明携带PTC的TCR和Ig mRNAs是否被下调 一种不同于作用于其他哺乳动物的常见机制 成绩单。国家导弹防御系统的研究将对我国的发展做出重大贡献。 对免疫系统的了解。阐明无意义密码子是如何 调控核事件可能会改变基因表达的主流观点 以及它在真核细胞中的区间化。

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.