Department of Genetics and Developmental Biology, University of Connecticut Health Center, 400 Farmington Avenue, Farmington, CT 06030-6403, USA.
Mol Cell Biol. 2010 Dec;30(23):5502-13. doi: 10.1128/MCB.00966-10. Epub 2010 Sep 27.
Genome annotation suggested that early-diverged kinetoplastids possess a reduced set of basal transcription factors. More recent work, however, on the lethal parasite Trypanosoma brucei identified extremely divergent orthologs of TBP, TFIIA, TFIIB, and TFIIH which, together with the small nuclear RNA-activating protein complex, form a transcription preinitiation complex (PIC) at the spliced leader (SL) RNA gene (SLRNA) promoter. The SL RNA is a small nuclear RNA and a trans splicing substrate for the maturation of all pre-mRNAs which is metabolized continuously to sustain gene expression. Here, we identified and biochemically characterized a novel TFIIH-associated protein complex in T. brucei (Med-T) consisting of nine subunits whose amino acid sequences are conserved only among kinetoplastid organisms. Functional analyses in vivo and in vitro demonstrated that the complex is essential for cell viability, SLRNA transcription, and PIC integrity. Molecular structure analysis of purified Med-T and Med-T/TFIIH complexes by electron microscopy revealed that Med-T corresponds to the mediator head module of higher eukaryotes. These data therefore show that mediator is a basal factor for small nuclear SL RNA gene transcription in trypanosomes and that the basal transcription function of mediator head is a characteristic feature of eukaryotes which developed early in their evolution.
基因组注释表明，早期分化的动基体目生物拥有一套较少的基础转录因子。然而，最近对致命寄生虫布氏锥虫的研究鉴定了 TBP、TFIIA、TFIIB 和 TFIIH 的极度分化的同源物，它们与小核 RNA 激活蛋白复合物一起，在剪接先导（SL）RNA 基因（SLRNA）启动子处形成转录起始前复合物（PIC）。SL RNA 是一种小核 RNA，是所有前体 RNA 成熟的转剪接底物，不断代谢以维持基因表达。在这里，我们在布氏锥虫中鉴定并生化表征了一种新型的 TFIIH 相关蛋白复合物（Med-T），该复合物由九个亚基组成，其氨基酸序列仅在动基体生物中保守。体内和体外的功能分析表明，该复合物对于细胞活力、SLRNA 转录和 PIC 完整性是必需的。通过电子显微镜对纯化的 Med-T 和 Med-T/TFIIH 复合物进行的分子结构分析表明，Med-T 对应于高等真核生物的中介体头部模块。因此，这些数据表明中介体是锥虫中小核 SL RNA 基因转录的基础因子，并且中介体头部的基础转录功能是真核生物的一个特征，在其进化的早期就已经发展起来了。
