Odom_tRNA_evolution. Odom_tRNA_evolution

Homologous vertebrate tissues express a highly conserved set of transcribed genes; paradoxically, expression of tRNAs that are required to translate mRNAs into proteins have been reported to be divergent. To resolve this paradox, we mapped the genome-wide occupancy of pol III in primary tissues isolated from six mammals. We confirmed that the specific tRNA genes bound by pol III, as well as the extent and stability of binding, can vary substantially among mammalian tissues, and we discovered that this divergence is far greater between species. We combined pol III occupancy from genomically discrete tRNA loci into collective binding into isoacceptor classes and then into amino acid-based isotype classes, and at each step we found increasing conservation. At the level of amino acid isotypes, pol III binding is almost invariant among all the tissues and species profiled. Thus, the basal transcriptional machinery is constrained collectively in its synthesis of functional tRNA isotypes, despite rapid divergence of polymerase binding to specific tRNA genes.

同源脊椎动物组织通常表达一组高度保守的转录基因；然而令人矛盾的是，据报道，负责将mRNA翻译为蛋白质的转运RNA（tRNA）的表达却呈现分化特征。为解析这一矛盾，我们对分离自6种哺乳动物的原代组织中的RNA聚合酶III（Pol III）的全基因组结合占据情况进行了图谱绘制与分析。我们证实，Pol III结合的特定tRNA基因，以及结合的程度与稳定性，在不同哺乳动物组织间存在显著差异；且进一步发现，这类差异在物种间的分化程度更为突出。我们将基因组上离散tRNA位点的Pol III结合占据数据，先整合为同工受体类别下的集体结合特征，再进一步整合为基于氨基酸的tRNA同型类别；且在每一级分类中均观察到保守性逐步提升。在tRNA氨基酸同型这一分类层级上，Pol III的结合情况在所有被分析的组织与物种中几乎完全一致。由此可见，尽管聚合酶与特定tRNA基因的结合存在快速分化，但基础转录机器在功能性tRNA同型的合成过程中受到了集体性的约束。