Present-day mitochondria derive from a single endosymbiosis of an α-proteobacterium into a proto-eukaryotic cell. Since this monophyletic event, mitochondria have evolved considerably, and unique traits have been independently acquired in the different eukaryotic kingdoms. Mitochondrial genome expression and RNA metabolism have diverged greatly. Here, Cyanophora paradoxa, a freshwater alga considered as a living fossil among photosynthetic organisms, represents an exciting model for studying the evolution of mitochondrial gene expression. As expected, fully mature tRNAs are released from primary transcripts to function in mitochondrial translation. We also show that these tRNAs take part in an mRNA processing punctuation mechanism in a non-conventional manner, leading to mRNA–tRNA hybrids with a CCA triplet at their 3′-extremities. In this case, tRNAs are probably used as stabilizing structures impeding the degradation of mRNA by exonucleases. From our data we propose that the present-day tRNA-like elements (t-elements) found at the 3′-terminals of mitochondrial mRNAs in land plants originate from true tRNAs like those observed in the mitochondria of this basal photosynthetic glaucophyte.

中文翻译：

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Cyanophora paradoxa 线粒体 tRNA 玩双重游戏

现代线粒体源自α-变形杆菌进入原真核细胞的单一内共生。自从这一单系事件以来，线粒体已经有了很大的发展，并且在不同的真核生物界中独立地获得了独特的特征。线粒体基因组表达和 RNA 代谢有很大差异。这里，蓝藻悖论，一种被认为是光合生物活化石的淡水藻类，代表了研究线粒体基因表达进化的令人兴奋的模型。正如预期的那样，完全成熟的 tRNA 从初级转录本中释放出来，在线粒体翻译中发挥作用。我们还表明，这些 tRNA 以非常规方式参与 mRNA 处理标点机制，导致 mRNA-tRNA 杂交体在其 3'-末端具有 CCA 三联体。在这种情况下，tRNA 可能用作稳定结构，阻止外切核酸酶降解 mRNA。根据我们的数据，我们提出在陆地植物线粒体 mRNA 的 3' 末端发现的现代 tRNA 样元件（t-元件）源自真正的 tRNA，就像在这种基础光合绿藻的线粒体中观察到的那些一样。