Over the course of evolution, land plant mitochondrial genomes have lost many transfer RNA (tRNA) genes and the import of nucleus-encoded tRNAs is essential for mitochondrial protein synthesis. By contrast, plastidial genomes of photosynthetic land plants generally possess a complete set of tRNA genes and the existence of plastidial tRNA import remains a long-standing question. The early vascular plants of the Selaginella genus show an extensive loss of plastidial tRNA genes while retaining photosynthetic capacity, and represent an ideal model for answering this question. Using purification, northern blot hybridization, and high-throughput tRNA sequencing, a global analysis of total and plastidial tRNA populations was undertaken in Selaginella kraussiana. We confirmed the expression of all plastidial tRNA genes and, conversely, observed that nucleus-encoded tRNAs corresponding to these plastidial tRNAs were generally excluded from the chloroplasts. We then demonstrated a selective and differential plastidial import of around forty nucleus-encoded tRNA species, likely compensating for the insufficient coding capacity of plastidial-encoded tRNAs. In-depth analysis revealed differential import of tRNA isodecoders, leading to the identification of specific situations. This includes the expression and import of nucleus-encoded tRNAs expressed from plastidial or bacterial-like genes inserted into the nuclear genome. Overall, our results confirm the existence of molecular processes that enable tRNAs to be selectively imported not only into mitochondria, as previously described, but also into chloroplasts, when necessary.

中文翻译：

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将细胞核编码的 tRNA 广泛输入到光合石松植物 Selaginella kraussiana 的叶绿体中


在进化过程中，陆地植物线粒体基因组丢失了许多转移 RNA （tRNA） 基因，细胞核编码的 tRNA 的输入对于线粒体蛋白质合成至关重要。相比之下，光合陆地植物的质体基因组通常具有一套完整的 tRNA 基因，质体 tRNA 输入的存在仍然是一个长期存在的问题。Selaginella 属的早期维管植物表现出质体 tRNA 基因的广泛丢失，同时保留了光合作用能力，代表了回答这个问题的理想模型。使用纯化、Northern blot 杂交和高通量 tRNA 测序，对克拉索黑卷柏 （Selaginella kraussiana） 的总和质体 tRNA 种群进行了全局分析。我们确认了所有质体 tRNA 基因的表达，相反，观察到与这些质体 tRNA 相对应的细胞核编码的 tRNA 通常被排除在叶绿体之外。然后，我们证明了大约 40 种细胞核编码的 tRNA 物种的选择性和差异质体输入，可能弥补了质体编码的 tRNA 编码能力不足。深入分析揭示了 tRNA 同种解码器的差异输入，从而确定了特定情况。这包括从插入核基因组的质体或细菌样基因表达的细胞核编码的 tRNA 的表达和输入。总体而言，我们的结果证实了分子过程的存在，这些过程使 tRNA 不仅可以选择性地输入到线粒体中，如前所述，还可以在必要时输入到叶绿体中。