In Saccharomyces cerevisiae intron-containing pre-tRNAs are exported from the nucleus to the cytoplasm for removal of the introns, and the spliced tRNAs are returned to the nucleus for reasons that are not understood. The re-imported spliced tRNAs are then subjected to aminoacylation in the nucleolus to ensure that they are functional prior to re-export to the cytoplasm. Previous studies have shown that re-imported spliced tRNAs and mature tRNAs made entirely in the nucleus from intronless precursors are retained in the nucleus of S. cerevisiae in response to glucose, amino acid, nitrogen or inorganic phosphate deprivation. Contrary to these studies, we recently reported that starvation of S. cerevisiae of amino acids or nitrogen results in nuclear accumulation of re-imported spliced tRNAs, but not tRNAs made from intronless precursors. This finding suggests that separate pathways are used for nuclear export of retrogradely transported spliced tRNAs and tRNAs made from intronless pre-tRNAs. In addition, the data support the conclusion that the nuclear re-export pathway for retrogradely transported spliced tRNAs, but not the pathway responsible for nuclear export of tRNAs derived from intronless precursors is regulated during amino acid or nitrogen starvation. This regulation appears to occur at a step after the re-imported spliced tRNAs have undergone aminoacylation quality assurance and, in part, involves the TORC1 signalling pathway. Moreover, it was established that Utp9p is an intranuclear component that only facilitates nuclear re-export of retrogradely transported spliced tRNAs by the β-karyopherin Msn5p. Utp9p acts in concert with Utp8p, a key player in nuclear tRNA export in S. cerevisiae, to translocate aminoacylated re-imported spliced tRNAs from the nucleolus to Msn5p and assist with formation of the Msn5p-tRNA-Gsp1p-GTP export complex. This pathway, however, is not the only one responsible for nuclear re-export of retrogradely transported spliced tRNAs.

中文翻译：

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酿酒酵母逆行转运 tRNA 核转口的来龙去脉

在酿酒酵母中，含有内含子的前 tRNA 从细胞核输出到细胞质以去除内含子，并且由于未知原因，剪接的 tRNA 返回细胞核。重新导入的剪接 tRNA 然后在核仁中进行氨酰化，以确保它们在重新导出到细胞质之前具有功能。先前的研究表明，重新导入的剪接 tRNA 和完全在细胞核中由无内含子前体制成的成熟 tRNA 保留在酿酒酵母的细胞核中，以响应葡萄糖、氨基酸、氮或无机磷酸盐的缺乏。与这些研究相反，我们最近报道了氨基酸或氮的酿酒酵母饥饿会导致重新导入的剪接 tRNA 的核积累，但不会导致由无内含子前体制成的 tRNA。这一发现表明，逆行转运的剪接 tRNA 和由无内含子前 tRNA 制成的 tRNA 的核输出使用了不同的途径。此外，数据支持这样的结论，即逆行转运的剪接 tRNA 的核再出口途径，而不是负责从无内含子前体衍生的 tRNA 核出口的途径在氨基酸或氮饥饿期间受到调节。这种调节似乎发生在重新导入的剪接 tRNA 经历氨酰化质量保证之后的一个步骤，部分涉及 TORC1 信号通路。此外，已确定 Utp9p 是一种核内成分，仅促进 β-核型细胞融合蛋白 Msn5p 逆行转运的剪接 tRNA 的核再出口。Utp9p 与 Utp8p 协同作用，酿酒酵母中核 tRNA 输出的关键参与者，将氨酰化重新输入的剪接 tRNA 从核仁转移到 Msn5p，并协助形成 Msn5p-tRNA-Gsp1p-GTP 输出复合物。然而，这条通路并不是唯一负责逆行转运的剪接 tRNA 的核再出口的通路。