The ribosome’s common core, comprised of ribosomal RNA (rRNA) and universal ribosomal proteins, connects all life back to a common ancestor and serves as a window to relationships among organisms. The rRNA of the common core is most similar to rRNA of extant bacteria. In eukaryotes, the rRNA of the common core is decorated by expansion segments (ESs) that vastly increase its size. Supersized ESs have not been observed previously in Archaea, and the origin of eukaryotic ESs remains enigmatic. We discovered that the large subunit (LSU) rRNA of two Asgard phyla, Lokiarchaeota and Heimdallarchaeota, considered to be the closest modern archaeal cell lineages to Eukarya, bridge the gap in size between prokaryotic and eukaryotic LSU rRNA. The elongated LSU rRNAs in Lokiarchaeota and Heimdallarchaeota stem from the presence of two supersized ESs, ES9 and ES39. We applied chemical footprinting experiments to study the structure of Lokiarchaeota ES39. Furthermore, we used covariation and sequence analysis to study the evolution of Asgard ES39s and ES9s. By defining the common eukaryotic ES39 signature fold, we found that Asgard ES39s have more and longer helices than eukaryotic ES39s. While Asgard ES39s have sequences and structures distinct from eukaryotic ES39s, we found overall conservation of a three-way junction across the Asgard species that matches eukaryotic ES39 topology, a result consistent with the accretion model of ribosomal evolution.

中文翻译：

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Asgard 古细菌中超大核糖体 RNA 扩增片段。

核糖体的共同核心由核糖体 RNA (rRNA) 和通用核糖体蛋白组成，将所有生命连接回一个共同的祖先，并作为生物体之间关系的窗口。共同核心的rRNA与现存细菌的rRNA最相似。在真核生物中，共同核心的 rRNA 被扩展片段 (ES) 修饰，从而大大增加了其大小。以前在古细菌中没有观察到超大的 ESs，真核 ESs 的起源仍然是个谜。我们发现两个 Asgard 门Lokiarchaeota和Heimdallarchaeota的大亚基 (LSU) rRNA ，被认为是最接近真核的现代古菌细胞谱系，弥合了原核和真核 LSU rRNA 之间的大小差距。Lokiarchaeota 中拉长的 LSU rRNA和Heimdallarchaeota源于两个超大 ESs，ES9 和 ES39 的存在。我们应用化学足迹实验来研究Lokiarchaeota ES39的结构。此外，我们使用协变和序列分析来研究 Asgard ES39s 和 ES9s 的进化。通过定义常见的真核 ES39 特征折叠，我们发现 Asgard ES39 比真核 ES39 具有更多和更长的螺旋。虽然 Asgard ES39s 具有不同于真核 ES39s 的序列和结构，但我们发现整个 Asgard 物种的三向连接的总体保守性与真核 ES39 拓扑相匹配，结果与核糖体进化的吸积模型一致。