The initial components of ribosomes first appeared more than 3.8 billion years ago during a time when many types of RNAs were evolving. While modern ribosomes are complex molecular machines consisting of rRNAs and proteins, they were assembled during early evolution by the association and joining of small functional RNA units. Introns may have provided the means to ligate many of these pieces together. All four classes of introns (group I, group II, spliceosomal, and archaeal) are present in many rRNA gene loci over a broad phylogenetic range. A survey of rRNA intron sequences across the three major life domains suggests that some of the classes of introns may have diverged from one another within rRNA gene loci. Analyses of rRNA sequences revealed self-splicing group I and group II introns are present in ancestral regions of the SSU (small subunit) and LSU (large subunit), whereas spliceosomal and archaeal introns appeared in sections of the rRNA that evolved later. Most classes of introns increased in number for approximately 1 billion years. However, their frequencies are low in the most recently evolved regions added to the SSU and LSU rRNAs. Furthermore, many of the introns appear to have been in the same locations for billions of years, suggesting an ancient origin for these sequences. In this Perspectives paper, I reviewed and analyzed rRNA intron sequences, locations, structural characteristics, and splicing mechanisms; and suggest that rRNA gene loci may have served as evolutionary nurseries for intron formation and diversification.

中文翻译：

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核糖体 RNA、内含子和内含子苗圃的综合进化

核糖体的初始成分首次出现在 38 亿多年前，当时许多类型的 RNA 正在进化。虽然现代核糖体是由 rRNA 和蛋白质组成的复杂分子机器，但它们是在早期进化过程中通过小功能 RNA 单元的关联和连接组装而成的。内含子可能提供了将许多这些片段连接在一起的手段。所有四类内含子（I 组、II 组、剪接体和古细菌）都存在于广泛的系统发育范围内的许多 rRNA 基因位点中。对三个主要生命域的 rRNA 内含子序列的调查表明，某些类别的内含子可能在 rRNA 基因位点内彼此不同。rRNA 序列分析表明，自剪接 I 组和 II 组内含子存在于 SSU（小亚基）和 LSU（大亚基）的祖先区域，而剪接体和古细菌内含子出现在后来进化的 rRNA 部分。大多数种类的内含子在数量上增加了大约 10 亿年。然而，在添加到 SSU 和 LSU rRNA 的最新进化区域中，它们的频率较低。此外，许多内含子似乎已经在同一个位置存在了数十亿年，这表明这些序列有一个古老的起源。在这篇 Perspectives 论文中，我回顾并分析了 rRNA 内含子序列、位置、结构特征和剪接机制；并表明 rRNA 基因座可能作为内含子形成和多样化的进化苗圃。