In eukaryotes, rRNAs and spliceosomal snRNAs are heavily modified post-transcriptionally. Pseudouridylation and 2′-O-methylation are the most abundant types of RNA modifications. They are mediated by modification guide RNAs, also known as small nucleolar (sno)RNAs and small Cajal body-specific (sca)RNAs. We used yeast and vertebrate cells to test guide activities predicted for a number of snoRNAs, based on their regions of complementarity with rRNAs. We showed that human SNORA24 is a genuine guide RNA for 18S-Ψ609, despite some noncanonical base-pairing with its target. At the same time, we found quite a few snoRNAs that have the ability to base-pair with rRNAs and can induce predicted modifications in artificial substrate RNAs, but do not modify the same target sequence within endogenous rRNA molecules. Furthermore, certain fragments of rRNAs can be modified by the endogenous yeast modification machinery when inserted into an artificial backbone RNA, even though the same sequences are not modified in endogenous yeast rRNAs. In Xenopus cells, a guide RNA generated from scaRNA, but not from snoRNA, could induce an additional pseudouridylation of U2 snRNA at position 60; both guide RNAs were equally active on a U2 snRNA-specific substrate in yeast cells. Thus, post-transcriptional modification of functionally important RNAs, such as rRNAs and snRNAs, is highly regulated and more complex than simply strong base-pairing between a guide RNA and substrate RNA. We discuss possible regulatory roles for these unexpected modifications.

中文翻译：

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SnoRNA引导活动：真实与模糊


在真核生物中，rRNA 和剪接体 snRNA 在转录后经过大量修饰。假尿苷化和 2′- O-甲基化是最丰富的 RNA 修饰类型。它们由修饰指导 RNA 介导，也称为小核仁 (sno)RNA 和小卡哈尔体特异性 (sca)RNA。我们使用酵母和脊椎动物细胞来测试根据与 rRNA 互补的区域预测的许多 snoRNA 的引导活性。我们证明人类 SNORA24 是 18S-Ψ609 的真正引导 RNA，尽管与其靶标存在一些非规范碱基配对。同时，我们发现相当多的snoRNA具有与rRNA碱基配对的能力，并且可以诱导人工底物RNA中的预测修饰，但不会修饰内源rRNA分子内的相同靶序列。此外，当插入人工主链RNA时，rRNA的某些片段可以被内源酵母修饰机制修饰，即使相同的序列在内源酵母rRNA中没有被修饰。在非洲爪蟾细胞中，由 scaRNA 而不是由 snoRNA 生成的引导 RNA 可以诱导 U2 snRNA 在 60 位额外的假尿苷化；两种引导 RNA 对酵母细胞中 U2 snRNA 特异性底物的活性相同。因此，功能重要的RNA（例如rRNA和snRNA）的转录后修饰受到高度调控，并且比指导RNA和底物RNA之间简单的强碱基配对更为复杂。我们讨论这些意想不到的修改可能发挥的监管作用。