Department of Biological Sciences, University of Maryland Baltimore County (UMBC), 1000 Hilltop Circle, Baltimore, MD 21250, USA.
Department of Biology, University of Rochester, Rochester, NY 14627, USA.
Int J Mol Sci. 2021 Jun 22;22(13):6690. doi: 10.3390/ijms22136690.
Processing of the RNA polymerase I pre-rRNA transcript into the mature 18S, 5.8S, and 25S rRNAs requires removing the "spacer" sequences. The canonical pathway for the removal of the ITS1 spacer involves cleavages at the 3' end of 18S rRNA and at two sites inside ITS1. The process can generate either a long or a short 5.8S rRNA that differs in the number of ITS1 nucleotides retained at the 5.8S 5' end. Here we document a novel pathway to the long 5.8S, which bypasses cleavage within ITS1. Instead, the entire ITS1 is degraded from its 5' end by exonuclease Xrn1. Mutations in RNase MRP increase the accumulation of long relative to short 5.8S rRNA. Traditionally this is attributed to a decreased rate of RNase MRP cleavage at its target in ITS1, called A3. However, results from this work show that the MRP-induced switch between long and short 5.8S rRNA formation occurs even when the A3 site is deleted. Based on this and our published data, we propose that the link between RNase MRP and 5.8S 5' end formation involves RNase MRP cleavage at unknown sites elsewhere in pre-rRNA or in RNA molecules other than pre-rRNA.
RNA 聚合酶 I 前 rRNA 转录本加工成成熟的 18S、5.8S 和 25S rRNA 需要去除“间隔序列”。ITS1 间隔序列去除的典型途径涉及 18S rRNA 3' 端和 ITS1 内两个位点的切割。该过程可以产生长或短的 5.8S rRNA，其在 5.8S 5' 端保留的 ITS1 核苷酸数量不同。在这里，我们记录了一种通向长 5.8S 的新途径，该途径绕过了 ITS1 内的切割。相反，整个 ITS1 由外切酶 Xrn1 从其 5' 端降解。RNase MRP 的突变会增加长 5.8S rRNA 的积累，相对于短 5.8S rRNA。传统上，这归因于 ITS1 中靶标 A3 处 RNase MRP 切割速率的降低。然而，这项工作的结果表明，即使 A3 位点缺失，MRP 诱导的长 5.8S rRNA 与短 5.8S rRNA 形成之间的转换也会发生。基于这一点和我们已发表的数据，我们提出 RNase MRP 与 5.8S 5' 端形成之间的联系涉及 RNase MRP 在 pre-rRNA 或 pre-rRNA 以外的 RNA 分子中的未知位点的切割。
