Abstract

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.

Highlights

  • Processing of the RNA polymerase I pre-rRNA transcript into the mature 18S, 5.8S, and 25S rRNAs requires removing the “spacer” sequences

  • Both strains harbor an 18S-5.8S-25S rRNA transcription unit on the high-copy plasmid pDK16 that is transcribed by RNA polymerase II (Pol II) from the Cu2+ induced

  • Previous studies showed that pre-rRNA transcripts produced by Pol

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Summary

Introduction

Processing of the RNA polymerase I pre-rRNA transcript into the mature 18S, 5.8S, and 25S rRNAs requires removing the “spacer” sequences. MRP increase the accumulation of long relative to short 5.8S rRNA This is attributed to a decreased rate of RNase MRP cleavage at its target in ITS1, called A3. 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. Ribosome formation is the most resource-requiring process in both pro- and eukaryotes [1,2,3] It involves complex pathways for regulated synthesis of rRNA and ribosomal proteins (r-proteins), and the assembly of these components into functional ribosomal subunits. The pathways differ between pro- and eukaryotes, but in eukaryotes the major steps are conserved from yeast to humans, the complexity has evolved [4,5,6]

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