A cluster of ribosome synthesis factors regulate pre-rRNA folding and 5.8S rRNA maturation by the Rat1 exonuclease

The 5′-exonuclease Rat1 degrades pre-rRNA spacer fragments and processes the 5′-ends of the 5.8S and 25S rRNAs. UV crosslinking revealed multiple Rat1-binding sites across the pre-rRNA, consistent with its known functions. The major 5.8S 5′-end is generated by Rat1 digestion of the internal transcribed spacer 1 (ITS1) spacer from cleavage site A3. Processing from A3 requires the ‘A3-cluster' proteins, including Cic1, Erb1, Nop7, Nop12 and Nop15, which show interdependent pre-rRNA binding. Surprisingly, A3-cluster factors were not crosslinked close to site A3, but bound sites around the 5.8S 3′- and 25S 5′-regions, which are base paired in mature ribosomes, and in the ITS2 spacer that separates these rRNAs. In contrast, Nop4, a protein required for endonucleolytic cleavage in ITS1, binds the pre-rRNA near the 5′-end of 5.8S. ITS2 was reported to undergo structural remodelling. In vivo chemical probing indicates that A3-cluster binding is required for this reorganization, potentially regulating the timing of processing. We predict that Nop4 and the A3 cluster establish long-range interactions between the 5.8S and 25S rRNAs, which are subsequently maintained by ribosomal protein binding.