Published on Jan. 1, 2020 in Science volume 369: 1477-1481.

PubMed ID: 32943522

DOI: 10.1126/science.aba9690

Abstract:

The 90 S preribosome is a large, early assembly intermediate of small ribosomal subunits that undergoes structural changes to give a pre-40 S ribosome. Here, we gained insight into this transition by determining cryo-electron microscopy structures of Saccharomyces cerevisiae intermediates in the path from the 90 S to the pre-40 S The full transition is blocked by deletion of RNA helicase Dhr1. A series of structural snapshots revealed that the excised 5' external transcribed spacer (5' ETS) is degraded within 90 S , driving stepwise disassembly of assembly factors and ribosome maturation. The nuclear exosome, an RNA degradation machine, docks on the 90 S through helicase Mtr4 and is primed to digest the 3' end of the 5' ETS. The structures resolved between 3.2- and 8.6-angstrom resolution reveal key intermediates and the critical role of 5' ETS degradation in 90 S progression.