Cryo-EM structure of 90<i>S</i> small ribosomal subunit precursors in transition states.

Abstract

The 90<i>S</i> preribosome is a large, early assembly intermediate of small ribosomal subunits that undergoes structural changes to give a pre-40<i>S</i> ribosome. Here, we gained insight into this transition by determining cryo-electron microscopy structures of <i>Saccharomyces cerevisiae</i> intermediates in the path from the 90<i>S</i> to the pre-40<i>S</i> 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<i>S</i>, driving stepwise disassembly of assembly factors and ribosome maturation. The nuclear exosome, an RNA degradation machine, docks on the 90<i>S</i> 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<i>S</i> progression.