Cryo-EM structure of 90 small ribosomal subunit precursors in transition states.

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