The complete structure of the small subunit processome

The small subunit processome represents the earliest stable precursor of the eukaryotic small ribosomal subunit. Here we present the cryo-EM structure of the Saccharomyces cerevisiae small subunit processome at an overall resolution of 3.8 Å, which provides an essentially complete atomic model of this assembly. In this nucleolar superstructure, 51 ribosome assembly factors and two RNAs encapsulate the 18S rRNA precursor and 15 ribosomal proteins in a state that precedes pre-rRNA cleavage at site A1. Extended flexible proteins are employed to connect distant sites in this particle. Molecular mimicry, steric hindrance as well as protein-and RNA-mediated RNA remodeling are used in a concerted fashion to prevent the premature formation of the central pseudoknot and its surrounding elements within the small ribosomal subunit.