Cryo-EM reconstructions of LiCl core particles show strong similarities between disassembly and assembly of the ribosomal 50S subunit

Bacterial ribosome assembly is a complex process. Purified 50S ribosomes subjected to high-salt wash removing a subset of ribosomal proteins (r-proteins), were early shown competent for in vitro assembly into functional 50S subunits. We here used cryo-EM to determine the structure of such LiCl core particles derived from E. coli 50S subunits. A wide range of complexes with large variation in extent of ordered 23S rRNA and occupancy of r-proteins could be identified, and resolved to between 2.8 Å and 9 Å resolution. Many of these particles showed high similarity to in vivo and in vitro assembly intermediates, supporting the inherent stability or metastability of these states. Similar to states in early ribosome assembly, the main class showed ordered density for 23S rRNA domains 0, I, II, III, VI and the 5’-half of domain IV. In addition, smaller core particles were discovered, which may mimic earlier stages of 50S biogenesis. Our data support a multi-pathway disassembly process based on independent folding blocks, similar but reverse to the assembly process. The study provides examples of dependencies between complex tertiary RNA structure and RNA-protein interactions, exemplifies the need for proteins to stabilize native RNA structure also after folding, and supports the relevance of salt-washed core particles as a mimics of early ribosome assembly intermediates. ### Competing Interest Statement The authors have declared no competing interest.