part of protein-RNA complex assembly 90S preribosome assembly https://github.com/geneontology/go-ontology/issues/23886 The aggregation, arrangement and bonding together of proteins and RNA molecules to form a 90S preribosome. The 90S preribosome represents the complex that forms on the primary rRNA transcript before it splits into the small subunit and large subunit portions. biological_process GO:0034463 cytosolic ribosome assembly