Abstact

During eukaryotic ribosome biogenesis, nascent ribosomal RNA (rRNA) forms pre-ribosomal particles containing ribosomal proteins and assembly factors. Subsequently, these immature rRNAs are processed and remodelled. Little is known about the premature assembly states of rRNAs and their structural rearrangement during ribosome biogenesis. Using cryo-EM we characterize a pre-60S particle, where the 5S rRNA and its associated ribosomal proteins L18 and L5 (5S ribonucleoprotein (RNP)) are rotated by almost 180° when compared with the mature subunit. Consequently, neighbouring 25S rRNA helices that protrude from the base of the central protuberance are deformed. This altered topology is stabilized by nearby assembly factors (Rsa4 and Nog1), which were identified by fitting their three-dimensional structures into the cryo-EM density. We suggest that the 5S RNP performs a semicircular movement during 60S biogenesis to adopt its final position, fulfilling a chaperone-like function in guiding the flanking 25S rRNA helices of the central protuberance to their final topology.