SRA

Assembly of eukaryotic ribosomes begins in the nucleolus, a compartmentalized membraneless organelle. Although the two ribosomal subunits, 40S and 60S, assemble independently, it remains unknown if these particles are physically sorted as they assemble and how they partition from the central chromatin compartment into the outer nucleolar regions, where maturation occurs. In this study, we show that nucleophosmin specifically mediates the assembly of nascent 60S subunits and that this specificity is determined by its chromatin localization at the rDNA sites encoding for 60S subunit rRNA. Nucleophosmin dissociates from chromatin to bind nascent 60S subunits, causing their partitioning away from chromatin and from nascent 40S subunits through liquid-liquid phase separation. This directs translocation of nascent 60S subunits towards the nucleophosmin-rich granular component, where biogenesis continues. Notably, this compartmentalization increases the efficiency of 60S subunit assembly, specifically the incorporation of the 60S domain III. Our data reveal that the chromatin localization of nucleophosmin determines its specificity in sorting and coordinates the movement of ribosomal subunits into specialized assembly compartments. Overall design: ChIP sequencing, nascent and total RNA sequencing in wild type cells and different S. pombe mutants