How the concentric organization of the nucleolus and chromatin ensures accuracy of ribosome biogenesis and drives transport.

The article focuses on the biogenetic transport and maturation of ribosomal subunit precursors within the nucleus of higher eukaryotic cells, emphasizing the concentric organization of nucleolar subcompartments—fibrillar center (FC), dense fibrillar component (DFC), granular component (GC), and surrounding chromatin—and their roles in ribosome biogenesis. It discusses how inhibition of ribosomal RNA (rRNA) synthesis leads to a reorganization or "inversion" of these subcompartments, likely driven by changes in rDNA packaging and chromatin compatibility. The authors propose a vectorial relay mechanism based on a Brownian ratchet model, whereby assembly factors characteristic of each subcompartment bind maturing ribonucleoprotein particles to facilitate directional transport and quality control through successive nuclear phases. This framework integrates thermodynamic incompatibility and kinetic access to explain the sequential maturation and transport of ribosomal precursors, highlighting the nucleolus as a series of distinct but contiguous phases that ensure efficient ribosome assembly.