Active RNA synthesis patterns nuclear condensates

Biomolecular condensates organize intracellular chemistry by forming membraneless compartments. Through theory, simulation, and experiment, we investigate how RNA, a key regulator of condensate formation and dissolution, influences condensate patterning. Using nucleolar fibrillar centers (FCs) as a model condensate, we found both in theory and in experiment that inhibiting ribosomal RNA synthesis significantly alters the patterning of FCs and causes droplet ripening, whereas resuming RNA synthesis leads to droplet anti-coarsening and to a non-equilibrium steady state pattern. Moreover, altering FC condensate patterning by expression of the FC component TCOF1 impaired ribosomal RNA processing. These results show how processes crucial for biological function control condensate patterning and, in turn, how patterning affects function. This is likely a generic motif in intracellular organization.