RNA as a crowding agent.
The crowded intracellular milieu shapes the thermodynamics and kinetics of biochemical reactions. RNA, an abundant and structurally versatile polymer, contributes to this crowding by acting both as a physical agent that restricts diffusion and enhances excluded volume, and as a sequence- and structure-specific scaffold driving multivalent RNA-RNA and RNA-protein interactions. These properties position RNA as a biologically active determinant of intracellular organization, distinct from inert synthetic crowders and purely structural scaffolds. This minireview examines how RNA shapes the formation, composition, and material properties of biomolecular condensates, highlighting the molecular grammar encoded in RNA sequence, length and valency, structure, and chemical modifications. We discuss how concentration-dependent, biphasic effects of RNA on condensate assembly can tip the balance between functional compartmentalization and pathological liquid-to-solid transitions implicated in neurodegenerative disease and cancer. Finally, we outline challenges in defining RNA-specific thresholds and translating structural insights into therapeutic strategies for mitigating aberrant RNA-mediated crowding.