Programmable artificial RNA condensates in mammalian cells.

Artificial biomolecular condensates have emerged as powerful tools for controlling cellular behaviour. Here we introduce a method to build artificial condensates within living mammalian cells by designing modular RNA motifs composed of a single short RNA strand. These condensates emerge spontaneously, creating RNA-rich compartments that remain separated from their surrounding environment. The RNA sequences include stem-loop domains that fold as the RNA is transcribed, and then condense in the nucleus and cytoplasm through loop-loop interactions. These sequences can be optimized and diversified, enabling the generation of distinct, non-mixing condensate populations and the programmable control of their subcellular localization. The RNA motifs can also be modified to recruit small molecules, proteins and RNA molecules in a sequence-specific manner to the RNA-rich phase. By introducing RNA linkers, we can build condensates with multiple subcompartments, whose organization can be controlled by tuning the linker stoichiometry. These artificial condensates provide a versatile platform for studying and manipulating molecular functions inside living cells.