Liao, J.; Ahn, S. Y.; Obermeyer, A. C.

At sites of active gene expression, dynamic compartments known as transcriptional condensates assemble and dissolve on timescales relevant to RNA synthesis and degradation. Yet how the non-equilibrium dynamics of these condensates emerge from the coupling of RNA concentration and phase separation remains poorly understood. Here we engineer synthetic active condensates in which T7 RNA polymerase transcribes RNA in situ, triggering phase separation with a cationic scaffold protein. By using RNA concentration as a tunable parameter, we drive condensates along defined paths through a characterized phase diagram. This reaction-phase separation coupling gives rise to three emergent dynamic phenomena not accessible in passive systems: a rapid switch-like nucleation burst, RNA-mediated positive and negative feedback regulation of transcription, and oscillatory condensate formation in which RNA degradation spontaneously renucleates condensates. Together, these results show that the dynamic functions of transcriptional condensates emerge from their reaction-driven paths through phase space, providing a quantitative framework for understanding how RNA flux governs condensate dynamics in living cells.