Figure 1.

Schematic phase diagram of protein-nucleic acid (NA) mixtures. Increasing the NA concentration (e.g., along the gray arrow) drives an initial phase transition from one phase to two phases and the formation of heterotypic protein-NA condensates. Further increasing the NA concentration drives the second phase transition from two phases to one phase and dissolution of the condensate. Such a process is termed as RNA-mediated reentrant phase transition, which is driven by electrostatic interactions. Right column: the interfacial tension, viscosity, and diffusivity of protein-RNA condensates across the two-phase regime can be quantified using a combined method, including PTM, FCS, and optical tweezer-induced droplet fusion (OTF). In OTF, one optically trapped condensate is forced to move toward another stationary droplet to induce droplet fusion in solution. OTF enables us to probe the ratio of viscosity over surface tension.