Re-entrant condensation of DNA due to the competition between self-assembly and phase separation

Biomolecular condensates formed by Liquid-Liquid Phase Separation (LLPS) carry out critical physiological functions of cells, such as controlling stress responses, maintaining homeostasis, biochemical reactions, transport, and signaling. Model systems made up of simple constituents, such as multivalent assemblies of DNA strands called "DNA nanostars (NSs)", can be used to probe the fundamental physics governing phase separation. In this talk, I will show that the phase behavior of DNA NSs is far richer than anticipated. At room temperature, DNA NSs phase separate due to attractive interactions between their sticky ends. The sticky-end interactions weaken with increasing temperature, leading to a transition to a single phase. Interestingly, upon further increasing the temperature, we find that the NSs again phase separate. We hypothesize that this re-entrant transition results from the disassembly of NSs and phase separation of the constituent DNA oligomers. To test this hypothesis, we construct the temperature-dependent phase diagram using droplet-based microfluidics and compare our measurements to predictions from a simple theoretical model. Quantitative agreement between our experiments and predictions suggests that the very same molecules can separate into condensates at both low and high temperatures and that the re-entrant transition between the two condensate regions is modulated by the competition between self-assembly and phase separation.