Membraneless organelles or condensates form through liquid–liquid phase separation^1,2,3,4, which is thought to underlie gene transcription through condensation of the large-scale nucleolus^5,6,7 or in smaller assemblies known as transcriptional condensates^8,9,10,11. Transcriptional condensates have been hypothesized to phase separate at particular genomic loci and locally promote the biomolecular interactions underlying gene expression. However, there have been few quantitative biophysical tests of this model in living cells, and phase separation has not yet been directly linked with dynamic transcriptional outputs^12,13. Here, we apply an optogenetic approach to show that FET-family transcriptional regulators exhibit a strong tendency to phase separate within living cells, a process that can drive localized RNA transcription. We find that TAF15 has a unique charge distribution among the FET family members that enhances its interactions with the C-terminal domain of RNA polymerase II. Nascent C-terminal domain clusters at primed genomic loci lower the energetic barrier for nucleation of TAF15 condensates, which in turn further recruit RNA polymerase II to drive transcriptional output. These results suggest that positive feedback between interacting transcriptional components drives localized phase separation to amplify gene expression.

无膜细胞器或缩合物通过液-液相分离^1,2,3,4形成，这被认为是基因转录的基础，通过大规模核仁^5,6,7或称为转录缩合物的较小组装体的缩合^{8,9 ,10,11}。已假设转录凝聚物在特定基因组位点相分离并局部促进基因表达的生物分子相互作用。然而，在活细胞中对该模型的定量生物物理测试很少，相分离尚未与动态转录输出直接相关^12,13. 在这里，我们应用光遗传学方法来证明 FET 家族的转录调节因子在活细胞内表现出强烈的相分离趋势，这一过程可以驱动局部 RNA 转录。我们发现 TAF15 在 FET 家族成员中具有独特的电荷分布，增强了它与 RNA 聚合酶 II 的 C 端结构域的相互作用。初生基因组位点的新生 C 末端域簇降低了 TAF15 凝聚物成核的能量屏障，从而进一步募集 RNA 聚合酶 II 以驱动转录输出。这些结果表明相互作用的转录成分之间的正反馈驱动局部相分离以放大基因表达。