Liu, W., Deng, L., Wang, M., Liu, X., Ouyang, X., Wang, Y., Miao, N., Luo, X., Wu, X., Lu, X., Xv, X., Zhang, T., Li, Y., Ji, J., Qiao, Z., Wang, S., Guan, L., Li, D., Dang, Y., Liu, C., Li, W., Zhang, Y., Wang, Z., Chen, F.X., Chen, C., Lin, C., Goh, W.S.S., Zhou, W., Luo, Z., Gao, P., Li, P., Yu, Y. (2025). Pcf11/Spt5 condensates stall RNA polymerase II to facilitate termination and piRNA-guided heterochromatin formation.  Mol. Cell 85(5): 929--947.e10.

FBrf0261810

Research paper

The PIWI-interacting RNA (piRNA) pathway plays a crucial role in protecting animal germ cells by repressing transposons. However, the mechanism of piRNA-guided heterochromatin formation and its relationship to transcriptional termination remains elusive. Through RNA interference screening, we discovered Pcf11 and PNUTS as essential for piRNA-guided silencing in Drosophila germ line. Enforced tethering of Pcf11 leads to co-transcriptional repression and RNA polymerase II (RNA Pol II) stalling, and both are dependent on an α-helical region of Pcf11 capable of forming condensates. An intrinsically disordered region can substitute for the α-helical region of Pcf11 in its silencing capacity and support animal development, arguing for a causal relationship between phase separation and Pcf11's function. Pcf11 stalls RNA Pol II by preferentially forming condensates with the unphosphorylated Spt5, promoted by the PP1/PNUTS phosphatase during termination. We propose that Pcf11/Spt5 condensates control termination by decelerating polymerase elongation, a property exploited by piRNAs to silence transposons and initiate RNA-mediated heterochromatin formation.