Ecdysone-Induced 3D Chromatin Reorganization Involves Active Enhancers Bound by Pipsqueak and Polycomb.

Evidence suggests that Polycomb (Pc) is present at chromatin loop anchors in Drosophila. Pc is recruited to DNA through interactions with the GAGA binding factors GAF and Pipsqueak (Psq). Using HiChIP in Drosophila cells, we find that the psq gene, which has diverse roles in development and tumorigenesis, encodes distinct isoforms with unanticipated roles in genome 3D architecture. The BR-C, ttk, and bab domain (BTB)-containing Psq isoform (PsqL) colocalizes genome-wide with known architectural proteins. Conversely, Psq lacking the BTB domain (PsqS) is consistently found at Pc loop anchors and at active enhancers, including those that respond to the hormone ecdysone. After stimulation by this hormone, chromatin 3D organization is altered to connect promoters and ecdysone-responsive enhancers bound by PsqS. Our findings link Psq variants lacking the BTB domain to Pc-bound active enhancers, thus shedding light into their molecular function in chromatin changes underlying the response to hormone stimulus.