The YY1 C-terminal half is sufficient for rescue of DNA loops between the Eμ and 3′RR enhancers, and for CSR in endogenous YY1-deleted primary splenic B cells. (A) Maps of YY1 constructs used to rescue YY1-conditionally deleted splenic B cells. The transcriptional activation domain resides near the amino terminus spanning amino acids 16-99.²⁵  The REPO domain involved in YY1 recruitment of PcG proteins to DNA lies between residues 201 and 226.¹⁷  The 4 zinc fingers that constitute the DNA binding domain lie at the C-terminus, spanning amino acids 298-414.²⁵  (B) 3D FISH results with the probes shown in Figure 1A in primary yy1^f/f splenic B cells that were mock treated, TAT-CRE treated, or TAT-CRE treated plus rescue with either vector alone or vector expressing full-length YY1, YY1 1-200, or YY1 201-414. Transduced cells were isolated by FACS for expression of green fluorescent protein (linked in the vector), and the Eμ-3′RR DNA loop was measured by 3D FISH. Equivalent expression of each construct was verified (supplemental Figure 4A) (C) Separation between probes was measured after image deconvolution from at least 100 nuclei. Bar graphs show the percentage of alleles in which the distance between probes fell in the ranges shown by different colors. (D) Cumulative frequencies of FISH probe spatial distances. (E) Rescue of CSR with each construct is shown relative to the level of CSR observed with mock-treated yy1^f/f splenic B cells (no YY1 deletion). Asterisks denote significant change from vector alone (P < .001). All experiments were repeated 3 to 4 times. pMX, Moloney murine leukemia virus vector X; WT, wild type.