Loss of GATA1 results in the loss of looping interactions within the TAL1 active hub. Bar diagrams of interaction patterns across the human TAL1 locus after siRNA transfection (48 hours and 96 hours) with GATA1 (KD) or with luciferase (LUC) as determined by 3C. (A) Interaction between TAL1 promoter 1b (P^TAL) and the stem cell enhancer (+19). (B) Interaction between TAL1 promoter 1b (P^TAL) and the erythroid enhancer (+51). (C) Interaction between the TAL1 erythroid enhancer (+51) and the stem cell enhancer (+19). Interactions, measured as relative ligation frequencies, are shown with SEs. The +19 and +51 enhancers are highlighted as black bars in the histograms. Locations of 3C “bait” regions are denoted by vertical gray arrows. P values are indicated for relative ligation frequencies which are significantly reduced between the bait and enhancers in KD conditions when compared with the corresponding LUC controls. Significant loss of interactions due to GATA1 knockdown are shown by the dotted gray lines with arrowheads (and marked with an “X”). Control regions are as in Figure 1. (D) Schematic model of looping interactions at the TAL1 locus, and during GATA1 siRNA knockdown, in K562 cells. Loss of GATA1 and the TEC from TAL1 cis-regulatory elements (Figure 2) is accompanied by loss of TAL1 transcription (percentage of mRNA remaining relative to control shown in brackets) and loss of looping interactions, initially most evident between +19 and the TAL1 promoters (48 hours GATA1 knockdown), and then between +19, +51 and the TAL1 promoters (96 hours GATA1 knockdown). *P < .05; **P < .01; *****P < .00001.