Chromatin boundary elements organize genomic architecture and developmental gene regulation in Drosophila Hox clusters

Figure 3 Hypothetical function of the SF1-SF2 chromatin loop in Scr and ftz gene regulation (adapted from Li et al[27], 2015). A: Segmental expression patter of a Drosophila Hox gene Scr (green). A neighbor non-Hox gene ftz is expressed in pair-rule stripes (red); B: A model for developmentally regulated SF1-SF2 chromatin loop in modulating enhancer-promoter communications and preventing spread of silent chromatin in the Drosophila Hox complex. The chromatin loop anchored by the interaction between SF1 and SF2 (green and red vertical ovals, respectively) during early Drosophila embryogenesis can restrict the access of ftz and Scr enhancers (red and green ovals, respectively) to neighboring gene promoters (curved block signs). The SF1-SF2 loop also prevents the spreading of the repressive H3K9Me3 and H3K27Me3 histone modification (grey forbidden circles) into the active ftz gene. Pairing of SF1 and SF2 may facilitate “enhancer bypass”, connect the Scr with its distal enhancers (green ovals) in 3D in the early Drosophila embryo. The ftz gene domain is present in opposite orientation in various Drosophila species (grey long arrows, bottom). FD: Ftz distal enhancer; F1/5: Ftz stripe 1/5 enhancer; Antp: Antennapedia, a Hox gene located distal to ftz.