Model of Zld's role on TF specific binding and enhancer activation. When Zld is absent before ZGA, enhancers (red bold line) bearing Zld motifs (blue bold line) and other TF motifs (yellow bold line) are covered by nucleosomes (gray circle) due to intrinsic nucleosomal preference (as far as nucleosome formation can occur during the rapid nuclear division cycles). Once Zld is present (dark blue oval), Zld binding to its motifs leads to local nucleosome depletion and possibly nucleosome shifting, which exposes the motifs of other TFs within the same enhancer. Patterning TFs (orange oval) can now access their motifs, which may lead to enhancer activation (cyan arrow). In zld− embryos, TFs are occluded from binding due to the high intrinsic nucleosome barrier at enhancers. Instead, excess TFs now bind nonspecifically (fuzzy orange oval) to open regions without cognate motifs, such as promoters. As development proceeds and Zld protein levels diminish in most late embryonic tissues, Zld decommissions from binding and TF motifs are again occluded from TF access.
