Replacement of Fab-7 by the gypsy or scs insulator disrupts long-distance regulatory interactions in the Abd-B gene of the bithorax complex

Chromatin domain boundaries, like scs or gypsy insulators in Drosophila, ha ve been identified in transgene assays through their enhancer-blocking acti vity. Boundary elements in the bithorax complex (BX-C), such as Fab-7 and F ab-8, have been identified genetically and been shown to have insulator act ivity in transgene assays. However, it is not clear whether boundary elemen ts identified in transgene assays will function appropriately in chromosoma l contexts such as BX-C. Using gene conversion, we have substituted the scs or gypsy insulators for Fab-7. We find that both scs and gypsy are very po tent insulators in the ectoderm, but surprisingly, the insulating activity of gypsy (but not scs) is lost in the CNS. Our results reveal that the Fab- 7 boundary must have special properties that scs and gypsy lack, which allo w it to function appropriately in BX-C regulation.