The Fab-8 boundary defines the distal limit of the bithorax complex iab-7 domain and insulates iab-7 from initiation elements and a PRE in the adjacent iab-8 domain

The Drosophila bithorax complex Abdominal-B (Abd-B) gene specifies parasegm ental identity at the posterior end of the fly. The specific pattern of Abd -B expression in each parasegment (PS) determines its identity and, in PS10 -13, Abd-B expression is controlled by four parasegment-specific cis-regula tory domains, iab-5 to iab-8, respectively, In order to properly determine parasegmental identity, these four cis-regulatory domains must function aut onomously during both the initiation and maintenance phases of BX-C regulat ion. The studies reported here demonstrate that the (centromere) distal end of iab-7 domain is delimited by the Fab-8 boundary. Initiators that specif y PS12 identity are located on the proximal iab-7 side of Fab-8, while init iators that specify PS13 identity are located on the distal side of Fab-8, in iab-8. We use transgene assays to demonstrate that Fab-8 has enhancer bl ocking activity and that it can insulate reporter constructs from the regul atory action of the iab-7 and iab-8 initiators. We also show that the iab-8 boundary defines the realm of action of a nearby iab-8 Polycomb Response E lement, preventing this element from ectopically silencing the adjacent dom ain. Finally, we demonstrate that the insulating activity of the Fab-8 boun dary in BX-C is absolutely essential for the proper specification of parase gmental identity by the iab-7 and iab-8 cis-regulatory domains. Fab-8 toget her with the previously identified Fab-7 boundary delimit the first genetic ally defined higher order domain in a multicellular eukaryote.