Long range repression conferring boundaries of Ultrabithorax expression in the Drosophila embryo.

In an attempt to reconstruct the embryonic expression pattern of the homeotic gene Ultrabithorax (Ubx) by stable integration of fusion constructs, we identified three key control regions called PBX, ABX and BXD. Each of these confers an expression pattern mimicking certain aspects of Ubx expression. The PBX and ABX patterns are limited to the Ubx domain with anterior boundaries at parasegments 6 and 5. In contrast, the BXD pattern extends from head to tail. PBX or ABX expression boundaries are imposed on the BXD pattern, if PBX or ABX is linked to BXD. These boundaries, although not the PBX and ABX expression limits themselves, are dependent on Polycomb function. We conclude that PBX and ABX are recognized by repressors which act across large distances to suppress BXD activity. Stable and heritable Ubx expression boundaries are thus mediated by this process of long range repression.