A Stage-Specific Factor Confers Fab-7 Boundary Activity during Early Embryogenesis in Drosophila

TheFab-7 boundary is required to ensure that theiab-6 andiab-7 cis -regulatory domains in theDrosophila Bithorax complex can function autonomously. ThoughFab-7 functions as a boundary from early embryogenesis through to the adult stage, this constitutive boundary activity depends on subelements whose activity is developmentally restricted. In the studies reported here, we have identified a factor, called early boundary activity (Elba), that confersFab-7 boundary activity during early embryogenesis. The Elba factor binds to a recognition sequence within aFab-7 subelement that has enhancer-blocking activity during early embryogenesis, but not during mid-embryogenesis or in the adult. We found that the Elba factor is present in early embryos but largely disappears during mid-embryogenesis. We show that mutations in the Elba recognition sequence that eliminate Elba binding in nuclear extracts disrupt the early boundary activity of theFab-7 subelement. Conversely, we find that early boundary activity can be reconstituted by multimerizing the Elba recognition site.