Fig. 6. Tentative model for the regulation of the Fbp1 EcRU activity in the fat body during development. Before the third larval instar, AEF-1 bound to element A (red bar) silences the activity of the Fbp1 enhancer (green bar). During the third larval instar, GATAb bound to GBS1 and GBS3 and associated with unidentified co-factors (C?) counteracts the repressing activity of AEF-1 in the fat body. Final activation of the Fbp1 promoter during late third larval instar results from synergistic transactivation by the GATAb multiprotein complex at GBS1 and the EcR/USP ecdysone receptor activated by its 20-hydroxyecdysone (20E) ligand.