Differential target gene responses to Dpp during early Drosophila embryogenesis. M. Xu , N. Kirov , C. Rushlow. Dept Biol, New York Univ, New York, NY.

   During blastoderm formation, Dpp functions as a morphgen specifying different cell fates in a concentration-dependent manner by directing differential target gene expression. Dpp functions through the downstream transducer phosphorylated Mad (pMad), which has peak levels in the dorsal-most region of the embryo. Three types of target genes have been categorized based on their requirement for Dpp. High-level targets such as Racerequire the highest levels of Dpp and are thus expressed only in the dorsal-most region specifying amnioserosa. Intermediate targets such as pannierhave a broader expression pattern spanning both amnioserosa and part of the dorsal ectoderm. And finally, low-level targets such as early zenare expressed most widely covering the entire dorsal area. Previous studies have prompted us to propose a model whereby the differential responses are due to a combinatorial mechanism involving both spatially restricted activators and repressors. Specifically, low-level targets are activated indirectly by Dpp through derepression of the Brinker (Brk) repressor in the ventral ectoderm region, while high-level targets are dependent on pMad-mediated activation. In contrast, the expression of intermediate targets relies on the net effect of competitive interactions between activators such as pMad and repressors such as Brk. One way to test such a model is to dissect the response elements in the enhancer regions of different target genes, and characterize the sequences that interact with specific activators and/or repressors. Here we report our preliminary results comparing target promoters. For example, in the high-level target gene Racewe have identified multiple activator-responsive regions but no repressor-responsive regions. Mutations in the activator-responsive regions abolish normal expression, confirming their functional relevance. We are further characterizing these responsive regions for specific activator-binding sites.