Synergistic activation of a Drosophila enhancer by a HTH/LAB/EXD triple complex and DPP signaling. T. Marty 1, M.A. Viganò 1, H.D. Ryoo 2, U. Nussbaumer 1, C. Ribeiro 1, N.C. Grieder 1, R. Mann 2, M. Affolter 1. 1) Dept. of Cell Biology, Biozentrum, University of Basel, Basel, Switzerland; 2) Dept. of Biochemistry & Molecular Biophysics, Columbia University, New-York, NY.
The cellular response to signaling molecules is developmentally regulated but the molecular mechanisms underlying this regulation remain obscure. To investigate the specificity of the response to DPP signaling, we analyzed the endodermal DPP-responsive enhancer of the labial gene, lab550. We identified a single, conserved LAB/EXD binding site on lab550, which is essential for the strong and timely induction of the lab550 enhancer. By sequence comparison with Drosophila hydei, we identified two blocks of homology, which are crucial for enhancer function. The first block contains a HOMOTHORAX site close to the LAB/EXD site and represents a Homeotic Response Element (HOMRE). The second block contains several modules which are essential for the response to DPP signaling. This DPP Response Element (DPPRE) contains a repressor site, which, when mutated, renders the element strongly DPP responsive irrespective of the presence or absence of the HOMRE. We conclude that the activity of the lab550 enhancer depends on a strong synergy between the HOMRE and the DPPRE. Indeed, neither the HOMRE nor the DPPRE are able to drive substantial expression on their own, and only the combination of the two leads to strong induction. The repressor site blocks DPP induction of the DPPRE unless the element can synergize with the HOMRE containing an intact LAB/EXD site. Therefore the homeotic protein LABIAL imposes specificity to the DPP signaling pathway by binding directly to a DPP-responsive enhancer. These results illustrate how homeotic and signaling inputs can act synergistically on a single, small enhancer to achieve a cell-specific response. Moreover, we will present a general model for the transcriptional response to DPP signaling, based on recent findings about the role of the different nuclear factors involved in this response.