Deciphering the cis-trans regulatory circuit mediating RTK/RAS signaling in visceral muscle founder cell specification. Yiyun Zhou^1,2, Emily Deutschman^1,2, Jean-Daniel Feuz⁵, Korneel Hens⁵, Bart Deplancke⁵, Marc S. Halfon^1,2,3,4. 1) Dept. of Biochemistry, SUNY-Buffalo, Buffalo, NY; 2) NYS Center of Excellence in Bioinformatics & Life Sciences; 3) Dept. of Biological Sciences, SUNY-Buffalo, Buffalo, NY; 4) Mollecular and Cellular Biology Depatment, Roswell Park Cancer Institute, Buffalo, NY; 5) Laboratory of Systems Biology and Genetics, Institute of Bioengineering, School of Life Sciences, Ecole Polytechnique Fédérale de Lausanne, Lausanne, Switzerland.

   Receptor Tyrosine Kinase (RTK) signaling plays a crucial role in many developmental processes and diseases. In both the somatic and visceral mesoderm of the Drosophila embryo, the RTK/Ras/MAP Kinase signaling pathway is required to specify the muscle founder cells. These founder cells (FCs) then fuse with fusion-competent myoblasts in order to form multinucleate muscle fibers. Although the well-established Ras effector Pnt is typically assumed to be the primary transcription factor acting in FC specification, we have established that FC specification in the visceral mesoderm is mediated by a Ras-dependent but Pnt-independent pathway. Analysis of an FC-specific transcriptional enhancer for the mib2 gene has identified putative transcription factor binding sites required for Ras-dependent FC specification. Mutagenesis of these sites leads to an expansion of FC markers throughout the trunk visceral mesoderm, suggesting a possible derepression model for RTK-based induction of FC fates. Recently-performed high-throughput yeast one-hybrid screening has yielded a number of potential transcription factors that bind to these sites as well as elsewhere on the mib2 enhancer and on FC enhancers from additional FC-specific genes including duf/kirre, org-1, and Hand. We are continuing to investigate the roles of these factors in RTK/RAS signaling during visceral mesoderm development and to detail the cis-trans circuitry of this important developmental pathway.