Roles played by Enhancer of split transcription factors in Drosophila R7 photoreceptor specification.

When a cell receives multiple developmental signals simultaneously, the intracellular transduction pathways triggered by those signals are coincidentally active. How then, do the cells decode the information contained within those multiple active pathways to derive a precise developmental directive? The specification of the Drosophila R7 photoreceptor is a classic model system for investigating such questions. The R7 fate is specified by the combined actions of the Notch (N) and receptor tyrosine kinase (RTK) signaling pathways. The two pathways cross-communicate in an integrative mechanism and also supply information independently of each other. Collectively, this information is summed to provide an unambiguous directive for the R7 fate. Our goal is to understand these mechanisms. Here, we examine how N activity represses transcription of the phyllopod gene in the process of information integration with the RTK pathway, and how it represses expression of the seven-up gene in an independent mechanism needed for R7 fate. We describe how N activity achieves these transcriptional repressions and identify Enhancer of Split transcription factors as the mediators of its functions.