Ras1 signaling and transcriptional competence in the R7 cell of Drosophila.

The receptor tyrosine kinase Sevenless determines R7 cell fate by activation of the Ras1 pathway in a subset of equivalent cells competent to respond in the Drosophila eye. We show that the prospero gene becomes transcriptionally activated at a low level in all Sevenless-competent cells prior to Sevenless signaling, and this requires the activities of Ras1 and two Ras1/MAP kinase-responsive ETS transcription factors. Restriction of high-level prospero expression to the R7 cell appears as a subsequent event, which requires Sevenless activation of the Ras1/MAP kinase pathway. We show that Phyllopod, a nuclear factor whose expression is induced by Sevenless, interacts with another nuclear factor, Sina, to form a complex, and that both factors are involved in upregulating transcription of the prospero gene in the eye. Ultimately, prospero expression is required for proper connectivity of R7 photoreceptor axons to their synaptic targets. Our results suggest that specific transcriptional responses are linked to the mode of activation of the Ras1/MAP kinase signal transduction pathway.