TRANSCRIPTIONAL REGULATION OF ATONAL DURING DEVELOPMENT OF THE DROSOPHILA PERIPHERAL NERVOUS-SYSTEM

atonal is a proneural gene for the development of Drosophila chordoton al organs and photoreceptor cells. We show here that atonal expression is controlled by modular enhancer elements located 5' or 3' to the at onal-coding sequences. During chordotonal organ development, the 3' en hancer directs expression in proneural clusters; whereas successive mo dular enhancers located in the 5' region drive tissue-specific express ion in chordotonal organ precursors in the embryo and larval leg, wing and antennal imaginal discs. Similarly, in the eye disc, the 3' enhan cer directs initial expression in a stripe anterior to the morphogenet ic furrow These atonal-expressing cells are then patterned through a N otch-dependent process into initial clusters, representing the earlies t patterning event yet identified during eye morphogenesis. A distinct 5' enhancer drives expression in intermediate groups and R8 cells wit hin and posterior to the morphogenetic furrow. Both enhancers are requ ired for normal atonal function in the eye. The 5' enhancer, but not t he 3' enhancer, depends on endogenous atonal function, suggesting a sw itch from regulation directed by other upstream genes to atonal autore gulation during the process of lateral inhibition. The regulatory regi ons identified in this study can thus account for atonal expression in every tissue and essentially in every stage of its expression during chordotonal organ and photoreceptor development.