Direct control of the proneural gene atonal by retinal determination factors during Drosophila eye development

The determination of neuronal identity in Drosophila cells depends on the accurate expression of proneural genes. The proneural gene atonal (ato) encodes a basic-HLH protein required for photoreceptor and chordotonal organ formation. The initial expression of ato in imaginal discs is regulated by sequences that lie 3′ to its open reading frame. In this report, we show that the initial ato transcription in different imaginal discs is regulated by distinct 3′ cis-regulatory sequences. The eye-specific ato 3′ cis-regulatory sequence consists of two distinct elements we term 2.8PB and 3.6BP that regulate ato transcription during different stages of eye development. The 2.8PB enhancer contains a highly conserved consensus binding site for the retinal determination (RD) factor Sine oculis (So). Mutation of this So binding site abolishes 2.8PB enhancer activity. Furthermore the RD factors So and Eyes absent (Eya) are required for 2.8PB enhancer activity and can induce ectopic 2.8PB reporter expression. In contrast, ectopic Dpp signaling is not sufficient to induce ato 3′ enhancer activation but can induce increased levels of RD factor Dachshund (Dac) and synergize with So and Eya to increase ato 3′ enhancer activity. These results demonstrate a direct mechanism by which the RD factors regulate ato expression and suggest an important role of Dpp in the activation of ato 3′ enhancer is to regulate the levels of RD factors.