Direct downstream targets of proneural activators in the imaginal disc include genes involved in lateral inhibitory signaling

In Drosophila imaginal discs, the spatially restricted activities of the achaete (ac) and scute (sc) proteins, which are transcriptional activators of the basic-helix-loop-helix class, define proneural clusters (PNCs) of potential sensory organ precursor (SOP) cells. Here, we report the identification of several genes that are direct downstream targets of ac-sc activation, as judged by the following criteria. The genes are expressed in the PNCs of the wing imaginal disc in an ac-sc-dependent manner; the proximal promoter regions of all of these genes contain one or two high-affinity ac- sc binding sites, which define the novel consensus GCAGGTG(T/G)NNNYY; where tested, these binding sites are required in vivo for PNC expression of promoter-reporter fusion genes. Interestingly, these ac-sc target genes, including Bearded, Enhancer of split m7, Enhancer of split m8, and scabrous, are all known or believed to function in the selection of a single SOP from each PNC, a process mediated by inhibitory cell-cell interactions. Thus, one of the earliest steps in adult peripheral neurogenesis is the direct activation by proneural proteins of genes involved in restricting the expression of the SOP cell fate.