Tramtrack69 is positively and autonomously required for Drosophila photoreceptor development

Cell-fate specification and cellular differentiation are tightly controlled by both positive and negative transcriptional factors during development. The Drosophila BTB/POZ (Bric-a-brac Tramtrack Broad complex/Pox virus and Zinc finger) domain-containing Tramtrack (Ttk) proteins have been previously shown to be transcriptional repressors and inhibitors of the neuronal fate of cells such as photoreceptors. Here we provide evidence that one of the Ttk proteins, Ttk69, also plays a positive and autonomous role in promoting or maintaining differentiation of photoreceptor neurons at the late stages of Drosophila eye development. Consistent with this notion, the Ttk69 protein, but not Ttk88, is expressed in all photoreceptor cells during pupal stage. Thus, Ttk69 appears to play a dual function by serving negative and positive regulatory roles at different stages of photoreceptor development.