DESCRIPTION (provided by applicant): Throughout development, and in the mature retina, precise control of gene expression is essential for photoreceptor integrity and function. This includes control of time, place and level of expression. Transcription factors provide the activation and repression inputs that control gene expression in response to signaling pathways. While much progress has been made towards our understanding of transcriptional activators, our knowledge of the repressors that contribute to the precise regulation of rod-specific genes remains sparse. This proposal will study the role of a novel repressor (Fiz1) that was discovered, by the P.I., from its association with an essential activator (NRL) of rod differentiation and rod-specific gene expression. Stringent regulation of the rhodopsin gene offers a useful paradigm of rod-specific gene expression to explore the interactions between activators, repressors, and signaling pathways. Several transcription factors (NRL. NR2E3. CRX) are shown to be activators of rhodopsin expression. Mutations in these transcription factors cause retinal degeneration. Fiz1 is a zinc-finger protein that was discovered binding to NRL, and behaves as a transcriptional repressor of the rhodopsin gene promoter. Recent studies in the P.l.'s laboratory have shown that Fiz1 protein levels rise in the retina with the onset of rod-specific gene expression. Fiz1 also binds to NR2E3, another transcription factor that is essential for rod development. Fiz1 is detected throughout the cell and can interact with the intracellular domain of a membrane receptor tyrosine-kinase (Flt-3). Levels of the extracellular protein (Flt-3-Ligand, FL) that activates this receptor, also rise during rod-maturation. We hypothesize that Fiz1 is a transcriptional repressor of rod-specific genes and is required for generating quantitatively precise levels of expression in response to extracellular signals. Furthermore, we suggest that Fiz1 assists in the recruitment of proteins that regulate physical access of the transcriptional machinery to DNA. The specific aims of this proposal are: 1. Determine the expression patterns of Fiz1, Flt-3 and FL during retinal development. 2. Identify Fiz1-binding proteins from the retina. 3. Examine the effect of Fiz1 gain and loss of function mutations on mouse rod-photoreceptor development. These aims will elucidate the physiological role of Fiz1 in rod-photoreceptors, and discover additional components of rod-specific transcriptional regulatory networks.