Our long term objective is to understand the cellular mechanisms which underly vertebrate photoreceptor renewal. Renewal of photoreceptors in the vertebrae retina consists of many diverse cellular processes which are orchestrated both in time and space to effect the sequential replacement of photoreceptive outer segment discs. We have recently demonstrated that in lower vertebrates, the level of rod opsin mRNA fluctuates with a daily rhythm, and is regulated both by light and by a circadian oscillator. Here we propose to build on these results by investigating the role of the rod opsin gene expression in causing these fluctuations. We will use nuclear run-on experiments to establish whether gene transcription alone can account for these fluctuations or if modification of opsin mRNA transcript lifetime is also required. We also intend to analyze the production and regulation of cone opsin mRNA for which we will clone cone opsin cDNA from fish retina and analyze the daily fluctuations in specific cone opsin mRNA levels. We will analyze the rhythms of mRNA production for other proteins specific to rod photoreceptors, transducin and interphotoreceptor retinal binding protein, to understand more clearly how the renewal process is regulated. We will also examine the role, if any, of the retinal neuromodulator dopamine in causing these fluctuations. We intend to ascertain the pathway by which light elicits an increase in opsin mRNA level during the dark phase of diurnal illumination. In particular, to understand whether or not this regulation depends on directly on phototransduction and if it is controlled locally or globally. Taken together, the proposed experiments should allow us to understand how photoreceptor renewal is regulated at a cellular and molecular level.