There are two major types of cellular photoresponses in plants and certain unicellular algae mediated by the photoreceptor, phytochrome. The molecular mechanisms of these photoresponses are largely unknown. The long term objective of the proposed study, therefore, is to understand the mechanisms of phytochrome action at the molecular level. Our specific aims of the proposed research are two-fold: (1) to investigate the molecular topography of phytochrome as affected by the reversible photoactivation of the inactive to the active form of phytochrome, and (2) to characterize the mode of interactions between the active phytochrome form (Pfr) and its potential/putative "receptors" and models in terms of molecular topographic probes. Since the gene-regulatory role and the photosensory transduction processes mediated by phytochrome are likely to reveal a useful contrast and/or similarity to the hormonal gene regulation and sensory transduction (e.g., vision) in animals, the proposed study is health-related in understanding the basic principles of regulatory and sensory biology.