Plants use sunlight as a source of energy for photosynthesis and as an environmental cue to coordinate their development to the ambient light conditions and seasonal cycles. A number of photoreceptors are responsible for detecting these light signals. The most influential are the phytochromes, a family of chromophore-bearing proteins that absorbs red and far-red light. They act as light-regulated switches for a number of agriculturally important processes, including seed germination, chloroplast maturation, pigmentation, stem growth, flowering, and senescence to name a few. Despite their importance, we still do not fully understand how phytochromes allow plants to sense the intensity, direction, and duration of the light signal. A major barrier to this understanding has been the dearth of simple genetically-tractable models for the study of phytochromes without the complications of other photoreceptors and photosynthesis. We have simultaneously overcome these problems with the recent discovery of a single phytochrome-like photoreceptor system in the non-photosynthetic bacterium, Deinococcus radiodurans [Davis et al. (1999) Science 286: 2517-2520]. Like phytochromes, the D. radiodurans bacteriophytochrome photoreceptor (BphP) absorbs red and far-red light. Studies on BphP suggest that it acts as a protein kinase that begins light sensing by phosphorylating other proteins. Ultimately this signal stimulates the bacterium to produce carotenoids, pigments typically used to protect plants from high light conditions. The goal of this project is to study the D. radiodurans BphP phytochrome to understand its structure and mechanism of action. The BphP photoreceptor will be characterized at the biochemical level to determine the identity of its natural chromophore and the way it is linked to the protein. The pathway that synthesizes the chromophore will be determined. Attempts will be made to crystallize the protein so that its structure can be defined. Via a number of methods, the biochemical steps that lead from BphP to light-stimulated carotenoid production will be defined. Completion of this work will begin to provide the first description of a phytochrome signal transduction pathway that can then be used as a framework for understanding how phytochromes work in crop plants.
