Cyanophytochromes, Bacteriophytochromes, and Plant Phytochromes. Light-Regulated Kinases Related to Bacterial Two-Component Regulators

Phytochromes (phys) are a family of photoreceptors used by higher plants to coordinate their growth and development to the ambient light environment. Through the unique photochromic properties of their bilin chromophore, phys behave as light-modulated switches sensitive to red and far-red light. The idea that phys are light-regulated protein kinases has been strongly supported recently with the discovery of phy-like proteins in cyanobacteria and eubacteria called cyanophytochromes (CphPs) and bacteriophytochromes (BphPs), respectively. CphPs and BphPs both contain an N-terminal sensor module homologous to higher plant phys that binds the bilin chromophore followed by a C-terminal transmitter module with sequence similarity to two-component histidine kinases common among other bacterial signaling systems. While phys use phytochromobilin as the chromophore, CphPs and BphPs likely use phycocyanobilin and biliverdin, respectively. Several of these CphPs and BphPs behave in vitro as histidine kinases using an associated response regulator as the phosphoacceptor. In the few cases studied, the CphP/BphP sensory chain ultimately affects motility or pigmentation, presumably as a way to enhance photosynthetic light capture or to protect the bacterium from light damage. In vitro studies with recombinant phys suggest that they are also light-regulated kinases. However, phys appear more related biochemically to serine threonine kinases, despite their evolutionary ancestory. This kinase activity could have multiple functions in plants that include initiating signal transduction as well as affecting localization, activity, and/or degradation of the photoreceptor.