The photosynthetic bacterium Rhodopseudomonas palustris is an extremely adaptable bug that can survive by producing energy through photosynthesis, aerobic respiration, anaerobic respiration, or fermentation. Now studies of bacteriophytochrome photoreceptor proteins (BphPs) by Vuillet et al. appear to show an example of evolution in action in which different strains of Rps. palustris use a nearly identical protein for either light or redox sensing. The BphPs get their light-sensing ability from a chromophore that is covalently linked to the protein through a Cys residue. Indeed, the product of the RpBphP4 protein can sense light and activates histidine kinase activity of the protein, which is coupled to a two-component regulatory system. However, most strains of Rps. palustris express a RpBphP4 protein that lacks the critical Cys where the chromophore would be linked. Instead, the authors show that the achromo-RpBphP4 protein is sensitive to changes in redox state. The strains that express achromo-RpBphP4 can respond to low oxygen tension, through the two-component system, to regulate synthesis of a set of light-harvesting complexes. This is the same response produced when specific wavelengths of light are detected in strains expressing the chromo-RpBphP4 protein. Thus, the authors propose that the system appears to display an example of evolution of the nature of signal perception by a histidine kinase sensor protein that may enhance the adaptability of the species to rapid changes in its environment.
L. Vuillet, M. Kojadinovic, S. Zappa, M. Jaubert, J.-M. Adriano, J. Fardoux, L. Hannibal, D. Pignol, A. Verméglio, E. Giraud, Evolution of a bacteriophytochrome from light to redox sensor. EMBO J. 26, 3322-3331 (2007). [PubMed]