Certain cyanobacteria can change the absorbing light colors for photosynthesis using a green- and red-light sensing photosensor protein. A Japanese research group revealed the unique conformation of the bilin chromophore and the unique protein structure that potentially functions as a proton transfer route to bilin. They also demonstrated that RcaE undergoes protonation and deprotonation of the bilin chromophore during the green and red photoconversion.