Evolution of cyanobacterial and plant phytochromes

Phytochromes are broadly distributed photochromic photoreceptors that are most sensitive in the red and far-red region of the visible spectrum. Three different bilins can be used as chromophores: plant phytochromes incorporate phytochromobilin, while phycocyanobilin serves as a chromophore of some cyanobacterial phytochromes, whereas all other phytochromes, including cyanobacterial orthologs incorporate biliverdin. During the evolution of plant and cyanobacterial phytochromes, the chromophore binding site has changed from a cysteine close to the N-terminus of the protein, the biliverdin attachment site, to a cysteine which lies within the so-called GAF domain and serves as phytochromobilin or phycocyanobilin attachment site. Since phylogenetic analyses imply that plant phytochromes are not direct successors of cyanobacterial phytochromes, chromophore exchange and the switch of the chromophore binding site has probably occurred at least twice in evolution. This may be regarded as an example for convergent evolution at the molecular level.