Abstract
Light is one of the most significant signals for phototrophic organisms. In plants, linear tetrapyrrole-based photoreceptors, phytochromes, serve as red/far-red light sensors to regulate physiological responses such as flowering. Cyanobacteriochromes are a recently emerging photoreceptor superfamily that is distantly related to the phytochromes and found only in cyanobacteria. They have unique spectral properties distinct from the phytochromes. Phototaxis regulators SyPixJ1/TePixJ covalently bind phycoviolobilin and show blue/green reversible photoconversion. Putative phototaxis regulator AnPixJ and chromatic acclimation regulator SyCcaS both covalently bind phycocyanobilin and show similar green/red reversible photoconversion. However, relationship of chromophore structure and absorption is opposite between AnPixJ and SyCcaS. Putative circadian input regulator SyCikA shows unidirectional photoconversion from violet-absorbing form to yellow-absorbing form. Recently, we determined the crystal structure of the chromophore-binding domain of AnPixJ and performed site-directed mutagenesis. We will discuss about structure-function relationship of the tetrapyrrole-based photoreceptors including the phytochromes.
