Cyanobacteriochromes: photoreceptors covering the entire UV-to-visible spectrum.

Cyanobacteriochrome photoreceptors are linear tetrapyrrole-binding photoreceptors that are distantly related to the canonical phytochrome photoreceptors. The chromophore-binding region of the cyanobacteriochromes consists of only a cGMP-phosphodiesterase/adenylate cyclase/FhlA (GAF) domain, while that of the phytochromes consists of three domains, including the GAF domain. Most of the canonical phytochromes homogenously show red/far-red reversible photoconversion. Conversely, the cyanobacteriochrome photoreceptors are highly diverse in the colors of light they sense. Since the discovery of the first cyanobacteriochrome photoreceptor around 15 years ago, physiological, biochemical, and biophysical studies on cyanobacteriochromes have been extensively performed to date. In this review, we focus on color-tuning mechanisms of diverse cyanobacteriochromes.