Cyanobacteriochromes are linear tetrapyrrole-binding photoreceptors produced by cyanobacteria. Their chromophore-binding GAF domains categorized into many lineages. Among them, dual Cys-type cyanobacteriochrome possessing not only a highly conserved ‘first Cys' but also ‘second found from multiple The first Cys stably attaches to C31 of the A-ring, while second mostly shows reversible ligation ...