Bacteriophytochromes constitute a light-sensing subgroup of sensory kin-ases with a chromophore-binding motif in the N-terminal half and a C-ter-minally located histidine kinase activity. The cyanobacteriumFremyella diplosiphon(also designated Calothrixsp.) expresses two sequentially very similar bacteriophytochromes, cyanobacterial phytochrome A (CphA) and cyanobacterial phytochrome B (CphB).