Phytochromes are red/far-red photochromic biliprotein photoreceptors which regulate plant development. They are widely represented in both photosynthetic and non-photosynthetic bacteria and are known in a variety of fungi. Although sequence similarities are low, this domain is structurally related to pfam01590, which is generally located immediately N-terminal to this domain. Compared with pfam01590, this domain carries an additional tongue-like hairpin loop between the fifth beta-sheet and the sixth alpha-helix which functions to seal the chromophore pocket and stabilize the photoactivated far-red-absorbing state (Pfr). The tongue carries a conserved PRxSF motif, from which an arginine finger points into the chromophore pocket close to ring D forming a salt bridge with a conserved aspartate residue.