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 Pfam:PF01590 [1], which is generally located immediately N-terminal to this domain. Compared with Pfam:PF01590, 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 stabilise the photoactivated far-red-absorbing state (Pfr) [1]. 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 [1].

1: The structure of a complete phytochrome sensory module in the Pr ground state. Essen LO, Mailliet J, Hughes J; Proc Natl Acad Sci U S A. 2008;105:14709-14714. PMID:18799745