Five regulatory proteins with alternating absorbance peaks in red and far-red light (see Fig. P72). Through their absorbance peaks (red [R] 660 nm and far-red [FR] 730), they control various photomorphogenic processes, such as short- and long-day onset of flowering, hypocotyl elongation, apical hooks, pigmentation, etc. These chromoproteins are homodimers of 124 kDa subunits and a tetrapyrrole complex, joined covalently through a cystein residue at about 1/3 distance from the NH₂ end. The molecule exists in two conformations corresponding to the R and FR absorption states. The interconversion between these states is mediated very rapidly by light of R and FR emission peaks. In etiolated plant tissue, the inactive P_r conformation may constitute up to 0.5% of the protein. The transition from the P_r conformation into the active P_fr form also entails the degradation of this receptor. The apoprotein, coded by different genes (PHYA and PHYB) in Arabidopsis may have only about 50% homology in amino acid sequences, although they bind the same chromophore. The specificity of PhyA (far-red) and PhyB (red) resides in the N-termini. The C-terminal domain of phytochrome B attenuates the transducing signals (Matsishita T et al 2003 Nature [Lond] 424:571). Phytochromes can induce and silence the expression of genes in a specific selective manner. The transcription of the phytochrome genes is also light regulated; R light reduces the transcription more effectively than FR. Phy-A perceives continuous FR, whereas phy-B responds to continuous red light.

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Phytochrome B is also a photoreceptor in the circadian rhythm. Phytochrome A appears to be serine/threonine kinase. Phytochrome C is a light-stable molecule. SPA1 (suppressor of phy-A), a WD-protein with sequence similarity to protein kinases, mediates, among other factors, the photomorphogenic reactions. The phytochrome responses are under complex genetic regulatory systems involving light response elements, transcription factors, and components of the signal transduction circuits. PIF3 (phytochrome-inducing factor) is a basic helix-loop-helix protein that attaches to the non-photoactive C-terminus of phytochromes A and B and mediates their conversion into active forms. PIF3 also binds to a G-box in the promoter and thus regulates transcription. Nucleoside diphosphate kinase 2 (NDPK2) preferentially binds to the red light activated form of phytochrome and appears to play a role in eliciting light responses. In photomorphogenic responses, phytochromes interact with cryptochromes. Although phytochrome is known as a ubiquitous plant product, the yeast Pichia also synthesizes phytochromobilin (PΦB), a precursor of this plant chromophore. PΦB deficient plants can be complemented by the insertion of the algal phycocyanobilin gene (Kami C et al 2004 Proc Natl Acad Sci USA 101:1099). Also, a phytochrome-like protein (Ppr) has been identified in non-photosynthetic prokaryotes (Deinococcus radiodurans, Pseudomonas aeruginosa). In the Rhodospirillum centenum, a purple photosynthetic bacterium, a photoreactive yellow (PYP) pigment has been identified with a central domain resembling phytochromes.

In cyanobacteria, the circadian input kinase (CikA), a bacteriophytochrome, mediates the circadian oscillations.  photoperiodism,  photomorphogenesis,  signal transduction,  phycobilins,  cryptochromes,  brassinosteroids,  WD-40,  G box; Neff MM et al 2000 Genes Dev 14:257; Martinez-Garcia JF et al 2000 Science 288:859; Smith H 2000 Nature [Lond] 407:585; Bhoo S-H. et al 2001 Nature [Lond] 414:776; Nagy F, Schäfer E 2002 Annu Rev Plant Biol 53:329.