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Phytochromes: Structure, Function and Mechanisms of Action

#plants #phytochrome #photoreceptors Phytochromes are photoreceptors that respond to environmental light conditions. They are dimeric chromoproteins with each monomer possessing a covalently linked open tetrapyrrole phytochromobilin as a chromophore. In higher plants, phytochromes are encoded by small gene families, for example, five members (phyA to phyE) in Arabidopsis thaliana. They are further classified into light-labile type I (phyA) and light-stable type II (phyB to phyE) species. Phytochrome is a soluble chromo-protein with a molecular mass of 250 kDa. Phytochrome occurs as a dimer made up of two subunits, each with molecular mass of 125 kDa. Each subunit consists of two components: light- absorbing pigment molecule, chromophore and a polypeptide chain, apo protein. Apo protein and chromophore together make up the holochrome. The chromophore is a linear tetrapyrrole phytochromobilin and it is a ring attached to the protein through thioether-linkage to a cysteine residue. The chromophore undergoes cis-trans isomerization at the carbon 15 in response to red and far-red light.