<p>Phytochrome [<cite idref="PUB00000104"/>, <cite idref="PUB00000736"/>] is a plant protein that acts as a regulatory photoreceptor and which mediates red-light effects on a wide variety of physiological and molecular responses. Phytochrome can undergo a reversible photochemical conversion between a biologically inactive red light-absorbing form and the active far-red light-absorbing form. Phytochrome is a dimer of identical 124 Kd subunits, each of which contains a covalently attached linear tetrapyrrole chromophore. The chromophore is attached to a cysteine which is located in a highly conserved region that can be used as a signature pattern. Synechocystis strain PCC 6803 hypothetical protein slr0473 contains a domain similar to that of plants phytochrome and seems to also bind a chromophore. </p> Phytochrome chromophore binding site