photoreceptor protein photoreceptor protein photoreceptor proteins are light-sensitive proteins involved in the sensing and response to light in a variety of organisms. some examples are rhodopsin in the photoreceptor cells of the vertebrate retina, phytochrome in plants, and bacteriorhodopsin and bacteriophytochromes in some bacteria. they mediate light responses as varied as visual perception, phototropism and phototaxis, as well as responses to light-dark cycles such as circadian rhythm and other photoperiodisms including control of flowering times in plants and mating seasons in animals. structure photoreceptor proteins typically consist of a protein attached to a non-protein chromophore (sometimes referred as photopigment, even so photopigment may also refer to the photoreceptor as a whole). the chromophore reacts to light via photoisomerization or photoreduction, thus initiating a change of the receptor protein which triggers a signal transduction cascade. chromophores found in photoreceptors include retinal (retinylidene proteins, for example rhodopsin in animals), flavin (flavoproteins, for example cryptochrome in plants and animals) and bilin (biliproteins, for example phytochrome in plants). the plant protein uvr8 is exceptional amongst photoreceptors in that it contains no external chromophore. instead, uvr8 absorbs light through tryptophan residues within its protein coding sequence. photoreceptors in animals