Concept

Cryptochrome

Summary
Cryptochromes (from the Greek κρυπτός χρώμα, "hidden colour") are a class of flavoproteins found in plants and animals that are sensitive to blue light. They are involved in the circadian rhythms and the sensing of magnetic fields in a number of species. The name cryptochrome was proposed as a portmanteau combining the chromatic nature of the photoreceptor, and the cryptogamic organisms on which many blue-light studies were carried out. The genes Cry1 and Cry2 encode the two cryptochrome proteins CRY1 and CRY2, respectively. Cryptochromes are classified into plant Cry and animal Cry. Animal Cry can be further categorized into insect type (Type I) and mammal-like (Type II). CRY1 is a circadian photoreceptor whereas CRY2 is a clock repressor which represses Clock/Cycle (Bmal1) complex in insects and vertebrates. In plants, blue-light photoreception can be used to cue developmental signals. Besides chlorophylls, cryptochromes are the only proteins known to form photoinduced radical-pairs in vivo. These appear to enable some animals to detect magnetic fields. Cryptochromes have been the focus of several current efforts in optogenetics. Employing transfection, initial studies on yeast have capitalized on the potential of CRY2 heterodimerization to control cellular processes, including gene expression, by light. Although Charles Darwin first documented plant responses to blue light in the 1880s, it was not until the 1980s that research began to identify the pigment responsible. In 1980, researchers discovered that the HY4 gene of the plant Arabidopsis thaliana was necessary for the plant's blue light sensitivity, and, when the gene was sequenced in 1993, it showed high sequence homology with photolyase, a DNA repair protein activated by blue light. Reference sequence analysis of cryptochrome-1 isoform d shows two conserved domains with photolyase proteins. Isoform d nucleotide positions 6 through 491 show a conserved domain with deoxyribodipyrimidine photolyase, and positions 288 through 486 show a conserved domain with the FAD binding domain of DNA photolyase.
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