Cryptochrome

Cryptochrome is a name used historically for the blue light photoreceptors of plants. It is now used to describe a specific subset of blue light receptors, a family of flavoproteins that regulate germination, elongation, photoperiodism and other responses in higher plants. Blue light also mediates phototropism, but this response is now known to have its own set of photoreceptors, the phototropins.

Cryptochromes are found in all plant species. Two similar cryptochromes exist in most plants: CRY1 and CRY2. Cryptochrome is evolutionarily ancient, and is derived from photolyase, a bacterial enzyme activated by light and participating in DNA damage repair. In eukaryotes the chryptochromes lost their enzymatic activity.

Cryptochrome possesses two chromophores: pterin and flavin (a chemical relative of pterin). Not much is currently known about the way cryptochromes exercise their physiological effects. Pterin absorbs a photon, which causes it to emit an electron; the latter is absorbed by flavin, which probably mediates the phosphorylation of a certain domain in cryptochrome. This triggers a signal transduction chain which affects gene regulation in the cell nucleus.

Cryptochromes are also found in insects and mammals. The two cryptochromes found in mammals play a pivotal role in the generation and maintenance of circadian rhythms. New evidence suggests that cryptochromes allow migratory birds to navigate by sensing magnetic fields. Other theories suggest that this ability lies dormant in all mammals. In the human genome the genes coding for CRY1 and CRY2 are found on chromosomes 12 and 11, respectively.