cryptochrome

Cryptochromes are a family of flavoproteins that function as blue-light photoreceptors and as core components of circadian clocks in plants and animals. They are evolutionarily related to the DNA repair enzymes known as photolyases, but modern cryptochromes generally lack photolyase activity due to changes in catalytic residues.

Structurally, cryptochromes contain a photolyase homology region that binds the light-absorbing flavin adenine dinucleotide (FAD) cofactor, and a variable C-terminal extension that mediates signaling. Upon blue-light absorption, the FAD chromophore undergoes a conformational change that promotes interactions with clock or signaling proteins.

In plants, the Arabidopsis thaliana genome encodes CRY1 and CRY2, blue-light receptors that regulate photomorphogenesis and timing of development. Activation by blue light inhibits hypocotyl elongation, promotes seedlings’ light-grown morphology, and influences flowering time. CRYs interact with COP1–SPA complexes to stabilize transcription factors such as HY5.

In animals, including mammals and many insects, cryptochromes are integral clock components. Mammalian CRY1 and CRY2 act as transcriptional repressors in the CLOCK–BMAL1 feedback loop, helping to maintain circadian rhythms. In Drosophila, a distinct CRY functions as a blue-light photoreceptor that resets the clock by triggering TIM degradation.

Cryptochromes were first described as blue-light receptors in plants in the early 1990s; mammalian cryptochromes were identified a few years later. Beyond light sensing, cryptochromes are also studied for potential roles in magnetoreception and metabolic regulation, though these areas remain active and debated.