Cryptochrome function

FIG. 1. Circadian light input Phytochromes and cryptochromes function as the... 

Molecular modeling of mammalian cryptochromes on the structure of photolyase indicates conservation of the chromophore-binding regions, the DNA binding groove, as weU as the damage-binding pocket. However, the functional implications of these structural features for cryptochrome function in circadian photoreception are unknown. The major difference between cryptochrome and photolyase Hes in the presence of an extended C-terminal tail in cryptochrome, which is absent in photolyase. Based on... 

Sancar, A., Structure and function of DNA photolyase and cryptochrome blue-light... 

Ahmad M, Grancher N, Heil M et al 2002 Action spectrum for cryptochrome-dependent hypocotyl growth inhibition in Arabidopsis. Plant Physiol 129 774-785 Bellingham J, Whitmore D, Philp AR, Wells DJ, Foster RG 2002 Zebrafish melanopsin isolation, tissue localisation and phylogenetic position. Brain Res Mol Brain Res 107 128-136 Crawford BH 1949 The scotopic visibility function. Proc Phys Soc Lond B62 321—334 Miyashita Y, Moriya T, Yamada K et al 2001 The photoreceptor molecules in Xenopus tadpole tail fin, in which melanophores exist. Zool Sci 18 671-674 Wald G 1945 The spectral sensitivity of the human eye a spectral adaptometer. J Opt Soc Am 35 187... 

It is theoretically possible that cryptochromes do not function as photopigments, but are required either for the production of another photopigment, or for the signal transduction pathway of another photopigment. Such a pigment would need to be fully resistant to severe vitamin A depletion (since photic immediate-early gene induction in the SCN is fully preserved in RBP I mice but lost in mice) and so is unlikely to be opsin-... 

Sargent ML, Briggs WR, Woodward DO 1966 Circadian nature of a rhythm expressed by an invertaseless strain of Neurospora crassa. Plant Physiol 41 1343—1349 Selby CP, Thompson C, Schmitz TM, Van Gelder RN, Sancar A 2000 Functional redundancy of cryptochromes and classical photoreceptors for nonvisual ocular photoreception in mice. Proc Natl Acad Sci USA 97 14697-14702... 

Helfrich-Forster C, Winter C, Hofbauer A, Hall JC, Stanewsky R 2001 The circadian clock of fruit flies is blind after elimination of all known photoreceptors. Neuron 30 249-261 Lin FJ, Song W, Meyer-Bernstein E, Naidoo N, Sehgal A 2001 Photic signaling by cryptochrome in the Drosophilacitcidiaa system. Mol Cell Biol 21 7287-7294 Mas P, Devlin PF, Panda S, Kay SA 2000 Functional Interaction of phytochrome B and cryptochrome 2. Nature 408 207-211... 

Ivanchenko M, Stanewsky R, Giebultowicz JM 2001 Circadian photoreception in Drosophila-, functions of cryptochrome in peripheral and central clocks. J Biol Rhythms 16 205-215... 

Figure 23.2. Reaction mechanism of PD-DNA photolyase. A photon of blue light is absorbed by the MTHF chromophore that acts as a photoantenna. The excited energy is transferred to the flavin chromophore (FADFF). The excited flavin (FADFI ) acts as a photocatalyst and transfers an electron to a CPD in DNA. The thymines are restored to their native state and the electron is transferred back to the flavin. (Reproduced with permission from Sancar, A. Structure and function of DNA photolyase cryptochrome blue-light photoreceptors. Chem. Rev. 103, 2203-2237, 2003.)...

Cryptochrome genes have been found in many organisms. In the fly Drosophila cryptochrome appears to interact directly with the clock proteins that control the circadian cycle. Most important are products of two genes per (period) and tim (timeless). They are helix-loop-helix DNA binding proteins that form heterodimers, are translocated to the nucleus, and repress their own transcription. Morning light leads to a rapid disappearance of e TIM protein. The cryptochrome CRY appears to react directly with TIM to inactivate it. However, details remain to be learned. " The circadian clock mechanism appears to be universal and the cryptochrome-2 mcryl gene) appears to function in the mouse. A human cDNA clone was found to have a 48% identity with a relative of cryptochromes, the (6-4) photolyase of Drosophila. 

Redox reactions have been proposed to play a key role in light-responsive activities of cryptochromes [98, 99], blue-light photoreceptors in plants, animals, and bacteria with widespread functions ranging from the regulation of circadian rhythms of plants and animals [13] to the sensing of magnetic fields in a number of species... 

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