In avian pinealocytes, an environmental light signal resets the phase of the endogenous circadian pacemaker that controls the rhythmic production of melatonin1-6. Investigation of the pineal phototransduction pathway should therefore reveal the molecular mechanism of the biological clock. The presence of rhodopsin-Iike photoreceptive pigment4,5 7-9, transducin-like immunoreaction10, and cyclic GMP-dependent cation-channel activity11 in the avian pinealocytes suggests that there is a similarity between retinal rod cells and pinealocytes in the phototransduction pathway. We have now cloned chicken pineal cDNA encoding the photoreceptive molecule, which is 43-48% identical in amino-acid sequence to vertebrate retinal opsins. Pineal opsin, produced by transfection of complementary DNA into cultured cells, was reconstituted with 11-cis-retinal, resulting in formation of a blue-sensitive pigment λmax ≈470 nm). In the light of this functional evidence and because the gene is specifically expressed only in the pineal gland, we con-clude that it is a pineal photosensor and name it pinopsin.
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