Many tissues in non-mammalian vertebrates contain both photoreceptors and circadian clock systems. Among these photosensitive clock structures, the chick pineal gland has been characterized in detail from cellular and molecular aspects of the clock oscillation and entrainment. Analyses of the pineal photic-input pathway revealed a phase-shifting mechanism mediated by activation of G11, one of the Gq-type G-proteins. A major photoreceptive molecule, pinopsin, likely triggers this pathway by transmitting the light signal to the circadian oscillator. In the chick pineal oscillator, the transcription/translation-based autoregulatory feedback loop is composed of positive and negative elements (clock gene products) that are homologous to those identified in mammals. In the molecular cycling, a CACGTG E-box located in the promoter region of the negative element genes plays a central role in the transcriptional regulation. The phase of the molecular cycling is modulated by many regulatory components, among which E4BP4 and extracellular signal-regulated kinase (ERK) are closely associated with the photic entrainment. A light-responsive element was found in the promoter region of the Pinopsin gene, and the element included a CACGTG E-box, suggesting a novel role of the E-box as a point of convergence of light and circadian signals. These observations together point to general and unique features of the chick pineal circadian system among animal clocks.
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