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WIREs Syst Biol Med
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Circadian oscillators in eukaryotes

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Abstract The biological clock, present in nearly all eukaryotes, has evolved such that organisms can adapt to our planet's rotation in order to anticipate the coming day or night as well as unfavorable seasons. As all modern high‐precision chronometers, the biological clock uses oscillation as a timekeeping element. In this review, we describe briefly the discovery, historical development, and general properties of circadian oscillators. The issue of temperature compensation (TC) is discussed, and our present understanding of the underlying genetic and biochemical mechanisms in circadian oscillators are described with special emphasis on Neurospora crassa, mammals, and plants. Copyright © 2010 John Wiley & Sons, Inc. This article is categorized under: Models of Systems Properties and Processes > Cellular Models

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Schematic representation of a molecular mechanism for circadian oscillations with negative and positive feedback loops. Positive components/transcription factors interact with the promoter regions of clock genes leading to their expression and forming corresponding mRNAs and proteins. Some clock gene activation mechanisms may involve positive feedback loops. As supported by model calculations,37–39 the crucial element for getting oscillations is the presence of one (or several) negative feedback loop(s), in which a clock protein inhibits its own transcription. Environmental influences affect the clock mechanism through a series of receptors, which alter the properties of clock proteins and their transcription factors through kinases and phosphatases, where some of phosphorylation and dephosphorylation pathways appear to be mechanistically conserved.40.

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Feedback loops of the plant circadian network. Three loops are presently considered, the dawn‐phased CCA1/LHY containing loop, which negatively regulates TOC1, a morning‐phased loop containing the PRR proteins inhibiting the formation of CCA1/LHY, and an evening‐phased loop, probably through GIGANTEA (GI) activating TOC1.

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Model of the circadian core network in mammals. The heterodimer CLOCK/BMAL activates genes containing an E‐box. CRY, the PER proteins, and REV‐ERBα are negative elements, whereas the ROR proteins together with CLOCK and BMAL1 define positive elements. For a more detailed discussion, see main text.

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Scheme of the circadian core network in Neurospora crassa. Several negative feedback loops have been identified. The FREQUENCY (FRQ) protein plays a central role. Its highly regulated stability defines period length and TC of the conidiation rhythm.64, 65 Additional feedback loops are also indicated. They seem to serve special purposes, i.e., when nitrate ion is the only source for nitrogen, or, as in the case of VIVID (VVD), playing a role in the phasing of the rhythm.

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Growth tubes monitoring the free‐running circadian rhythm in Neurospora. The sterile tubes contain growth medium (agar) and are sealed on each side with cotton plugs allowing air exchange. Inoculation with mycelium or conidia occurs at one side of the tube. Under free‐running conditions, generally in darkness or in a red safety light, the mycelium then grows along the tube with approximately constant speed.116 Approximately every 22 h conidia are formed shown as the patches on the tube, reflecting the output of the circadian clock. The period of the free‐running rhythm can be determined by measuring the distance between the conidial patches and dividing this distance by the growth speed.

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