Article Figures & Data
Figures
- Fig. 1.
Depiction of the flower clock (eine Blumen-Uhr) designed by Linneaus. The left half of the figure (6 am–12pm) shows when the petals of different species are opening; the right half (12 pm–6 pm) shows times of petal closing (except evening primrose, which starts to open its flowers after 5 pm). Note that some species can act to time both morning and afternoon events. (From Moore-Ede et al., 1982.)
- Fig. 2.
Four characteristics of circadian rhythms (see text). An idealized rhythm is shown under entraining conditions (light/dark cycles) and during free-run in constant conditions (continuous light). In this example the endogenous free-running period is longer than the cycle of the entrainment schedule. As long as light/dark cycles continue, the period of the oscillator is identical to the period length of the entrainment schedule cycle, resulting in a consistent difference in time (i.e. phase relationship) between any chosen point on the curve (e.g. peak expression) and a given phase marker (e.g. the dark-to-light transition; dotted line). During free-run, this constant relationship breaks down as the oscillator reverts to its longer, endogenous period and the peak of expression drifts further from “subjective” dawn. Dark (shaded bar) and light (white bar) are indicated below the trace. (Adapted from Edmunds, 1988.)
- Fig. 3.
Model of a simple circadian system. The three primary components include: an input (entrainment) pathway(s), the central oscillator, and an output pathway(s). The phytochromes (PHY) and the cryptochromes (CRY) are two classes of photoreceptors known to mediate the first step of the light entrainment pathway (Somers et al., 1998a). Interactions among the components (A–D) of the central oscillator create the autoregulatory negative-feedback loop that generates the approximately 24-h oscillations. Three different hypothetical couplings of the central oscillator to possible output pathways are shown to indicate that differently phased overt rhythms with the same period (E–G) can arise from a single pacemaker.
- Fig. 4.
Current working model of a eukaryotic circadian clock. This generalized scheme is drawn largely from work withDrosophila and mouse, although all of the available evidence from Neurospora supports a similar mechanism. PROT1 and PROT2 are two nuclear-localized proteins that act to negatively regulate their own transcription by blocking the activity of two bHLH-containing DNA-binding proteins (PP1 and PP2). PP1 and PP2 share a common domain (PAS) that facilitates protein-to-protein interactions and may contribute to their heterodimerization and binding to the circadian clock element (CCE) present on the promoters of PROT1 and PROT2. In the absence of PROT1 and PROT2, PP1 and PP2 activate the transcription of both PROT genes. This results in increased protein levels of both PROT gene products in the cytoplasm, which are further regulated by light (PROT2 is light labile) and/or phosphorylation state (PROT1 is destabilized by phosphorylation). Association of PROT2 with PROT1 helps stabilize PROT1 levels and facilitates the transport of the heterodimer to the nucleus. PP1/PP2-dependent transcription is inhibited by the pair until turnover of PROT1 and PROT2 depletes levels, allowing transcription to proceed, and then the cycle begins again. Degraded proteins are indicated as small, grouped ovals. P, Phosphorylation.
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- Article
- WHAT ARE CIRCADIAN RHYTHMS?
- THE CIRCADIAN CLOCK IS A SIGNALING SYSTEM LINKING THE ENVIRONMENT TO PHYSIOLOGY AND DEVELOPMENT
- THE “HANDS” OF THE CLOCK: THE OUTPUT PATHWAY LEADS TO MANY AND VARIED CLOCK-REGULATED PROCESSES IN PLANTS
- ENTRAINMENT OF THE CENTRAL OSCILLATOR OCCURS VIA THE LIGHT INPUT PATHWAY
- ARRHYTHMICS: WHERE DO THEY BELONG?
- THE CIRCADIAN OSCILLATOR IS AN AUTOREGULATORY NEGATIVE FEEDBACK LOOP
- PERSPECTIVE
- ACKNOWLEDGMENTS
- Footnotes
- LITERATURE CITED
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