per mRNA cycling is locked to lights-off under photoperiodic conditions that support circadian feedback loop function.

Circadian fluctuations in per mRNA and protein are central to the operation of a negative feedback loop that is necessary for setting the free-running period and for entraining the circadian oscillator to light-dark cycles. In this study, per mRNA cycling and locomotor activity rhythms were measured under different light and dark cycling regimes to determine how photoperiods affect the molecular feedback loop and circadian behavior, respectively. These experiments reveal that per mRNA peaks in abundance 4 h after lights-off in photoperiods of < or = 16 h, that, phase shifts in per mRNA cycling and behavioral rhythmicity occur rapidly after flies are transferred from one photoperiod to another, and that photoperiods longer than 20 h abolish locomotor activity rhythms and leave per mRNA at a median constitutive level. These results indicate that the per feedback loop uses lights-off as a phase reference point and suggest (along with previous findings for per01 and tim01) that per mRNA cycling is not regulated via simple negative feedback from the per protein.     

果蝇昼夜节律的分子机制研究进展

果蝇由于遗传易操作性而成为一个研究昼夜节律分子机制的理想模式生物 . 到目前为止,通过遗传学和生物化学方法已经鉴定到 10 多个时钟基因 (clock genes) 和许多时钟相关基因,包括时钟输入基因和钟控基因 . 这些时钟基因以及它们的相应产物组成两个互相依赖的转录 / 翻译反馈环路,从而调节行为和生理的昼夜节律 . 果蝇这种核心钟的工作原理同样见于哺乳动物 .     

Circadian biology: clocks for the real world.

The circadian system of Neurospora crassa includes a molecular feedback loop that is entrainable by light. A recent study has shown that a second, elusive oscillator interacts with the feedback loop to drive output rhythms.     

Photic signaling by cryptochrome in the Drosophila circadian system

Oscillations of the period (per) and timeless (tim) gene products are an integral part of the feedback loop that underlies circadian behavioral rhythms in Drosophila melanogaster. Resetting this loop in response to light requires the putative circadian photoreceptor cryptochrome (CRY). We dissected the early events in photic resetting by determining the mechanisms underlying the CRY response to light and by investigating the relationship between CRY and the light-induced ubiquitination of the TIM protein. In response to light, CRY is degraded by the proteasome through a mechanism that requires electron transport. Various CRY mutant proteins are not degraded, and this suggests that an intramolecular conversion is required for this light response. Light-induced TIM ubiquitination precedes CRY degradation and is increased when electron transport is blocked. Thus, inhibition of electron transport may "lock" CRY in an active state by preventing signaling required either to degrade CRY or to convert it to an inactive form. High levels of CRY block TIM ubiquitination, suggesting a mechanism by which light-driven changes in CRY could control TIM ubiquitination.     

A novel circadianly expressed Drosophila melanogaster gene dependent on the period gene for its rhythmic expression.

The Drosophila melanogaster period (per) gene is required for expression of endogenous circadian rhythms of locomotion and eclosion. per mRNA is expressed with a circadian rhythm that is dependent on Per protein; this feedback loop has been proposed to be essential to the central circadian pacemaker. This model would suggest the Per protein also controls the circadian expression of other genetic loci to generate circadian behavior and physiology. In this paper we describe Dreg-5, a gene whose mRNA is expressed in fly heads with a circadian rhythm nearly identical to that of the per gene. Dreg-5 mRNA continues to cycle in phase with that of per mRNA in conditions of total darkness and also when the daily feeding time is altered. Like per mRNA, Dreg-5 mRNA is not expressed rhythmically in per null mutant flies. Dreg-5 encodes a novel 298 residue protein and Dreg-5 protein isoforms also oscillate in abundance with a circadian rhythm. The phase of Dreg-5 protein oscillation, however, is different from that of Per protein expression, suggesting that Dreg-5 and per have common translational but different post-translational control mechanisms. These results demonstrate that the per gene is capable of modulating the rhythmic expression of other genes; this activity may form the basis of the output of circadian rhythmicity in Drosophila.     

Bifurcations in a mathematical model for circadian oscillations of clock genes

Circadian oscillations with a period of about 24h are observed in nearly all living organisms as conspicuous biological rhythms. In this paper, we investigate various kinds of bifurcation phenomena produced in a circadian oscillator model of Drosophila. In Drosophila, it is known that circadian oscillations in the levels of two proteins, PER and TIM, result from the negative feedback exerted by a PER-TIM complex on the expression of the per and tim genes that code for the two proteins. For studying circadian oscillations of proteins in Drosophila, a mathematical model has been proposed. The model cannot only account for regular circadian oscillations in environmental conditions such as constant darkness, but also give rise to more complex oscillatory phenomena including chaos and birhythmicity. By calculating bifurcations using Kawakami's method, we obtain detailed bifurcation diagrams related to stable and unstable invariant sets, and identify parameter regions in which the model generates complex oscillations as well as regular circadian oscillations. Moreover, we study bifurcations observed in the model incorporating the effect on a light-dark (LD) cycle and show that the waveform of the periodic variation in the light-induced parameter has a marked influence on the global bifurcation structure or the type of dynamic behavior resulting from the forcing term of the circadian oscillator by the LD cycles.     

A minimal model of light-induced circadian rhythms of nitrate reductase activity in leaves of barley

Observed circadian rhythms of nitrate reductase (NR) (EC 1.6.6.1) activity in leaves of barley ( Hordeum vulgare L. cv. Herta) under continuous light conditions are described by a simple kinetic model. The oscillatory mechanism has been decomposed into the negative and positive feedback loops which are necessary according to present theories of chemical oscillating systems. Our results indicate that the decrease of NR activity in darkness can be considered as a reversible unimolecular conversion of the active form of NR into an inactive form, forming a negative stabilizing feedback loop. The light-induced increase of NR activity is related to a positive destabilizing feedback loop. In our treatment this process is represented as an autocatalytical reaction.     

Circadian systems and metabolism

Circadian systems direct many metabolic parameters and, at the same time, they appear to be exquisitely shielded from metabolic variations. Although the recent decade of circadian research has brought insights into how circadian periodicity may be generated at the molecular level, little is known about the relationship between this molecular feedback loop and metabolism both at the cellular and at the organismic level. In this theoretical paper, we conjecture about the interdependence between circadian rhythmicity and metabolism. A mathematical model based on the chemical reactions of photosynthesis demonstrates that metabolism as such may generate rhythmicity in the circadian range. Two additional models look at the possible function of feedback loops outside of the circadian oscillator. These feedback loops contribute to the robustness and sustainability of circadian oscillations and to compensation for long- and short-term metabolic variations. The specific circadian property of temperature compensation is put into the context of metabolism. As such, it represents a general compensatory mechanism that shields the clock from metabolic variations.