蓝藻生物钟分子机理的研究进展

蓝藻是具有内源性生物钟的简单生物.虽然蓝藻生物钟具有跟真核生物同样的基础特征,但其相关基因和蛋白质与真核生物没有同源性.蓝藻生物钟的核心是kai基因簇及其编码的蛋白KaiA,KaiB和KaiC.这三种Kai蛋白相互作用调节KaiC的磷酸化状态,从而产生昼夜节律信息.KaiC的磷酸化循环是昼夜节律的起博器,调控包括kai基因在内的相关基因的节律性表达.组氨酸蛋白激酶的磷酸化传递可将环境信息输入和将节律信息输出生物钟核心.     

Timing of Photoperiodic Flowering：Light Perception and Circadian Clock

Flowering symbolizes the transition of s plant from vegetative phase to reproductive phase and is controlled by fairly complex and highly coordinated regulatory pathways. Over the last decade, genetic studies in Arabidopsis have aided the discovery of many signaling components involved in these pathways. In this review, we discuss how the timing of flowering is regulated by photoperiod and the involvement of light perception and the circadian clock in this process. The specific regulatory mechanisms on CONSTANS expression and CONSTANS stability by the circadian clock and photoreceptors are described in detail. In addition, the roles of CONSTANS, FLOWERING LOCUS T, and several other light signaling and circadian-dependent components in photoperiodic flowering are also highlighted.     

Timing of Photoperiodic Flowering:Light Perception and Circadian Clock

Flowering symbolizes the transition of a plant from vegetative phase to reproductive phase and is controlled by fairly complex and highly coordinated regulatory pathways. Over the last decade, genetic studies in Arabidopsis have aided the discovery of many signaling components involved in these pathways. In this review, we discuss how the timing of flowering is regulated by photoperiod and the involvement of light perception and the circadian clock in this process. The specific regulatory mechanisms on CONSTANS expression and CONSTANS stability by the circadian clock and photoreceptors are described in detail. In addition, the roles of CONSTANS, FLOWERING LOCUS T, and several other light signaling and circadiandependent components in photoperiodic flowering are also highlighted.     

Circadian Clock Regulation of the Cell Cycle in the Zebrafish Intestine

The circadian clock controls cell proliferation in a number of healthy tissues where cell renewal and regeneration are critical for normal physiological function. The intestine is an organ that typically undergoes regular cycles of cell division, differentiation and apoptosis as part of its role in digestion and nutrient absorption. The aim of this study was to explore circadian clock regulation of cell proliferation and cell cycle gene expression in the zebrafish intestine. Here we show that the zebrafish gut contains a directly light-entrainable circadian pacemaker, which regulates the daily timing of mitosis. Furthermore, this intestinal clock controls the expression of key cell cycle regulators, such as cdc2, wee1, p21, PCNA and cdk2, but only weakly influences cyclin B1, cyclin B2 and cyclin E1 expression. Interestingly, food deprivation has little impact on circadian clock function in the gut, but dramatically reduces cell proliferation, as well as cell cycle gene expression in this tissue. Timed feeding under constant dark conditions is able to drive rhythmic expression not only of circadian clock genes, but also of several cell cycle genes, suggesting that food can entrain the clock, as well as the cell cycle in the intestine. Rather surprisingly, we found that timed feeding is critical for high amplitude rhythms in cell cycle gene expression, even when zebrafish are maintained on a light-dark cycle. Together these results suggest that the intestinal clock integrates multiple rhythmic cues, including light and food, to function optimally.     

Evidence for Multiple Lateral Transfers of the Circadian Clock Cluster in Filamentous Heterocystic Cyanobacteria Nostocaceae

Cyanobacteria are the first prokaryotes reported to show circadian rhythmicity, which is regulated by a cluster of three genes: kaiA, kaiB, and kaiC. Phylogenetic analysis of the kaiBC cluster in filamentous cyanobacteria of the family Nostocaceae including Nodularia spumigena and Nostoc linckia from Arubotaim Cave, Mt. Sedom, Israel, indicated that this cluster has experienced multiple lateral transfers. The transfers have occurred in different periods of the species evolution. The data obtained suggest that lateral transfers of the circadian clock cluster in filamentous cyanobacteria have been common and might have adaptive significance.     

Sirt1与生物钟的相互调控

生物钟调控机制广泛存在于各种类型的细胞中,控制着细胞代谢的节律性变化.最近的研究发现,NAD+依赖的组蛋白去乙酰化酶Sirt1参与了生物钟调控过程,对维持正常的生物钟节律具有重要作用;另一方面,Sirt1的表达也受到生物钟系统的调控,呈现出昼夜节律性的表达.因此Sirt1能与生物钟进行相互调控,并且这一作用机制很可能广泛参与了不同类型细胞内的信号转导和能量代谢过程.本文总结了Sirt1与生物钟之间相互调控的一些研究进展,对它们之间的分子调控机制进行了概述.     

The ATP-Mediated Regulation of KaiB-KaiC Interaction in the Cyanobacterial Circadian Clock

The cyanobacterial circadian clock oscillator is composed of three clock proteins—KaiA, KaiB, and KaiC, and interactions among the three Kai proteins generate clock oscillation in vitro. However, the regulation of these interactions remains to be solved. Here, we demonstrated that ATP regulates formation of the KaiB-KaiC complex. In the absence of ATP, KaiC was monomeric (KaiC^1mer) and formed a complex with KaiB. The addition of ATP plus Mg²⁺ (Mg-ATP), but not that of ATP only, to the KaiB-KaiC^1mer complex induced the hexamerization of KaiC and the concomitant release of KaiB from the KaiB-KaiC^1mer complex, indicating that Mg-ATP and KaiB compete each other for KaiC. In the presence of ATP and Mg²⁺ (Mg-ATP), KaiC became a homohexameric ATPase (KaiC^6mer) with bound Mg-ATP and formed a complex with KaiB, but KaiC hexamerized by unhydrolyzable substrates such as ATP and Mg-ATP analogs, did not. A KaiC N-terminal domain protein, but not its C-terminal one, formed a complex with KaiB, indicating that KaiC associates with KaiB via its N-terminal domain. A mutant KaiC^6mer lacking N-terminal ATPase activity did not form a complex with KaiB whereas a mutant lacking C-terminal ATPase activity did. Thus, the N-terminal domain of KaiC is responsible for formation of the KaiB-KaiC complex, and the hydrolysis of the ATP bound to N-terminal ATPase motifs on KaiC^6mer is required for formation of the KaiB-KaiC^6mer complex. KaiC^6mer that had been hexamerized with ADP plus aluminum fluoride, which are considered to mimic ADP-Pi state, formed a complex with KaiB, suggesting that KaiB is able to associate with KaiC^6mer with bound ADP-Pi.     

Intramolecular Regulation of Phosphorylation Status of the Circadian Clock Protein KaiC

Background

KaiC, a central clock protein in cyanobacteria, undergoes circadian oscillations between hypophosphorylated and hyperphosphorylated forms in vivo and in vitro. Structural analyses of KaiC crystals have identified threonine and serine residues in KaiC at three residues (T426, S431, and T432) as potential sites at which KaiC is phosphorylated; mutation of any of these three sites to alanine abolishes rhythmicity, revealing an essential clock role for each residue separately and for KaiC phosphorylation in general. Mass spectrometry studies confirmed that the S431 and T432 residues are key phosphorylation sites, however, the role of the threonine residue at position 426 was not clear from the mass spectrometry measurements.

Methodology and Principal Findings

Mutational approaches and biochemical analyses of KaiC support a key role for T426 in control of the KaiC phosphorylation status in vivo and in vitro and demonstrates that alternative amino acids at residue 426 dramatically affect KaiC''s properties in vivo and in vitro, especially genetic dominance/recessive relationships, KaiC dephosphorylation, and the formation of complexes of KaiC with KaiA and KaiB. These mutations alter key circadian properties, including period, amplitude, robustness, and temperature compensation. Crystallographic analyses indicate that the T426 site is phosphorylatible under some conditions, and in vitro phosphorylation assays of KaiC demonstrate labile phosphorylation of KaiC when the primary S431 and T432 sites are blocked.

Conclusions and Significance

T426 is a crucial site that regulates KaiC phosphorylation status in vivo and in vitro and these studies underscore the importance of KaiC phosphorylation status in the essential cyanobacterial circadian functions. The regulatory roles of these phosphorylation sites–including T426–within KaiC enhance our understanding of the molecular mechanism underlying circadian rhythm generation in cyanobacteria.     

昼夜生物钟、负反馈调节与翻译后修饰

到目前为止 ,几乎在所有类型的生物中发现了昼夜生物钟。它们以约 2 4小时的周期控制着众多的分子、生理和行为过程[1、2 ] 。一个典型的例子是 ,我们的睡眠、清醒过程受昼夜钟控制 ,并受光、温对相位的重拨 ,以及地球 2 4小时昼夜的约束。不仅如此 ,我们的中心体温、某些激素分泌、生化过程均受生物钟控制。研究发现 ,对某些药物而言 ,一天中某个时候给药会得到最好的效率。乘飞机的时差反应、三班倒都是生物钟系统对环境表现的不适应 ,尽管昼夜钟的相位能即时得到调整 ,但生物钟控制的过程需要一定的时间才能得到适应。生物钟的普遍性、基…     

Circadian Clock Genes Contribute to the Regulation of Hair Follicle Cycling

Hair follicles undergo recurrent cycling of controlled growth (anagen), regression (catagen), and relative quiescence (telogen) with a defined periodicity. Taking a genomics approach to study gene expression during synchronized mouse hair follicle cycling, we discovered that, in addition to circadian fluctuation, CLOCK–regulated genes are also modulated in phase with the hair growth cycle. During telogen and early anagen, circadian clock genes are prominently expressed in the secondary hair germ, which contains precursor cells for the growing follicle. Analysis of Clock and Bmal1 mutant mice reveals a delay in anagen progression, and the secondary hair germ cells show decreased levels of phosphorylated Rb and lack mitotic cells, suggesting that circadian clock genes regulate anagen progression via their effect on the cell cycle. Consistent with a block at the G1 phase of the cell cycle, we show a significant upregulation of p21 in Bmal1 mutant skin. While circadian clock mechanisms have been implicated in a variety of diurnal biological processes, our findings indicate that circadian clock genes may be utilized to modulate the progression of non-diurnal cyclic processes.