Circadian Clock in Synechococcus
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Circadian Clock in Synechococcus
Circadian rhythms are endogenous biological programs that allow organisms to anticipate changes in their environment, such as the onset of dawn and dusk, thereby coordinating temporal phases of physiology with the external environment. In plants, the circadian clock organizes a wide array of daily rhythms, including the expression of genes involved in photosynthesis, sugar metabolism, nutrient assimilation, stomatal opening, and leaf movements. In addition, two other clock properties, entrainment (setting the clock to local time with respect to environmental cycles) and temperature compensation (the ability of the clock to run at the same rate at different temperatures) ensure synchrony with the environment (Ref.1).

At the core of a circadian system is a molecular oscillator that generates a period of rhythmicity of about 24h. The oscillator possesses a resetting mechanism to receive timing cues from the environment. Resetting of the clock by light enables the oscillator to maintain a stable phase relationship with the external photoperiod and to detect gradual shifts at the onset of dawn as seasons change. The oscillator and the light-resetting mechanism work in concert to measure photoperiod and thereby coordinate photoperiod-dependent events such as nitrogen fixation in Cyanobacteria, scent emission in plants, conidiation in Neurospora crassa, olfactory responses of Drosophila melanogaster, luteinizing hormone levels in birds and wheel running activity in hamsters (Ref.2). In the Cyanobacterium, Synechococcus elongatus, three Kai genes (KaiA, KaiB, and KaiC) are essential for circadian function and form a gene cluster. Transcription of all three genes is rhythmic; with mRNA levels peaking near the end of the day. KaiA is transcribed as a monocistronic mRNA, and KaiB and KaiC are co-transcribed as a dicistronic message from the KaiBC promoter. All three Kai proteins interact with each other to form protein complexes. Environmental information, such as light and temperature, is transduced through the histidine protein kinase, CikA (Circadian Input Kinase), to its yet-to-be-identified cognate response regulator CikR. This input most likely continues through a signal transduction pathway leading to the receiver-like amino-terminus of KaiA. Interaction between KaiA and KaiC stimulates autophosphorylation of KaiC and the formation of a hexameric state. In the early evening, SasA (Synechococcus Adaptive Sensor) joins the KaiA/KaiC complex and KaiC stimulates the autophosphorylation of SasA, which transfers its phosphoryl group to its yet-to-be-identified response regulator, SasR. SasR, as well as other proteins in the output pathway, transduce the temporal information from the oscillator throughout the genome. Meanwhile, KaiB binds to KaiC late in the evening to abrogate the positive effect of KaiA on KaiC phosphorylation. By mid-morning, the complex disassociates into its components. Ectopic over expression of KaiC suppresses expression from the KaiBC promoter, whereas over expression of KaiA enhances expression of KaiBC. KaiC also physically associates with SasA and SasA work in conjunction with an unidentified RR (Response Regulator) (Ref.3). Varying physical associations of KaiB, KaiC and SasA influence phosphorylation and dephosphorylation of such RR. KaiC has ATP-binding and autokinase activities and a certain state of KaiA-KaiC complex activates the KaiBC promoter activity. Alternatively, KaiA may derepress KaiBC expression by affecting an inhibitory KaiC-containing complex. Moreover, KaiA is partly involved in KaiC-dependent repression of KaiBC expression. The state of interaction between KaiA and KaiC determines both positive and negative feedback effects on KaiBC expression (Ref.4). Although the biochemical functions of the Kai proteins remain unknown, the amino acid sequence of KaiC contains two ATP-/GTP-binding motifs in the tandem duplicated domains of KaiC (CI and CII domains).

The circadian clock in Synechococcus controls processes such as photosynthesis, nitrogen fixation, cell division, respiration, carbohydrate synthesis, and amino acid uptake. Another function that has been suggested for the clock is to allow organisms to program activities so that they occur at a specific part of the diurnal cycle. This programming may serve to ensure that incompatible reactions, such as nitrogen fixation and photosynthesis in cyanobacteria are temporally spaced. Circadian clocks may also allow the organism to anticipate night/day changes. For example, some molecular processes important for photosynthesis are initiated before dawn so that by sunrise the plants are ready to take maximum advantage of available light for photosynthesis. In addition, organisms may produce screening pigments before the sun rises to avoid damage by visible and UV light (Ref.5).