The Circadian Clock—A Molecular Tool for Survival in Cyanobacteria

Abstract

Cyanobacteria are photosynthetic organisms that are known to be responsible for oxygenating Earth’s early atmosphere. Having evolved to ensure optimal survival in the periodic light/dark cycle on this planet, their genetic codes are packed with various tools, including a sophisticated biological timekeeping system. Among the cyanobacteria is Synechococcus elongatus PCC 7942, the simplest clock-harboring organism with a powerful genetic tool that enabled the identification of its intricate timekeeping mechanism. The three central oscillator proteins—KaiA, KaiB, and KaiC—drive the 24 h cyclic gene expression rhythm of cyanobacteria, and the “ticking” of the oscillator can be reconstituted inside a test tube just by mixing the three recombinant proteins with ATP and Mg2+. Along with its biochemical resilience, the post-translational rhythm of the oscillation can be reset through sensing oxidized quinone, a metabolite that becomes abundant at the onset of darkness. In addition, the output components pick up the information from the central oscillator, tuning the physiological and behavioral patterns and enabling the organism to better cope with the cyclic environmental conditions. In this review, we highlight our understanding of the cyanobacterial circadian clock and discuss how it functions as a molecular chronometer that readies the host for predictable changes in its surroundings.

Shift in Conformational Equilibrium Underlies the Oscillatory Phosphoryl Transfer Reaction in the Circadian Clock

“…KaiB binding‘s allosteric induction of A-loop burial also had a potential role in the entrainment process since maintaining the A-loop’s buried conformation during the falling phase of the clock was crucial not only for maintaining the KaiC–KaiB–CikA complex, but also because KaiA could not tether the A-loop in its exposed conformation and run the rhythm backward toward phosphorylation. If the A-loop became exposed at this stage, KaiA, not CikA, would have been inactivated by oxidized quinone, which would have advanced the phase instead of delaying it [ 43 , 45 , 46 ]. It is through this process that KaiB initiated the dephosphorylation phase at the correct timing of day and ensured that the CikA binding took place accordingly.…”

CBC‐Clock Theory of Life – Integration of cellular circadian clocks and cellular sentience is essential for cognitive basis of life

“…[ 13 ] In order to survive, ancient as well as present circadian clocks had to have been coordinated and synchronized with the environmental time. [ 14 ] It is well known that circadian clocks are flexible and adapt effectively to change in the parameters of relevant environmental cues.…”

“…[103][104][105][106] As both the redox-based circadian clocks and anesthetics-induced loss of sentience are closely related to electron transfers and ROS homeostasis, it can be expected that these elusive phenomena will be closely linked. The cognitive aspects of circadian clocks, allowing cellular and organismal survival in the fluctuating environment, as well as environmental awareness via the CBC theory [1][2][3][4][5]14] provides us with a foundation for the novel CBC-clock theory of life.…”

Damped Oscillating Phosphoryl Transfer Reaction in the Cyanobacterial Circadian Clock

“…This is also true for cyanobacteria, in which its metabolic activities are controlled by an accurate circadian clock, an internal time-keeping system in a cell. 2 However, in a cyanobacterial strain, Synechococcus elongatus PCC 7942, the 24 h rhythm has been found to be generated by the circadian oscillator, composed of only three major proteins—KaiA (uniProt ID: Q79PF6 ), KaiB (uniProt ID: Q79PF5 ), and KaiC (uniProt ID: Q79PF4 )—which runs in a manner somewhat independent of TTFL. 3 The biochemical aspects of the proteinaceous circadian clock have been studied in detail using the in vitro reconstitution method, which has revealed the chemical mechanism among the clock components.…”