## 3 Physiological Rhythm through the Output Pathway  
At dawn, the de novo synthesis of photosystems, ATP synthase, and carbon fixation-related proteins peaks [23; 48; 49]; at dusk, more than 30% of _S. elongatus_' transcription activity is shut off [23; 49]. It was proposed that cyanobacteria enter this period of dormancy to conserve reducing power and survive under an energy-limited nocturnal setting [50]. Then, how does the central oscillator exert its effect on the metabolic activity of the prokaryote such that not only the right proteins get expressed at the right time but also energy is more efficiently spent? It has been reported that KaiC activates the autokinase activity of histidine kinase SasA [51]. SasA is known to interact with KaiC directly, forming the KaiC-SasA complex [18]; this activates its autokinase activity. Through a sequence analysis, SasA was found to be composed of a receiver domain and a histidine kinase domain; though, the only structural data available for SasA by this point was that of a thioredoxin-like N-terminal domain [52]. Interestingly, KaiB is known to flip between an inactive (ground) and active (fold-switched) state [18], and the structural similarity between the fold-switched form of KaiB and N-terminal domain of SasA has previously been suggested [18], hinting at their competition for binding KaiC [51; 53]. Although KaiC-SasA binding peaks before that of KaiC-KaiB, there is a significant overlap between those binding activities [54], and it is difficult to conclude what accounts for this difference between SasA's and KaiB's affinities to KaiC at this point [55]. Then, phosphorylated SasA leads to RpaA phosphorylation, which is a master response regulator that binds more than 134 transcripts [3; 56]. The rhythmic change in the RpaA phosphorylation level brings about the activation of gene expression that peaks every 24 h, activating a sigma factor cascade that modulates cyanobacteria's physiological pattern in a polyphasic manner [57]. This simple output pathway accounts for cyanobacteria's ability to temporally control the production of proteins that are only needed at a certain time of day, such as enzymes that are needed for photosynthesis or glycogen breakdown [48]. A dogma in circadian clock research was that all timekeeping mechanisms run through the transcription-translation feedback loop (TTFL), where the abundance and gradual degradation of clock proteins renders a periodicity of 24 h. Cyanobacteria emerged as an exception and showed a free-running oscillator component that is embedded in, yet independent from, the TTFL known as the post-translational oscillator (PTO) [12]. This KaiABC-based PTO is suggested to account for the robustness of the cyanobacterial circadian clock, while TTFL is controlled by the oscillatory phosphorylation level of RpaA [58]. Aside from its input functionality, CikA also seems to be involved in other biologically pivotal processes. The cyanobacterial circadian clock controls when it is an optimal time to initiate or halt cell division, and there is a 6 h block of time during the subjective night when cytokinesis is limited. This narrow time frame of preventing reproduction is carefully modulated by the circadian clock such that cyanobacteria stop dividing when the cell is deprived of energy. \(\Delta\)_cikA_ and \(\Delta\)_kaiB_ display elongated cell phenotypes, and this was attributed to the defective clock circuit, which prevents them from initiating cytokinesis [59]. Ectopic expression of CikA made them quickly recover from their elongated state and start dividing. As if it was not prepared for dividing any time soon, \(\Delta\)_cikA_ showed diffuse patterns of FtsZ, which is a bacterial tubulin homolog that becomes localized to the mid-cell as the cell readies for cytokinesis [59].