Rhythmic oscillations in KaiC1 phosphorylation and ATP/ADP ratio in nitrogen-fixing cyanobacterium <i>Cyanothece</i> sp. ATCC 51142

<p>Cyanobacterial circadian clock composed of the Kai oscillator has been unraveled in the model strain <i>Synechococcus elongatus</i> PCC 7942. Recent studies with nitrogen-fixing <i>Cyanothece</i> sp. ATCC 51142 show rhythmic oscillations in the cellular program even in continuous light albeit with a cycle time of ~11 h. In the present study, we investigate correlation between cellular rhythms, KaiC1 phosphorylation cycle, ATP/ADP ratio, and the redox state of plastoquinone pool in <i>Cyanothece</i>. KaiC1 phosphorylation cycle of <i>Cyanothece</i> was similar to that of <i>Synechococcus</i> under diurnal cycles. However, under continuous light, the cycle time was shorter (11 h), in agreement with physiological and gene expression studies. Interestingly, the ATP/ADP ratio also oscillates with an 11 h period, peaking concomitantly with the respiratory burst. We propose a mathematical model with C/N ratio as a probable signal regulating the clock in continuous light and emphasize the existence of a single timing mechanism regardless of the cycle time.</p>