Fig. 8: Model of the two interconnected KaiABC systems in Synechocystis cells.

KaiC1 and KaiC3 display phase-locked phosphorylation rhythms. KaiA3 and KaiB3 regulate auto-phosphorylation and dephosphorylation of KaiC3 and form a second oscillator. The KaiA1B1C1 oscillator appears to be the main driver of backscatter rhythms, whereas KaiC3 and KaiA3 are required to maintain the amplitude and period. KaiB3 deletion abolished the rhythms to a similar extent as kaiA1B1C1 deletion. Because KaiB proteins were shown to also interact with the KaiC proteins of the other system²⁶, we assume that the absence of competing KaiB3 leads to enhanced KaiB1 binding to phosphorylated KaiC3 (dashed line), thereby disturbing KaiA1B1C1-based oscillations. Altogether, this implies that phosphorylated KaiC3 stabilizes backscatter oscillations. The interconnection between the KaiC1 and KaiC3 systems is also metabolically relevant. Deletion of kaiA1B1C1 and both up- and downregulation of kaiA3 reduced mixotrophic growth, whereas deletion of the entire KaiA3B3C3 system and kaiC3 had no effect. This implies that KaiA1B1C1 mainly contributes to the switch from autotrophic to heterotrophic growth, but the phosphorylation rhythms of KaiC3 can interfere with it. Whether this interference occurs directly via the KaiC1-KaiC3 interaction²⁶ or indirectly via output pathways needs to be clarified. This figure was created with BioRender.com, released under a Creative Commons Attribution-NonCommercial-NoDerivs 4.0 International license.