Fig. 1.

Simplified scheme of the central circadian oscillator system in S. elongatus. The central oscillator of the posttranslational circadian system of S. elongatus is composed of three proteins called KaiA, KaiB, and KaiC. KaiC, shown in its hexameric form, starting in the nonphosphorylated state (Left) coming out of the dark (gray). KaiC then proceeds through a series of phosphorylation steps (black filled P) involving the sequential phosphorylation of Thr432 and Ser431, returning back to the unphosphorylated state via dephosphorylation of p-Thr432 and p-Ser431 (6). The phosphorylation states of KaiC are indicated by different colors, with green CII domains representing the KaiC-SpT state and orange the KaiC-pST state. KaiA, shown in its dimeric form (purple), promotes the phosphorylation of KaiC whereas KaiB, shown in its tetrameric form (red), inhibits the effect of KaiA. The phosphorylation/dephosphorylation events of KaiC within the Kai complex determine timekeeping. The phosphomimetics used in this study are indicated in parentheses next to the state that each best mimics. Each state was queried for its potential participation in Kai complex output activity (designated as “Output?”).