Revealing a Two-Loop Transcriptional Feedback Mechanism in the Cyanobacterial Circadian Clock
Figure 3
Initial dynamics of the transcriptional KaiC feed-back species in simulated kaiA knockout mutants.
Each panel depicts the simulated expression dynamics of the positive transcriptional regulator, the negative transcriptional regulator and kaiBC mRNA for the first days in LL after kaiA transcription was removed. (A and B) Predicted time-series for two models of Group I. The HTD+-HU− model (A) predicts decreased kaiBC mRNA levels in the absence of kaiA transcription. In this simulation, TD-KaiC phosphorylation ceases and U-KaiC constitutively accumulates. As a result, kaiBC transcription is suppressed. Down-regulation of kaiBC was predicted from all models in which U-KaiC hexamers are assumed to suppress kaiBC. The HU+-HT− model (B) predicts an enhanced kaiBC level when the kaiA gene is absent. In this kaiA-knockout simulation the threonine phosphorylated KaiC hexamer level drops immediately. There are no T-KaiC hexamers, which could negatively feed back on kaiBC. In addition, U-KaiC hexamers increase steadily. As a result, kaiBC expression is not reduced. All models in which D-KaiC, T-KaiC, and S-KaiC hexamers negatively feed back on kaiBC cannot reproduce suppression of kaiBC when kaiA is absent. After removing kaiA transcription KaiC phosphorylation ceases abruptly such that the negative feedback loop is not functional to down-regulate kaiBC transcription. However, three exceptions suggest that the peak amplitude rhythms of the transcriptional activator and the transcriptional repressor species are crucial (Figure S8). (C and D) Predicted time-series for two models of Group II. The peak amplitude rhythms of the U-KaiBC complexes in the HT+-BU− model (C) are too low to fulfill the role as negative regulator of kaiBC transcription in the kaiA− mutant. Only the enhanced retention of the transcriptional activator (D-KaiC hexamers) in the HD+-BT− model (D) alone can suppress kaiBC expression rhythm in the simulated kaiA-knockout mutant. Note the different Y-scalings. The abbreviations are explained in Figure 1.