Distinguishing Feedback Mechanisms in Clock Models

Biological oscillators are very diverse but can be classified based on dynamical motifs such as the types of feedback loops present. The S. Elongatus circadian clock is a remarkable phosphorylation-based oscillator that can be reconstituted in vitro with only 3 different purified proteins: the clock proteins KaiA, KaiB, and KaiC. Despite a growing body of knowledge about the biochemistry of the Kai proteins, basic questions about how their interactions lead to sustained oscillations remain unanswered. Here, we compare models of this system that make opposing assumptions about whether KaiA sequestration introduces a positive or a negative feedback loop. We find that the two different feedback mechanisms can be distinguished experimentally by the introduction of a protein that binds competitively with KaiA. Understanding the dynamical mechanism responsible for oscillations in the Kai system may shed light on the broader question of what clock architectures have been selected by evolution and why.