Evolutionary Origins of Self-sustained Kai Protein Circadian Oscillators

Light–dark cycles affect photosynthetic efficiency in autotrophic cyanobacteria; therefore, determining whether ancient cyanobacteria possessed a self-sustained circadian clock when oxygenic photosynthetic systems were established is an important issue in chronobiology. Here we examined the oscillation of the clock protein KaiC in modern cyanobacteria, as well as the function and structure of ancestral Kai proteins, to determine the evolutionary origin of the self-sustained Kai-protein oscillators. The results show that the oldest double-domain KaiC in ancestral bacteria lacked the factors functionally and structurally essential for rhythmicity. The ancestral Kai proteins had acquired these factors through molecular evolution that occurred around Global Oxidation and Snowball Earth events, and were eventually inherited as a self-sustained circadian oscillator by the most recent common ancestor of cyanobacteria capable of oxygenic photosynthesis. This autonomous Kai protein oscillator was further inherited by most freshwater and marine cyanobacteria present today as an autotrophic basis for time-optimal acquisition and consumption of energy from oxygenic photosynthesis.