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Michael J. Rust and Erin K. O’Shea

Circadian clocks are biological oscillators that help organisms keep track of the time of the day and coordinate their behavior with the rising and setting of the sun. These clocks are found across a broad range of organisms from bacteria to flies to humans. Despite their diversity of origins, all circadian clocks tend to share certain defining features characteristic of their biological function. Importantly, these clocks are truly self-sustained, and if an organism is kept in the lab in a constant environment its circadian clock will continue to robustly oscillate. Despite this robustness, circadian clocks are also sensitive to appropriate light/dark cues and will adjust their phase to bring the oscillation in synchrony with the environment–a process familiar to anyone who has taken an airplane flight to a very different time zone.

Photosynthetic cyanobacteria have the simplest known circadian clock, and the core oscillator can be reconstituted in a test tube using three purified proteins. This test-tube oscillator has many similar properties to the circadian clock of a living cyanobacterium, including the ability to run robustly for many days without external input. We sought to identify changes in physiology that occur in cyanobacteria in response to light and darkness and then to study how these changes affect the core oscillator. Because many species of cyanobacteria cannot take up sugars from their environment and must rely entirely on photosynthesis to generate energy, darkness causes profound changes in their metabolism. We found that prolonged periods of darkness cause a decay in the pool of the energy carrier molecule ATP (adenosine triphosphate) and a concomitant rise in its spent form ADP (adenosine diphosphate). At the same time as these metabolic changes occur, the phase of the circadian clock shifts in a manner which depends on the original state of the clock.

We simulated these physiological changes in ATP and ADP in the test tube oscillator system, and found that one of the oscillator proteins, a kinase, is tuned to respond to the ratio of ATP to ADP and thus acts as a sensor for the amount of available biochemical energy. Remarkably, pulses of ADP and ATP in the test tube cause phase shifts in the clock that are very similar to those caused by darkness, thus suggesting that there is a strong connection between the cell’s need for energy and the function of the circadian clock in photosynthetic bacteria. We were able to quantitatively understand how different phase shifts are induced by changing ATP and ADP at different times using a mathematical model that accounts for the rates of multisite phosphorylation in the protein oscillator.

Read more in Science


[January 25th, 2011]

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