Spring 2016
Seminars are held at 11:30AM in Cullimore Hall, Room 611, unless noted otherwise. For questions about the seminar schedule, please contact Casey Diekman.
Date: March 8, 2016
Speaker: Yong-Ick Kim
Department of Chemistry and Environmental Science,
New Jersey Institute of Technology
Title: "The Molecular Mechanism of the Cyanobacterial Circadian Clock"
Abstract:
Circadian clock systems provide many advantages for organismal survival and productivity on Earth, allowing for many organisms to adjust their metabolism to the daily oscillations of light and dark cycles. The cyanobacterial circadian clock is composed of a three part signaling system: input, oscillator, and output. The input pathway detects changes in light and dark from the environment, and synchronizes the phase of the oscillator with day/night cycles. The central oscillator is encoded by three genes, kaiA, kaiB, and kaiC, whose protein products function together to generate a 24-hour rhythm of KaiC phosphorylation. The oscillator transmits the 24-hour KaiC phosphorylation signal to an output pathway resulting in the regulation of a wide variety of rhythmic behaviors including gene expression. The central oscillator of cyanobacterial circadian clock is encoded by three genes, kaiA, kaiB, and kaiC, whose protein products function together to generate a 24-hour rhythm of KaiC phosphorylation. KaiC has two residues (Ser431, Thr432) that can be phosphorylated and modulation of KaiC’s autokinase and autophosphatase activities generates a 24-hour period phosphorylation and dephosphorylation rhythm. KaiA activates the autophosphorylation of KaiC and KaiB attenuates KaiA’s function, resulting in KaiC dephosphorylation. The 24-hour KaiC phosphorylation rhythm is generated by timely association and dissociation amongst these three Kai proteins. The 24-hour rhythm of KaiC phosphorylation can be reconstituted in vitro by mixing purified KaiA, KaiB, and KaiC with adenosine triphosphate (ATP). Cyanobacteria can synchronize their circadian rhythm to the environmental light/dark cycle by sensing redox state in the cell. The transition from light to dark oxidizes plastoquinone, which is sensed by KaiA and changes the phase of KaiC phosphorylation. This redox sensing is an alternative to direct light sensing in cyanobacteria which lack of light receptors and is an important factor for entrainment. The pseudo-receiver domain of KaiA and CikA are the main components to sense redox state of quinone. Another well-known product of photosynthesis that entrain circadian clock is ADP (or ATP). ADP seems like directly inhibit the phosphorylation of KaiC. The detailed mechanism of the resetting on the circadian clock can be studied with a reconstituted in vitro oscillator system.