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Niwa, Y, Matsuo, T, Onai, K, Kato, D, Tachikawa, M and Ishiura, M (2013) Phase-resetting mechanism of the circadian clock in Chlamydomonas reinhardtii. Proc. Natl. Acad. Sci. U.S.A. 110:13666-71
Although the circadian clock is a self-sustaining oscillator having a periodicity of nearly 1 d, its period length is not necessarily 24 h. Therefore, daily adjustment of the clock (i.e., resetting) is an essential mechanism for the circadian clock to adapt to daily environmental changes. One of the major cues for this resetting mechanism is light. In the unicellular green alga Chlamydomonas reinhardtii, the circadian clock is reset by blue/green and red light. However, the underlying molecular mechanisms remain largely unknown. In this study, using clock protein-luciferase fusion reporters, we found that the level of RHYTHM OF CHLOROPLAST 15 (ROC15), a clock component in C. reinhardtii, decreased rapidly after light exposure in a circadian-phase-independent manner. Blue, green, and red light were able to induce this process, with red light being the most effective among them. Expression analyses and inhibitor experiments suggested that this process was regulated mainly by a proteasome-dependent protein degradation pathway. In addition, we found that the other clock gene, ROC114, encoding an F-box protein, was involved in this process. Furthermore, we demonstrated that a roc15 mutant showed defects in the phase-resetting of the circadian clock by light. Taken together, these data strongly suggest that the light-induced degradation of ROC15 protein is one of the triggers for resetting the circadian clock in C. reinhardtii. Our data provide not only a basis for understanding the molecular mechanisms of light-induced phase-resetting in C. reinhardtii, but also insights into the phase-resetting mechanisms of circadian clocks in plants.
Algal Proteins/metabolism; Base Sequence; Chlamydomonas reinhardtii/physiology; Circadian Clocks/physiology; Circadian Clocks/radiation effects; Circadian Rhythm/physiology; Gene Expression Regulation, Plant/physiology; Gene Expression Regulation, Plant/radiation effects; Light; Luciferases/genetics; Luciferases/metabolism; Luminescent Measurements; Molecular Sequence Data; Time Factors
|Gene product||Qualifier||GO Term||Evidence Code||with/from||Aspect||Extension||Notes||Status|
|GO:0009649: entrainment of circadian clock||
Figure 5A. Wildtype and roc15 mutant Chlamydomonas was entrained to a 12:12 light:dark cycle. The cultures were transferred to constant darkness and their tufA expression was measured with luminescence. They were then expressed to 5 minute long pulses of light. The wild type Chlamydomonas responded to the light pulses, evident by a phase shift in tufA expression. The roc15 mutant Chlamydomonas did not exhibit this phase shift. This indicate that roc15 is essential for circadian regulated phase shifts and entrainment.
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