Sweeney and Hastings reported in 1958 that cells of the dinoflagellate Gonyaulax polyedra are permitted to divide only during a particular time of a day and that the timing of cell division is regulated by an endogenous circadian clock (1). Since then, the circadian control of the timing, or the circadian gating of cell division has been found in many organisms and, the formal properties of the circadian gating have been extensively studied in the algal flagellate Euglena gracilis by Edmunds and his coworkers (2).
However, in the case of the multiple fission green alga Chlamydomonas reinhardtii, a circadian rhythm has not been considered to play a role for the timing of cell division (2,3). We (4) recently obtained an unequivocal evidence for the circadian gating, and also pointed out both the erroneous assumptions on which was based the anti-circadian experiments of the advocators (3) and the possibility that it was just a circadian transient that they observed since they followed the cell division behavior only a few cycle at most after perturbations.
Another challenge was put forward by Chisholm and her coworkers (5) and, it has been thought that the circadian gating is not a sole explanation for the ~24 h cell division rhythm (2). The Chisholm’s theory (5) can predict not only the ~24 h rhythmicity of cell division under a constant condition for the cell population with a mean generation time of ~24h but also the other properties of the circadian rhythms. Since a circadian rhythm is defined by its formal properties but not by its molecular mechanism, their theory (5) should not be considered against the involvement of the circadian rhythm. Instead, they raised a possibility that the cell division cycle is involved in the circadian timing of cell division.
However, as was pointed out by Sweeney and Hastings (1), not all the cells divide in a circadian cycle and those cells have to wait for dividing until the following subjective nights. Thus, the phenomenon has been called the circadian gating of the timing of cell division. Chisholm’s theory (5) can hold for an imaginary case in which all the cells divide in a circadian cycle, which has never been realized. In addition, we showed that the speed of the circadian rhythm of the timing of cell division is completely independent of the cell cycle speed in Chlamydomonas (4).
Finally, how does the circadian clock regulate the timing of cell cycle progression?. Sweeney was the first to tackle with this question and demonstrated the circadian gating of the timing of the morphological transitions associated with the cell cycle progression in the giant green alga Pyrocystis fusiformis (6). However, the manifestation of the circadian timing of a cell cycle event (e.g. cytokinesis) does not mean that the event is a regulatory point by the circadian rhythm. For example, if no mitotic cells exist in the subjective day, it is natural that cells do not divide at all in the subjective day. In this case, the cell division is not a regulatory point or a circadian checkpoint. A DNA flowcytometric study in Euglena (7) demonstrated that, in the subjective day (13 h in our case), cells do not divide while there are the cells at G2/M and while the progression through G2/M takes only about 3 h without a circadian control, indicating a circadian checkpoint at G2/M. Similarly, another circadian checkpoint was found to be located at S. On the other hand, while the number of cells exiting from G1 displayed a circadian rhythm, it never attains 0. Therefore, the timing of the G1 exit does not seem a circadian checkpoint but instead determined by the timing of cell division.
1. Sweeney, B.M & Hastings
(1958) J. Protozool. 5, 217-224. 2.
Edmunds, L.N.Jr. (1988) Cellular and Molecular Bases of Biological Clocks,
Springer-Verlag, NY. 3.
Donnan, L. & John, P.C.L. (1983) Nature 340, 630-633; Spudich, J.L.
& Sager, R. (1980) J.Cell Biol. 85, 136-145. 4.
Goto, K. and Johnson, C.H. (1995) J. Cell Biol. 129, 1061-1069. 5.
Vaulot, D. & Chisholm, S.W. 6.
Sweeney, B.M. (1982) Plant Physiol. 70, 272-276. 7.
Hagiwara, S. , Kiyota, M., Goto, K., unpublished.
In: The 4th International Symposium on Bioscience and Human-Technology,
Natl Inst Bioscience and Human-Technology, Tsukuba, Feb 5-6, 1997.
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