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Institute of Pharmacology and Toxicology Chronobiology and Sleep Research

The Cellular Mechanism of Biological Clocks


In most organisms studied to date, the circadian oscillator is based upon a feedback loop of transcription and translation, whereby the protein product of a clock gene will indirectly shut off its own expression. Such feedback loops are regulated and probably prolonged by post-translational modifications of most proteins involved (reviewed in Harms et al., 2004) In cyanobacteria, it has been shown that rhythms of phosphorylation can even occur in the absence of transcription and translation (Nakajima et al., 2005).

Species in which a circadian clock have been characterized are printed in blue or red in Figure 1 (previous page). For the organisms listed below, an overview of the molecular feedback loops that comprise the clock can be seen by clicking on the indicated links.

Mammals: Musculus musculus, Homo sapiens
Drosophila melanogaster
Neurospora crassa
Synechoccus aureus


Harms, E., Kivimae, S., Young, M. W., and Saez, L. (2004). Posttranscriptional and posttranslational regulation of clock genes. J Biol Rhythms 19:361-373.

Nakajima, M., Imai, K., Ito, H., Nishiwaki, T., Murayama, Y., Iwasaki, H., Oyama, T., and Kondo, T. (2005). Reconstitution of circadian oscillation of cyanobacterial KaiC phosphorylation in vitro. Science 308:414-415.