Turns out it’s a bit more complicated than a Swiss watch. Emphasis added.
Science 31 October 2008:
Vol. 322. no. 5902, pp. 697 – 701
Structural Insights into a Circadian Oscillator
Carl Hirschie Johnson,1* Martin Egli,2 Phoebe L. Stewart3
An endogenous circadian system in cyanobacteria exerts pervasive control over cellular processes, including global gene expression. Indeed, the entire chromosome undergoes daily cycles of topological changes and compaction. The biochemical machinery underlying a circadian oscillator can be reconstituted in vitro with just three cyanobacterial proteins, KaiA, KaiB, and KaiC. These proteins interact to promote conformational changes and phosphorylation events that determine the phase of the in vitro oscillation. The high-resolution structures of these proteins suggest a ratcheting mechanism by which the KaiABC oscillator ticks unidirectionally. This posttranslational oscillator may interact with transcriptional and translational feedback loops to generate the emergent circadian behavior in vivo. The conjunction of structural, biophysical, and biochemical approaches to this system reveals molecular mechanisms of biological timekeeping.
Selected snips from the paper (subscription required for the full article):
Cogs and Gears: The Kai Proteins
The clockwork mechanism that controls these global rhythms of transcription, chromosomal topology, and cell division is composed of three essential proteins—KaiA, KaiB, and KaiC—which were identified in 1998 (15). Their three-dimensional structures, which became available in 2004 (16–21), are the only full-length structures of core circadian clock proteins that have been determined.
Therefore, the posttranslational cyanobacterial clockwork is composed of biochemical reactions such as phosphorylation, ATP hydrolysis, monomer exchange, and conformational changes among thousands of molecules per cell (~10,000 KaiC monomers per cell) (37), permitting robust oscillations of high precision and synchrony…..
The benefit of a clockwork that is imperturbable even when buffeted by the massive intracellular changes of cell division could have provided an evolutionary driving force for convergent circadian clock mechanisms among diverse organisms.
We now recognize KaiABC as a dynamically oscillating nanomachine that has evolved to precess unidirectionally and robustly. The challenges ahead are to delve deeper into the molecular nature of its temperature compensation, to examine the place of the PTO in the larger cellular program, and to discover if the clocks in our own cells have attributes that are similar to those of bacteria.