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Publication Abstract from PubMed

Circadian rhythms play an essential part in many biological processes, and only three prokaryotic proteins are required to constitute a true post-translational circadian oscillator(1). The evolutionary history of the three Kai proteins indicates that KaiC is the oldest member and a central component of the clock(2). Subsequent additions of KaiB and KaiA regulate the phosphorylation state of KaiC for time synchronization. The canonical KaiABC system in cyanobacteria is well understood(3-6), but little is known about more ancient systems that only possess KaiBC. However, there are reports that they might exhibit a basic, hourglass-like timekeeping mechanism(7-9). Here we investigate the primordial circadian clock in Rhodobacter sphaeroides, which contains only KaiBC, to elucidate its inner workings despite missing KaiA. Using a combination of X-ray crystallography and cryogenic electron microscopy, we find a new dodecameric fold for KaiC, in which two hexamers are held together by a coiled-coil bundle of 12 helices. This interaction is formed by the carboxy-terminal extension of KaiC and serves as an ancient regulatory moiety that is later superseded by KaiA. A coiled-coil register shift between daytime and night-time conformations is connected to phosphorylation sites through a long-range allosteric network that spans over 140 A. Our kinetic data identify the difference in the ATP-to-ADP ratio between day and night as the environmental cue that drives the clock. They also unravel mechanistic details that shed light on the evolution of self-sustained oscillators.

From primordial clocks to circadian oscillators.,Pitsawong W, Padua RAP, Grant T, Hoemberger M, Otten R, Bradshaw N, Grigorieff N, Kern D Nature. 2023 Apr;616(7955):183-189. doi: 10.1038/s41586-023-05836-9. Epub 2023 , Mar 22. PMID:36949197^[1]

From MEDLINE®/PubMed®, a database of the U.S. National Library of Medicine.