Trends in Microbiology
ReviewWinding up the cyanobacterial circadian clock
Section snippets
The cyanobacterial circadian clock system
Unlike most environmental variations that affect the fate of an organism, the rising and setting of the sun each day are profoundly predictable events. Many organisms have evolved internal timing mechanisms called ‘circadian clocks’ that enable them to anticipate the daily variations in light, temperature and humidity that result from Earth's rotation [1]. These endogenous biological clock systems are molecular mechanisms that control rhythmic gene expression and regulate metabolic and/or
The Kai oscillator: maintaining internal time
The cyanobacterial species in which the presence of a circadian clock was originally identified is not amenable to molecular genetic techniques [11]. S. elongatus PCC 7942 is used to investigate the molecular mechanisms that dictate timing in the cell because this bacterium offers many technical advantages, including a small fully sequenced genome and easy genetic manipulation 12, 13. The predominant circadian ‘behavior’ used to monitor clock activity in S. elongatus is the engineered
Input pathways: synchronizing the oscillator with daily cues
An oscillator, left to its own devices, would be poorly equipped as a clock. Slight imprecision in the oscillation would render the clock useless as it became continuously more out of synchronization with its surroundings. Even a spectacularly precise clock would be poorly adapted in most environments; an oscillator that is phase-locked to a specific time of day would not match the changing time of sunrise and sunset throughout the seasons.
The input pathway components recognize environmental
Output pathways: controlling cellular processes
The ability to maintain internal time is advantageous to an organism only if that information can be used to execute cellular processes at the time that provides the greatest benefit. In a competitive environment, the clock confers a growth advantage on S. elongatus45, 46. When cultures with different circadian periods are mixed in equal proportions and grown together in different LD cycles, the strain whose internal free-running period of gene expression most closely matches that of the
The periodosome
The in vivo S. elongatus clock system is much more complex than the beautiful simplicity of the in vitro oscillation in KaiC phosphorylation. The current model predicts the ∼24-h assembly and disassembly of a large, heteromultimeric complex termed a ‘periodosome’ [54] (Figure 4). Co-immunoprecipitation and gel-filtration experiments have shown that the KaiA, KaiB, KaiC and SasA proteins form a complex of 400–600 kDa [55], and the KaiA, KaiC and CikA proteins form complexes of just under 669 kDa
Concluding remarks and future directions
The circadian oscillator of the cyanobacterium S. elongatus, together with its intricate and complex input and output pathways, confers an adaptive advantage on this organism by enabling the anticipation of daily environmental cycles [46]. Despite the impressive advances in mechanistic details of the clock, many questions remain unanswered, some of which are listed in Box 3.
With each new discovery, it becomes increasingly difficult to separate the components into distinct divisions of the
Acknowledgements
The authors were supported by grants from the National Institutes of Health (R01 GM62419 and P01 NS39546) and the Department of Energy (DE-FG02–04ER15558) to S.S.G.
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