Abstract
The cyanobacterial circadian clock has been well-studied and shown to be both robust and a dominant factor in the control of gene expression in Synechococcus elongatus PCC7942. In Synechocystis sp. PCC6803, the circadian clock is assumed to function similarly, yet appears to control transcription to a far lesser extent and its circadian rhythm was reported to not be sustained, or at least rapidly damped, under continuous illumination. One of the feedback loops that governs the clock in S. elongatus in addition to the core oscillator, i.e., the transcriptional-translation regulation loop hinging on KaiC-dependent expression of kaiBC, appears to be missing in Synechocystis, which would account for this difference. Here, we show that the clock in Synechocystis fulfills all criteria of a circadian clock: 1) a free-running period of approximately 24 h, 2) temperature compensation, and 3) being able to be entrained. A remarkably stable rhythm is generated despite the fact that the organism grows with a doubling time of less than 24 h in a photobioreactor run in turbidostat mode. No damping of the free-running circadian oscillation was observed in 2 weeks, suggesting that the clock in individual cells stays synchronized within a culture despite the apparent lack of a transcriptional-translation regulation loop. Furthermore, the dependence of chlorophyll synthesis on the presence of O2 was demonstrated.
Highlights
Many organisms have a circadian timing mechanism to cope with daily fluctuations in light and nutrient availability and temporally separate mutually incompatible processes [1]
In order to establish whether or not Synechocystis has a circadian clock, a Synechocystis culture growing in a photobioreactor (PBR) in turbidostat mode was subjected to 12 h/12 h light/dark (LD) entrainment (Fig 1)
No growth was observed in the dark and a varying growth rate was observed in the light, which peaked in the afternoon of the subjective ‘day’
Summary
Many organisms have a circadian timing mechanism to cope with daily fluctuations in light and nutrient availability and temporally separate mutually incompatible processes [1]. The cyanobacteria are the only known phylum of prokaryotes to have a circadian clock, though there is evidence for the presence of circadian rhythms in other Bacteria [2] and Archaea [3]. Circadian clocks must fulfill three criteria to be called as such: 1) a period of approximately 24 h in the absence of environmental cues (the so-called free-running period); 2) compensation for environmental changes, most prominently temperature, to keep this period at approximately 24 h and 3) the ability to be entrained by environmental cues (zeitgebers) and maintaining its phase relationship with the zeitgeber [4,5]. PCC6803 alter the authors' adherence to PLOS ONE policies on sharing data and materials
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