Abstract
Organisms use circadian clocks to generate 24‐h rhythms in gene expression. However, the clock can interact with other pathways to generate shorter period oscillations. It remains unclear how these different frequencies are generated. Here, we examine this problem by studying the coupling of the clock to the alternative sigma factor sigC in the cyanobacterium Synechococcus elongatus. Using single‐cell microscopy, we find that psbAI, a key photosynthesis gene regulated by both sigC and the clock, is activated with two peaks of gene expression every circadian cycle under constant low light. This two‐peak oscillation is dependent on sigC, without which psbAI rhythms revert to one oscillatory peak per day. We also observe two circadian peaks of elongation rate, which are dependent on sigC, suggesting a role for the frequency doubling in modulating growth. We propose that the two‐peak rhythm in psbAI expression is generated by an incoherent feedforward loop between the clock, sigC and psbAI. Modelling and experiments suggest that this could be a general network motif to allow frequency doubling of outputs.
Highlights
Most organisms have evolved a circadian clock to anticipate the earth’s cycles of light and dark (Doherty & Kay, 2010)
To test whether our observed dynamics of psbAI are solely due to sigC and the circadian clock, we examined PpsbAI-YFP dynamics in a clock deletion background, and in a clock-sigC double deletion background
To test the generality of the motif, we examined the single-cell dynamics of the other group 2 sigma factors in cyanobacteria to test for signs of a two-peak oscillation
Summary
Most organisms have evolved a circadian clock to anticipate the earth’s cycles of light and dark (Doherty & Kay, 2010). Eukaryotic and prokaryotic algae have evolved a circadian clock to coordinate downstream processes (Cohen & Golden, 2015; Noordally & Millar, 2015). The product of pairs of circadian sinusoidal terms yields 12-h harmonics when the amplitudes of the two inputs are similar and their phases meet certain conditions (Korencic et al, 2012; Westermark & Herzel, 2013). It remains unclear how gene circuitry couples with the circadian clock to generate 12-h rhythms in vivo
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