Frequency doubling in the cyanobacterial circadian clock

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. image A combination of single‐cell imaging and mathematical modeling shows how the cyanobacterial circadian clock can double its frequency from one peak to two peaks every 24 h using an oscillatory incoherent feedforward loop circuit. The expression of a key photosynthesis gene, psb AI , displays two peaks of gene expression every 24 h. Double peaks of psbA1 expression are modulated by light and are correlated with double peaks in cellular growth. Genetic perturbations reveal that the observed frequency doubling requires both the circadian clock and the alternative sigma factor sigC . Modeling and experiments suggest that an incoherent feedforward loop can be a general mechanism for frequency doubling.