Frequency doubling in the cyanobacterial circadian clock

Area of Science:

• Cellular biology
• Molecular biology
• Systems biology

Background:

• Circadian clocks generate 24-h rhythms in gene expression.
• Interactions between the clock and other pathways can create shorter oscillations.
• The mechanisms for generating these different frequencies are not fully understood.

Purpose of the Study:

• To investigate how circadian clocks interact with other pathways to generate oscillations with frequencies other than 24 hours.
• To examine the coupling of the circadian clock to the alternative sigma factor sigC in Synechococcus elongatus.
• To understand the role of frequency doubling in modulating cellular processes like growth.

Main Methods:

• Single-cell microscopy to observe gene expression dynamics.
• Genetic manipulation to study the role of sigC.
• Mathematical modeling to elucidate network mechanisms.

Main Results:

• The photosynthesis gene psbAI exhibits two peaks of expression per circadian cycle under constant low light.
• This two-peak oscillation is dependent on the alternative sigma factor sigC.
• Absence of sigC leads to psbAI rhythms reverting to one peak per day.
• Two circadian peaks in elongation rate were observed, also dependent on sigC, suggesting a role in growth modulation.

Conclusions:

• An incoherent feedforward loop involving the clock, sigC, and psbAI likely generates the two-peak rhythm.
• This network motif may be a general mechanism for frequency doubling of outputs in biological systems.
• Frequency doubling of gene expression and growth rate by sigC influences cyanobacterial physiology.