Effects of the signaling molecule cyclic-di-GMP on cyanobacterial circadian rhythm in Synechococcus elongatus PCC 7942

In phototrophic cyanobacteria, the signaling molecule bis-(3'-5')-cyclic dimeric-guanosine monophosphate (c-di-GMP) plays important roles in a wide variety of functions associated with environmental conditions, including biofilm formation, motility, heterocyst development, cell size control, phototaxis, and flocculation. However, its role in circadian rhythms, which contribute to fitness under diel light conditions, remains unexplored. In this study, we investigated the impact of changes in intracellular c-di-GMP levels on the circadian clock of the model cyanobacterium Synechococcus elongatus PCC 7942. Using inducible expression systems for the Escherichia coli genes yhjH (encoding a c-di-GMP phosphodiesterase) and ydeH (encoding a diguanylate cyclase), we modulated intracellular c-di-GMP concentrations. The effects were analyzed by monitoring bioluminescence rhythms from a luciferase reporter under the control of the clock. Induction of yhjH lengthened the circadian period by 0.6 h, while ydeH induction caused a phase delay of 5.8 h and reduced peak amplitude on the third circadian day. Additionally, exogenous c-di-GMP administration at the onset of darkness in a 12-h light/12-h dark cycle delayed the peak of the rhythm by 1 h, whereas administration at light onset had no effect. These results demonstrate that c-di-GMP levels in S. elongatus influenced multiple properties of the cyanobacterial circadian clock, including period, phase, and amplitude during light-dark cycles, suggesting that c-di-GMP signaling networks likely integrate environmental cues into circadian regulation through as yet unidentified factors.IMPORTANCEbis-(3'-5')-Cyclic dimeric-guanosine monophosphate (c-di-GMP) is a widely conserved bacterial signaling molecule that controls diverse physiological processes such as biofilm formation and motility, yet its influence on the circadian clock in cyanobacteria has not been described. Using inducible expression of E. coli enzymes and exogenous administration to alter intracellular c-di-GMP in a model cyanobacterium Synechococcus elongatus PCC 7942, we demonstrate that c-di-GMP modulates key properties of the circadian clock, including period, phase, and amplitude. These findings uncover a novel regulatory layer linking bacterial signaling networks to circadian clock regulation in cyanobacteria and suggest that c-di-GMP signaling enhances cyanobacterial fitness in natural light-dark cycles by fine-tuning circadian dynamics.