An Electromechanical Relay-like Multi-enzymatic System Gates c-di-GMP-Dependent Cell Fate in a Cyanobacterium

Cyclic-di-GMP is a ubiquitous bacterial second messenger that regulates diverse cellular processes. Although many bacteria harbor multiple enzymes for metabolism, the mechanisms underlying coordinated regulation remain unclear. Using the cyanobacterium Anabaena PCC 7120, we generate mutant strains with varying numbers of c-di-GMP metabolic genes deleted, including cdg ⁰ (all degradation genes deleted) and cdg ^max (all synthesis genes deleted). We found that cyclic-di-GMP is essential for both cell viability and size regulation. Quantitative analysis identified two critical physiological thresholds: one for maintaining normal cell size and a lower, lethal threshold required for survival. The 16 cyclic-di-GMP metabolic enzymes function as an electromechanical-like dual relay system, where different enzyme groups maintain cyclic-di-GMP homeostasis or activate SOS-like alarm responses when cyclic-di-GMP concentrations drop below the lethal threshold. These regulatory effects are mediated by the cyclic-di-GMP receptor CdgR, depending on the fraction of apo-CdgR form. This dual-threshold system enables dynamic cellular adaptation while preventing lethal consequences, representing a fundamental growth-survival trade-off strategy in living organisms.