PII-like signaling protein SbtB links cAMP sensing with cyanobacterial inorganic carbon response

Life on Earth depends on photosynthetic CO₂ fixation to form organic carbon. This process evolved in cyanobacteria and was later conveyed to eukaryotes, giving rise to plastids in algae and plants. To cope with low atmospheric CO₂ concentrations that developed over the course of evolution, cyanobacteria evolved a CO₂-concentrating mechanism (CCM), which elevates CO₂ levels in the vicinity of RubisCO, the key enzyme of CO₂ fixation. Here we describe a conserved cyclic AMP receptor protein, SbtB, which participates in the sensing of fluctuating C_i levels to regulate the cyanobacterial CCM system. SbtB represents a new principle of C_i sensing, which is important for acclimation to varying C_i regimes in the ecological niches of cyanobacteria.