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Abstract

Cyanobacteria are phototrophic prokaryotes that evolved oxygenic photosynthesis ∼2.7 billion y ago and are presently responsible for ∼10% of total global photosynthetic production. To cope with the evolutionary pressure of dropping ambient CO2 concentrations, they evolved a CO2-concentrating mechanism (CCM) to augment intracellular inorganic carbon (Ci) levels for efficient CO2 fixation. However, how cyanobacteria sense the fluctuation in Ci is poorly understood. Here we present biochemical, structural, and physiological insights into SbtB, a unique PII-like signaling protein, which provides new insights into Ci sensing. SbtB is highly conserved in cyanobacteria and is coexpressed with CCM genes. The SbtB protein from the cyanobacterium Synechocystis sp. PCC 6803 bound a variety of adenosine nucleotides, including the second messenger cAMP. Cocrystal structures unraveled the individual binding modes of trimeric SbtB with AMP and cAMP. The nucleotide-binding pocket is located between the subunit clefts of SbtB, perfectly matching the structure of canonical PII proteins. This clearly indicates that proteins of the PII superfamily arose from a common ancestor, whose structurally conserved nucleotide-binding pocket has evolved to sense different adenyl nucleotides for various signaling functions. Moreover, we provide physiological and biochemical evidence for the involvement of SbtB in Ci acclimation. Collectively, our results suggest that SbtB acts as a Ci sensor protein via cAMP binding, highlighting an evolutionarily conserved role for cAMP in signaling the cellular carbon status.