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Abstract

The PII superfamily consists of widespread signal transduction proteins found in all domains of life. In addition to canonical PII proteins involved in C/N sensing, structurally similar PII-like proteins evolved to fulfill diverse, yet poorly understood cellular roles 1 . Cyanobacteria evolved highly specialized carbon concentrating mechanism (CCM) to cope with limiting atmospheric CO2 levels, augmenting intracellular inorganic carbon (Ci) levels to ensure efficient CO2-fixation2 . The sodium-dependent bicarbonate transporter SbtA is highly expressed under Ci limitation together with the conserved uncharacterized PII-like SbtB protein. Here, we provide the first structural, biochemical and physiological characterization of this unique PII-like protein from Synechocystis sp. PCC6803. SbtB can bind a variety of adenine nucleotides; structures revealed different complexes with cAMP, ADP and AMP, explaining the plasticity of effector binding. The nucleotide-binding pocket was identified to be located between the subunit clefts of SbtB, perfectly matching the structures of canonical PII proteins. This clearly indicates that proteins of the PII superfamily arose evolutionary from a common ancestor, whose structurally conserved nucleotide binding pocket has evolved to sense different adenyl nucleotides fulfilling various signaling functions. Moreover, physiological and biochemical characterization of wild-type and mutant cells provide evidence for involvement of SbtB in low Ci acclimation. Our results suggest that SbtB acts as Ci sensor protein via integrating the energy state of the cell and cAMP binding, highlighting an evolutionary conserved role for cAMP in signaling the cellular carbon status.