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The cyanobacterium Synechocystis sp. strain PCC 6803 possesses two CO₂ uptake systems and two HCO transporters. We transformed a mutant impaired in CO₂ uptake and in cmpA–D encoding a HCOtransporter with a transposon inactivation library, and we recovered mutants unable to take up HCO and grow in low CO₂ at pH 9.0. They are all tagged within slr1512 (designated sbtA). We show that SbtA-mediated transport is induced by low CO₂, requires Na⁺, and plays the major role in HCO uptake in Synechocystis. Inactivation of slr1509 (homologous to ntpJ encoding a Na⁺/K⁺-translocating protein) abolished the ability of cells to grow at [Na⁺] higher than 100 mm and severely depressed the activity of the SbtA-mediated HCO transport. We propose that the SbtA-mediated HCO transport is driven by ΔμNa⁺ across the plasma membrane, which is disrupted by inactivating ntpJ. Phylogenetic analyses …