Abstract

The cyanobacterium Synechocystis sp. strain PCC6803 possesses two CO₂ uptake systems; one constitutive, dependent on NdhD3/NdhF3/CupA (Sll1734), and one low-CO₂ inducible, dependent on NdhD4/NdhF4/CupB (Slr1302). Homologues of these genes are present in pairs in most cyanobacterial strains. Synechocystis PCC6803 also possesses two types of HCO₃^– transporters; an ATP-binding cassette (ABC)-type transporter encoded by the cmp operon, and a novel sodium-dependent transporter encoded byslr1512(sbtA) that plays a central role in HCO₃^– uptake. Mutants impaired for one of these four inorganic-carbon acquisition systems did not show mutant phenotype. Mutants inactivated for both CO₂ uptake systems were unable to grow at pH 7.0 in air, although they grew normally at pH 9.0 in air. Additional inactivation of the SbtA-type HCO₃^– transporter abolished growth at pH 9.0 in air. A fragment containing the promoter region of ndhF3 fused to the coding region of luxAB was inserted into a neutral site of the ΔndhD4 mutant to construct apF3-lux/ ΔndhD4 strain. The luminescence intensity of this strain was low in high-CO₂ grown cells, and was increased about 100 times after acclimation to air. Inactivation of the pF3-lux/ ΔndhD4 strain with a transposon-tagging library enabled us to isolate mutants incapable of acclimation to low CO₂.