Two CO2 uptake systems in cyanobacteria: four systems for inorganic carbon acquisition in Synechocystis sp. strain PCC6803.

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.