Genes essential to sodium-dependent bicarbonate transport in cyanobacteria - Function and phylogenetic analysis

The cyanobacterium. Synechocystis sp. strain PCC 6803 possesses two CO2 uptake systems and two HCO3- transporters. We transformed a mutant impaired in CO2 uptake and in cmpA-D encoding a HCO3- transporter with a transposon inactivation library, and we recovered mutants unable to take up HCO3- 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 HCO3- 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 m-m and severely depressed the activity of the SbtA-mediated HCO3- transport. We propose that the SbtA-mediated HCO3- transport is driven by DeltamuNa(+) across the plasma membrane, which is disrupted by inactivating ntpJ. Phylogenetic analyses indicated that two types of sbtA exist in various cyanobacterial strains, all of which possess ntpJ. The sbtA gene is the first one identified as essential to Na+-dependent HCO3- transport in photosynthetic organisms and may play a crucial role in carbon acquisition when CO2 supply is limited, or in Prochlorococcus strains that do not possess CO2 uptake systems or Cmp-dependent HCO3- transport.