The dc13 gene upstream of ictB is involved in rapid induction of the high affinity Na+ dependent HCO3- transporter in cyanobacteria

The phenomenon of rapid induction of high affinity HCO3- uptake was investigated in two cyanobacterial species, Synechococcus strain PCC 7942 and Synechocystis strain PCC 6803. For both strains, mass spectrometric analysis of HCO3- fluxes during steady state photosynthesis revealed that the high affinity HCO3- uptake system was rapidly induced only in the presence of Na+. In Synechococcus there was a correlation between the capability of rapid induction of the high affinity HCO3- uptake system and the appearance of the IctB protein. Neither fast induction of the high affinity HCO3- uptake system nor IctB accumulation were prevented by chloramphenicol but by K252a. Inactivation of the gene dc13 upstream of ictB in Synechococcus led to the inability of the cells to rapidly induce the high affinity Na+ dependent HCO3- uptake system although IctB was accumulated. The dc13 mutant was able to acclimate from high CO2 to 100 ppm CO2 by lowering the CO2 concentration step by step, while immediate decrease of the CO2 concentration to 100 ppm CO2 severely inhibited HCO3- uptake. In Synechocystis the rapid induction of the Na+ dependent high affinity HCO3- uptake system was not accompanied by an increase in sbtA RNA abundance, indicating that transcriptional regulation of sbtA is not responsible for the fast increase in substrate affinity of the HCO3- transporter. The results are discussed in terms of post-translational modification of constitutively expressed components (e.g., SbtA).