A Mutant of Synechococcus PCC7942 Incapable of Adapting to Low CO(2) Concentration

Some properties of a mutant (RK1) of Synechococcus PCC7942, which requires high CO(2) for growth, are described. The photosynthetic affinity for inorganic carbon (C(i)) in RK1 was about 40 times lower than that in the wild type (WT) when grown at 3% CO(2) (H-cells) and did not change during 10 hours of exposure to low CO(2) (air containing 0.04% CO(2)). The gas exchange of WT and RK1 cells was measured using an open gas-analysis system. All the measurements were performed at a CO(2) concentration of 400 microliters per liter under the conditions where photosynthetic CO(2) fixation is inhibited. When the suspension of H-cells of WT or RK1 was illuminated, the rate of CO(2) influx from the gas phase into the suspension was low and addition of carbonic anhydrase during illumination released only a small amount of CO(2) from the medium into the gas phase. The rate of CO(2) influx and the amount of CO(2) released by carbonic anhydrase were increased in WT during low CO(2) adaptation. These changes did not occur in RK1 during exposure to low CO(2). Cytoplasmic membrane from H-cells of WT or RK1 contained small amount of 42-kilodalton polypeptide. Exposure of RK1 to low CO(2) did not have significant effect on the amount of 42-kilodalton polypeptide, while the same treatment on WT resulted in a large increase of this polypeptide. The RK1 mutant appears to be defective in its ability to utilize the intracellular C(i) pool for photosynthesis and also to transmit a low CO(2) signal for inducing the functional and compositional changes observed in WT during low CO(2) adaptation.