A Mutant of Synechococcus PCC7942 Incapable of Adapting to Low CO2 Concentration

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Abstract
Some properties of a mutant (RK1) of Synechococcus PCC7942, which requires high CO2 for growth, are described. The photosynthetic affinity for inorganic carbon (Ci) in RK1 was about 40 times lower than that in the wild type (WT) when growth at 3% CO2 (H-cells) and did not change during 10 hours of expousre to low CO2 (air containing 0.04% CO2). The gas exchange of WT and RK1 cells was measured using an open gas-analysis system. All the measurements were performed at a CO2 concentration of 400 microliters per liter under the conditions where photosynthetic CO2 fixation is inhibited. When the suspension of H-cells of WT or RK1 was illuminated, the rate of CO2 influx from the gas phase into the suspension was low and addition of carbonic anhydrase during illumination released only a small amount of CO2 from the medium into the gas phase. The rate of CO2 influx and the amount of CO2 released by carbonic anhydrase were increased in WT during low CO2 adaptation. These changes did not occur in RK1 during exposure to low CO2. Cytoplasmic membrane from H-cells of WT or RK1 contained small amount of 42-kilodalton polypeptide. Exposure of RK1 to low CO2 did not have significant effect on the amount of 42-kilodalton polypeptide, whil 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 Ci pool for photosynthesis and also to transmit a low CO2 signal for inducing the functional and compositional changes observed in WT during low CO2 adaptation.