Mutants of Synechocystis PCC6803 Defective in Inorganic Carbon Transport

Abstract

Eighty mutants of Synechocystis PCC6803 that require high CO(2) for growth were examined with a mass spectrometer for their ability to take up CO(2) in the light. Two of these mutants (type A) did not show any CO(2) uptake while the rest of the mutants (type B) took up CO(2) actively. Type A mutants (RKa and RKb) and one type B mutant (RK11) were partially characterized. At 3% CO(2), growth rates of the mutants and the wild type (WT) were similar. Under air levels of CO(2), growth of RKa and RKb was very slow, and RK11 did not grow at all. The photosynthetic affinities for inorganic carbon (C(i)) in these three mutants were about 100 times lower than the affinity in WT. The following characteristics of type A mutants indicated that the mutants have a defect in their CO(2)-transport system: (a) the activity of (13)C(18)O(2) uptake in RKa and RKb in the light was less than 5% the activity in WT, and (b) each mutant had only a low level of activity of (14)CO(2) uptake as measured by the method of silicone oil-filtering centrifugation. The HCO(3) (-)-transport system was also impaired in these mutants. The activity of H(14)CO(3) (-) uptake was negligibly low in RKb and was one-third the activity of WT in RKa. On the other hand, the type B mutant, RK11, transported CO(2) and HCO(3) (-) into the intracellular C(i) pool as actively as WT but was unable to utilize it for photosynthesis. Complementation analysis of type A mutants indicated that RKa and RKb have mutations in different regions of the genome. These results suggested that at least two kinds of proteins are involved in the C(i)-transport system.