Mutants of Synechocystis PCC6803 defective in inorganic carbon transport

Abstract

Eighty mutants of Synechocystis PCC6803 that require high CO2 for growth were examined with a mass spectrometer for their ability to take up CO2 in the light. Two of these mutants (type A) did not show any CO2 uptake while the rest of the mutants (type B) took up CO2 actively. Type A mutants (RKa and RKb) and one type B mutant (RK11) were partially characterized. At 3% CO2, growth rates of the mutants and the wild type (WT) were similar. Under air levels of CO2, growth of RKa and RKb was very slow, and RK11 did not grow at all. The photosynthetic affinities for inorganic carbon (Ci) 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 CO2-transport system: (a) the activity of 13C18O2 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 14CO2 uptake as measured by the method of silicone oil-filtering centrifugation. The HCO3--transport system was also impaired in these mutants. The activity of H14CO3- 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 CO2 and HCO3- into the intracellular Ci 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 Ci-transport system.