Is There a Role for the 42 Kilodalton Polypeptide in Inorganic Carbon Uptake by Cyanobacteria?

Cyanobacterial cells accumulate substantial amounts of a membrane-associated 42 kilodalton polypeptide during adaptation to low CO(2) conditions. The role of this polypeptide in the process of adaptation and in particular in the large increase in the ability to accumulate inorganic carbon (C(i)), which accompanies this process, is not yet understood. We have isolated a mutant Synechococcus PCC7942 that does not accumulate the 42 kilodalton polypeptide. The mutant requires a high-CO(2) concentration for growth and exhibits a very low apparent photosynthetic affinity for extracellular C(i). The latter might be attributable to the observed defective ability of the mutant to utilize the intracellular C(i) pool for photosynthesis. The 42 kilodalton polypeptide does not appear to participate directly in the active transport of C(i), since the difference between the observed capabilities for CO(2) and HCO(3) (-) uptake of the mutant and the wild type is not sufficient to account for their different growth and photosynthetic performance. Furthermore, high CO(2)-grown wild-type cells, where we could not detect the 42 kilodalton polypeptide, transported CO(2) faster than the mutant. An analysis of the curves relating the rate of accumulation of C(i) to the concentration of CO(2) or HCO(3) (-) supplied, in the presence or absence of carbonic anhydrase, indicated that under the experimental conditions used here, CO(2) was the preferred C(i) species taken up by Synechococcus.