Identification of an ATP-binding cassette transporter involved in bicarbonate uptake in the cyanobacterium Synechococcus sp. strain PCC 7942

Exposure of cells of cyanobacteria (blue-green algae) grown under high- CO₂ conditions to inorganic C-limitation induces transcription of particular genes and expression of high-affinity CO₂ and HCO₃^- transport systems. Among the low-CO₂-inducible transcription units of Synechococcus sp. strain PCC 7942 is the cmpABCD operon, encoding an ATP-binding cassette transporter similar to the nitrate/nitrite transporter of the same cyanobacterium. A nitrogen-regulated promoter was used to selectively induce expression of the cmpABCD genes by growth of transgenic cells on nitrate under high CO₂ conditions. Measurements of the initial rate of HCO₃^- uptake after onset of light, and of the steady-state rate of HCO₃^- uptake in the light, showed that the controlled induction of the cmp genes resulted in selective expression of high-affinity HCO₃^- transport activity. The forced expression of cmpABCD did not significantly increase the CO₂ uptake capabilities of the cells. These findings demonstrated that the cmpABCD genes encode a high- affinity HCO₃^- transporter. A deletion mutant of cmpAB (M42) retained low CO₂-inducible activity of HCO₃^- transport, indicating the occurrence of HCO₃^- transporter(s) distinct from the one encoded by cmpABCD. HCO₃^- uptake by low-CO₂-induced M42 cells showed lower affinity for external HCO₃^- than for wild-type cells under the same conditions, showing that the HCO₃^- transporter encoded by cmpABCD has the highest affinity for HCO₃^- among the HCO₃^- transporters present in the cyanobacterium. This appears to be the first unambiguous identification and description of a primary active HCO₃^- transporter.