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-CO 2 conditions to inorganic C-limitation induces transcription of particular genes and expression of high-affinity CO 2 and HCO 3 − transport systems. Among the low-CO 2 -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 CO 2 conditions. Measurements of the initial rate of HCO 3 − uptake after onset of light, and of the steady-state rate of HCO 3 − uptake in the light, showed that the controlled induction of the cmp genes resulted in selective expression of high-affinity HCO 3 − transport activity. The forced expression of cmpABCD did not significantly increase the CO 2 uptake capabilities of the cells. These findings demonstrated that the cmpABCD genes encode a high-affinity HCO 3 − transporter. A deletion mutant of cmpAB (M42) retained low CO 2 -inducible activity of HCO 3 − transport, indicating the occurrence of HCO 3 − transporter(s) distinct from the one encoded by cmpABCD . HCO 3 − uptake by low-CO 2 -induced M42 cells showed lower affinity for external HCO 3 − than for wild-type cells under the same conditions, showing that the HCO 3 − transporter encoded by cmpABCD has the highest affinity for HCO 3 − among the HCO 3 − transporters present in the cyanobacterium. This appears to be the first unambiguous identification and description of a primary active HCO 3 − transporter.