Identification of an ATP-binding cassette transporter involved in bicarbonate uptake in the cyanobacterium Synechococcus sp. strain PCC7942

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