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 ge nes and expression of high-affinity CO2 and HCO3- transport systems. Among the low-CO2-inducible transcription units of Synechococcus sp, strain PCC 7 942 is the cmpABCD operon. encoding an ATP-binding cassette transporter sim ilar to the nitrate/nitrite transporter of the same cyanobacterium. A nitro gen-regulated promoter was used to selectively induce expression of the cmp ABCD genes by growth of transgenic cells on nitrate under high CO2 conditio ns. Measurements of the initial rate of HCO(3)(-)uptake after onset of ligh t, and of the steady-state rate of HCO3- uptake in the light, showed that t he controlled induction of the cmp genes resulted in selective expression o f high-affinity HCO3- transport activity. The forced expression of cmpABCD did not significantly increase the CO2 uptake capabilities of the cells. Th ese findings demonstrated that the cmpABCD genes encode a high-affinity HCO 3- transporter. A deletion mutant of cmpAB (M42) retained low CO2-inducible activity of HCO3- transport, indicating the occurrence of HCO3- transporte r(s) distinct from the one encoded by cmpABCD, HCO3- uptake by low-CO2-indu ced M42 cells showed lower affinity for external HCO3- than for wild-type c ells 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 ide ntification and description of a primary active HCO3- transporter.