Structure, function and regulation of the cyanobacterial high-affinity bicarbonate transporter, BCT1.

The cmpABCD operon of the cyanobacterium Synechococcus sp. strain PCC7942, which is transcribed specifically under CO2-limited growth conditions, encodes an ATP-binding cassette (ABC) transporter involved in HCO3uptake (designated BCT1). The product of the cmpA gene is a 42-kDa protein anchored to the plasma membrane, which binds HCO3 with high affinity (Kd= 5 µM) and acts as the substrate-binding protein of the transporter. The apparent Km(HCO3) of BCT1 is 15 µM. BCT1 has the highest affinity for HCO3 among the HCO3 transporters of the Synechococcus strain and is essential for competitive utilization of HCO3 under CO2-limited conditions. BCT1 is closely related to the cyanobacterial nitrate/nitrite transporter (NRT) encoded by the nrtABCD genes. The BCT1 and NRT transporters, together with the putative cyanate transporter of cyanobacteria, comprise a monoanion transporter subfamily in the family of ABC importers. BCT1 and NRT are present in most fresh-water strains of cyanobacteria but seem to be absent in marine cyanobacterial strains. The low CO2-responsive induction of thecmp operon requires a LysR family protein CmpR, which is similar to CbbR (RbcR), the activator of the CO2 fixation operons of chemoautotrophic and purple photosynthetic bacteria.