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 HCO3^-uptake (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.