Inorganic carbon transporters of the cyanobacterial CO2 concentrating mechanism.

Cyanobacteria possess an environmental adaptation known as a CO(2) concentrating mechanism (CCM) that evolved to improve photosynthetic performance, particularly under CO(2)-limiting conditions. The CCM functions to actively transport dissolved inorganic carbon species (Ci; HCO(3)(-) and CO(2)) resulting in accumulation of a pool of HCO(3)(-) within the cell that is then utilised to provide an elevated CO(2) concentration around the primary CO(2) fixing enzyme, ribulose bisphosphate carboxylase-oxygenase (Rubisco). Rubisco is encapsulated in unique micro-compartments known as carboxysomes and also provides the location for elevated CO(2) levels in the cell. Five distinct transport systems for active Ci uptake are known, including two types of Na(+)-dependent HCO(3)(-) transporters (BicA and SbtA), one traffic ATPase (BCT1) for HCO(3)(-) uptake and two CO(2) uptake systems based on modified NADPH dehydrogenase complexes (NDH-I(3) and NDH-I(4)). The genes for a number of these transporters are genetically induced under Ci limitation via transcriptional regulatory processes. The in-membrane topology structures of the BicA and SbtA HCO(3)(-) transporters are now known and this may aid in determining processes related to transporter activation during dark to light transitions or under severe Ci limitation.