Inorganic carbon transporters of the cyanobacterial CO2 concentrating mechanism

Cyanobacteria possess an environmental adaptation known as a CO2 concentrating mechanism (CCM) that evolved to improve photosynthetic performance, particularly under CO2-limiting conditions. The CCM functions to actively transport dissolved inorganic carbon species (Ci; HCO3- and CO2) resulting in accumulation of a pool of HCO3- within the cell that is then utilised to provide an elevated CO2 concentration around the primary CO2 fixing enzyme, ribulose bisphosphate carboxylase-oxygenase (Rubisco). Rubisco is encapsulated in unique micro-compartments known as carboxysomes and also provides the location for elevated CO2 levels in the cell. Five distinct transport systems for active Ci uptake are known, including two types of Na+-dependent HCO3- transporters (BicA and SbtA), one traffic ATPase (BCT1) for HCO3- uptake and two CO2 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 HCO3- 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.