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; HCO ₃^- 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 carboxylaseoxygenase (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 C_iuptake 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-I3 and NDH-I₄). The genes for a number of these transporters are genetically induced under C_ilimitation 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 C_ilimitation.