Inorganic carbon transporters of the cyanobacterial CO2 concentrating mechanism

Abstract

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 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 Ciuptake 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 Cilimitation 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 Cilimitation.