Modes of active inorganic carbon uptake in the cyanobacterium, Synechococcus sp. PCC7942.

Cyanobacteria (blue-green algae) have evolved a remarkable environmental adaptation for survival at limiting CO2 concentrations. The adaptation is known as a CO2 concentrating mechanism, and functions to actively transport and accumulate inorganic carbon (Ci; HCO3 and CO2) within the cell. Thereafter, this Ci pool is utilised to provide elevated CO2 concentrations around the primary CO2 fixing enzyme, Rubisco, which is encapsulated in a unique micro-compartment known as the carboxysome. Recently, significant progress has been gained in understanding the different types of Ci transport in cyanobacteria. This semi-review centres on the model cyanobacterium, Synechococcus sp. PCC7942, which possesses at least four distinct modes of Ci uptake when grown under Ci limitation, each possessing a high degree of functional redundancy. The four modes so far identified are: (i) BCT1, an inducible, high affinity HCO3 transporter of the bacterial ATP binding cassette transporter family, encoded by cmpABCD; (ii) a constitutive, Na-dependent HCO3 transport system that can be allosterically activated (possibly by phosphorylation) in as little as 10 min; (iii) and (iv) two CO2 uptake systems, one constitutive and the other inducible, based on specialised forms of thylakoid-based, type 1, NAD(P)H dehydrogenase complexes (NDH-1). Here, we forward a speculative model that proposes that two unique proteins, ChpX and ChpY, possess CO2 hydration activity in the light, and when coupled to photosynthetic electron transport through the two specialised NDH-1 complexes, result in net hydration of CO2 to HCO3 as a crucial component of the CO2 uptake process.