Advances in understanding the cyanobacterial CO2-concentrating-mechanism (CCM):: functional components, Ci transporters, diversity, genetic regulation and prospects for engineering into plants

Cyanobacteria have evolved a significant environmental adaptation, known as a CO2-concentrating-mechanism (CCM), that vastly improves photosynthetic performance and survival under limiting CO2 concentrations. The CCM functions to transport and accumulate inorganic carbon actively (Ci; HCO3-, and CO2) within the cell where the Ci pool is utilized to provide elevated CO2 concentrations around the primary CO2-fixing enzyme, ribulose bisphosphate carboxylase-oxygenase (Rubisco). In cyanobacteria, Rubisco is encapsulated in unique micro-compartments known as carboxysomes. Cyanobacteria can possess up to five distinct transport systems for Ci uptake. Through database analysis of some 33 complete genomic DNA sequences for cyanobacteria it is evident that considerable diversity exists in the composition of transporters employed, although in many species this diversity is yet to be confirmed by comparative phenomics. In addition, two types of carboxysomes are known within the cyanobacteria that have apparently arisen by parallel evolution, and considerable progress has been made towards understanding the proteins responsible for carboxysome assembly and function. Progress has also been made towards identifying the primary signal for the induction of the subset of CCM genes known as CO2-responsive genes, and transcriptional regulators CcmR and CmpR have been shown to regulate these genes. Finally, some prospects for introducing cyanobacterial CCM components into higher plants are considered, with the objective of engineering plants that make more efficient use of water and nitrogen.