Regulatory components of carbon concentrating mechanisms in aquatic unicellular photosynthetic organisms

This review provides an insight into the regulation of the carbon concentrating mechanisms (CCMs) in lower organisms like cyanobacteria, proteobacteria, and algae. CCMs evolved as a mechanism to concentrate CO₂ at the site of primary carboxylating enzyme Ribulose-1, 5-bisphosphate carboxylase oxygenase (Rubisco), so that the enzyme could overcome its affinity towards O₂ which leads to wasteful processes like photorespiration. A diverse set of CCMs exist in nature, i.e., carboxysomes in cyanobacteria and proteobacteria; pyrenoids in algae and diatoms, the C₄ system, and Crassulacean acid metabolism in higher plants. Prime regulators of CCM in most of the photosynthetic autotrophs belong to the LysR family of transcriptional regulators, which regulate the activity of the components of CCM depending upon the ambient CO₂ concentrations. Major targets of these regulators are carbonic anhydrase and inorganic carbon uptake systems (CO₂ and HCO₃^- transporters) whose activities are modulated either at transcriptional level or by changes in the levels of their co-regulatory metabolites. The article provides information on the localization of the CCM components as well as their function and participation in the development of an efficient CCM. Signal transduction cascades leading to activation/inactivation of inducible CCM components on perception of low/high CO₂ stimuli have also been brought into picture. A detailed study of the regulatory components can aid in identifying the unraveled aspects of these mechanisms and hence provide information on key molecules that need to be explored to further provide a clear understanding of the mechanism under study.