Regulation of photosynthetic carbon metabolism in aquatic and terrestrial organisms by Rubisco activase, redox-modulation and CP12

• Specific mechanisms to regulate photosynthetic carbon metabolism.
• Control of ribulose-1,5-bisphosphate carboxylase/oxygenase by Rubisco activase.
• Control of phosphoribulokinase and glyceraldehyde-3-phosphate dehydrogenase by CP12.
• Redox and CP12 regulation in aquatic environments.

Both aquatic and terrestrial photosynthetic organisms experience rapid changes in environmental conditions that can impact photosynthesis. To respond efficiently and quickly to variations in CO2, light and temperature, photoautotrophic organisms have developed various mechanisms that adjust photosynthetic carbon metabolism to the prevailing conditions in their habitat. In this review we describe how specific mechanisms for protein–protein interactions regulate photosynthetic carbon metabolism. These mechanisms include control of ribulose-1,5-bisphosphate carboxylase/oxygenase by its catalytic chaperone, Rubisco activase, as well as the regulation of phosphoribulokinase and glyceraldehyde-3-phosphate dehydrogenase through interaction with a small chloroplast protein, CP12. Redox regulation involving transient interactions with thioredoxins and their targets are also discussed. Many enzymes of the C3 Cycle (and other metabolic pathways) have been identified as targets of redox regulation in terrestrial plants, whereas only a few targets of thioredoxins have been identified in aquatic photoautotrophs. Similarly, the regulatory properties of Rubisco activase from aquatic plants have received little attention, even in well-studied organisms like Chlamydomonas. It seems that while carbon assimilation or metabolism has been widely studied in land plants, the focus in aquatic photoautotrophs has been on carbon acquisition via biophysical mechanisms. Further studies on the interrelationships among CP12, redox regulation and CO2 concentrating mechanisms are needed to give additional insights into the regulatory mechanisms in aquatic species.