Fig. 7. CO₂- and light-dependent signals converge to regulate photoprotection and CCM in Chlamydomonas.

The intracellular levels of CO₂, defined by the equilibrium between CO₂ fixation in chloroplasts and the generation of CO₂ by mitochondrial metabolism (e.g. acetate assimilation) is the key determinant of the regulation of gene expression controlling two major processes of photosynthetic organisms: CCM and photoprotection. Changes in light availability have a direct impact on intracellular CO₂ levels; exposure to HL increases CO₂ fixation rates leading to depletion of CO₂ and to activation of not only photoprotection- but also CCM-related genes. Conversely, depletion of CO₂ is sufficient to drive high expression levels of CCM genes and LHCSR3 even in complete darkness (indicated by the black arrows). High CO₂ levels, either exogenously supplied by sparging or metabolically produced via acetate metabolism or by inhibiting photosynthetic electron flow using DCMU, repress LHCSR3 and CCM genes while at the same time they stabilize LHCSR1 protein levels. The close interconnection of photoprotection and CCM is further corroborated by the fact that CIA5, the regulator of expression of genes associated with the CCM, also exerts control over LHCSR3 and to a lesser extent over PSBS mRNA levels and acts as repressor of LHCSR1 protein accumulation. Independent of CIA5, light strongly impacts expression of all of these photoprotective genes (yellow arrows). This impact can be the consequence of both photoperception (e.g. phototropin) and the production of reactive oxygen species.