Impact of CP12 deletion on inorganic carbon acquisition and Rubisco partitioning in <i>Chlamydomonas reinhardtii</i>

The small chloroplastic protein CP12 has multiple functions, including the regulation of enzymes in the Calvin-Benson-Bassham cycle. Here, we investigated its role in the acclimation of Chlamydomonas reinhardtii to varying CO2 availability. We showed that phosphoribulokinase can interact with CP12 in conditions where the Calvin-Benson-Bassham cycle is active. Compared to the wild type, at high CO2, C. reinhardtii CP12 deletion mutants, or partially complemented mutants, have less phosphoribulokinase and ribulose-1,5-bisphosphate (RuBP), indicating that the regeneration of RuBP is regulated, in part, by CP12. C. reinhardtii has a CO2 concentrating mechanism that increases the supply of CO2 to ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) and involves, among other features, the condensation of Rubisco within the pyrenoid via its interaction with a scaffold protein named Essential Pyrenoid Component 1 (EPYC1). In CP12 deletion mutants, the expected relocation of Rubisco towards the pyrenoid was not observed upon transition from high to very low CO2, contrary to WT cells. The CP12 deletion mutants are a unique example where the induction of CO2 concentrating mechanism at very low CO2 was not accompanied by Rubisco relocation. Altogether, these results suggest that CP12 contributes to the coordination between RuBP regeneration, Rubisco location, and CO2 acquisition.