CP12 controls ribulose 1,5 bisphosphate recycling and carbon acquisition in Chlamydomonas reinhardtii

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. This alga has a CO2 concentrating mechanism that increases the supply of CO2 to ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCO) and involves hallmarks such as HCO3- transporters and carbonic anhydrases as well as the condensation of RuBisCO within the pyrenoid via its interaction with a scaffold protein named Essential Pyrenoid Component 1 (EPYC1). We showed that 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 by CP12. In the absence of CP12, 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 hallmarks 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. HighlightCP12 regulates phosphoribulokinase amount and its product RuBP. The CP12 deletion mutants are unique example where the RuBisCO location was not correlated with the induction of CO2 concentration mechanism. This reveals a novel link between Calvin-Benson-Bassham cycle regulation and CO2 concentrating mechanisms in Chlamydomonas reinhardtii. Graphical abstract O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=115 SRC="FIGDIR/small/659560v1_ufig1.gif" ALT="Figure 1"> View larger version (39K): org.highwire.dtl.DTLVardef@1cdadb2org.highwire.dtl.DTLVardef@46ae27org.highwire.dtl.DTLVardef@538295org.highwire.dtl.DTLVardef@a214c7_HPS_FORMAT_FIGEXP M_FIG C_FIG