Calredoxin regulates the chloroplast NADPH-dependent thioredoxin reductase inChlamydomonas reinhardtii

Calredoxin (CRX) is a calcium (Ca 2+ )-dependent thioredoxin (TRX) in the chloroplast of Chlamydomonas reinhardtii with largely unclear physiological role. We elucidated the CRX functionality by performing in-depth quantitative proteomics of wild type cells in comparison with crx insertional mutant (IM crx ), two CRISPR/Cas9 KO mutants and CRX rescues. These analyses revealed that the chloroplast NADPH-dependent TRX reductase (NTRC) is co-regulated with CRX. Electron transfer measurements revealed that CRX inhibits NADPH-dependent reduction of oxidized chloroplast 2-Cys peroxiredoxin (PRX1) via NTRC and that the function of the NADPH-NTRC complex is under strict control of CRX. Via non-reducing SDS-PAGE assays and mass spectrometry, our data also demonstrated that PRX1 is more oxidized under high light (HL) conditions in the absence of CRX. The redox tuning of PRX1 and control of the NADPH-NTRC complex via CRX interconnects redox control with active photosynthetic electron transport and metabolism as well as Ca 2+ signaling. In this way, an economic use of NADPH for PRX1 reduction is ensured. The finding, that the absence of CRX under HL conditions severely inhibited light-driven CO 2 fixation underpins the importance of CRX for redox tuning as well as for efficient photosynthesis. One-sentence summary Calredoxin dependent redox regulation ensures efficient photosynthesis.