The <i>HIGH CHLOROPHYLL FLUORESCENCE 244</i> homolog <i>CrHCF244</i> is required for <i>psbA</i> (D1) translation in <i>Chlamydomonas reinhardtii</i>

Translation of psbA, the chloroplast gene that encodes the D1 subunit of PSII, is important for both PSII biogenesis and repair. The translation of psbA transcripts in the chloroplast is under the control of nuclear gene products. Using a forward genetic screen and whole-genome sequencing of the alga Chlamydomonas reinhardtii, we found a mutant defective in PSII activity and mapped the causative gene to be the homolog of Arabidopsis HIGH CHLOROPHYLL FLUORESCENCE 244 (HCF244), namely CrHCF244. We then demonstrated that CrHCF244 is required for psbA translation in the alga, consistent with the function of HCF244 in Arabidopsis, and found that AtHCF244 also partially complemented the algal mutant. These results experimentally support the functional conservation of the homologs in green algae and land plants. Intriguingly, the CrHCF244 mutant also exhibited a relatively high rate of suppressor mutants, pointing to the presence of alternative factor(s)/pathway(s) for D1 translational control. The establishment of CrHCF244 as a psbA translation factor in C. reinhardtii shows the similarities in psbA translation regulation in algae and plants. The future identification of the alternative factor(s) in this alga will provide insights on psbA translation in plants.