Studies of CrHCF244 reveal similarities and differences in psbA translation between Chlamydomonas reinhardtii and Arabidopsis thaliana

Translation of psbA , the chloroplast gene that encodes the D1 subunit of Photosystem II (PSII), is important for both PSII biogenesis and repair. The translation of the psbA transcript in the chloroplast is under the control of nuclear gene products. Using a Chlamydomonas forward genetic screen and whole genome sequencing, we found a mutant defective in PSII activity and mapped the causative gene to be the homolog of Arabidopsis High Fluorescence ( HCF244 ) gene, or CrHCF244 . We then demonstrated that CrHCF244 is required for psbA translation in the alga, consistent with the function of HCF244 in Arabidopsis . The Arabidopsis HCF244 gene also complemented the algal mutant. These results experimentally support the functional conservation of the homologs in green algae and land plants. However, these studies also revealed differences in psbA translation in Chlamydomonas and Arabidopsis . Loss of HCF244 in Arabidopsis results in a large decrease in chlorophyll. In contrast, there is no significant loss of chlorophyll in Chlamydomonas when CrHCF244 is knocked out. This observation supports the uncoupling of D1 translation and chlorophyll association in algae as reported recently in an ohp2 mutant, which is defective in chlorophyll delivery. Intriguingly, the CrHCF244 mutant also exhibited a relatively high rate of suppressor mutants, pointing to the presence of alternative pathway(s) for D1 translation control. The characterization of both the conserved aspects and the differences in psbA translation control between algae and plants will help elucidate how this process is regulated. Highlight We identified CrHCF244 as a translation factor of psbA in Chlamydomonas . Characterization of this protein and genetic examinations of other previously identified psbA translation factors in Chlamydomonas reveal similarities and differences in psbA translation between Chlamydomonas and Arabidopsis .