Doctor of Philosophy (PhD)


Biological Sciences

Document Type



Photosystem II (PSII) is the first component in the photosynthetic electron transport chain that enables the capture and conversion of light energy into storable chemical energy. The biogenesis and repair of PSII is complicated. We aim to better understand these processes, specifically D1 subunit translation and degradation. To identify protein factors that are involved in the translation of psbA, encoding D1, we used a forward genetic screen of a Chlamydomonas reinhardtii mutant pool and found a mutant defective in PSII biogenesis. We then mapped the causative gene to be the homolog of Arabidopsis High Fluorescence 244(HCF244) gene, or CrHCF244. We show CrHCF244 is required for psbA translation in the alga. TheArabidopsis HCF244 gene complemented the algal mutant. These results experimentally prove the functional conservation of the homologs in algae and land plants. Chlorophyll biosynthesis and apo-D1 translation have been shown to be linked in cyanobacteria and plants. However, there was a lack of chlorophyll changes in ∆CrHCF244, supporting the uncoupling of D1 translation and chlorophyll association in algae. Interestingly, suppressor mutants of the CrHCF244 mutant were observed, pointing to the presence of alternative pathway(s) for D1 translational control.

PSII is known to undergo frequent photodamage and repair. D1 turnover is key to PSII repair. In plants, the degradation of D1 is carried out by cooperation between Deg and FtsH proteases. In algae, it’s not known whether Degs played a role in PSII repair. To study the functions of Degs in algae including any role in D1 degradation, we characterized Deg9 in Chlamydomonas. In vitro protease assays showed that recombinant Deg9 has serine protease activity and immunofluorescence results revealed that Deg9 is localized to the chloroplast. However, CRISPR-Cas9 knockout mutants of deg9 did not exhibit a photoinhibition phenotype and D1 degradation rate is not affected in the mutants, indicating that Deg9 is not required for PSII repair. The lack of phenotype could be accounted for by genetic redundancy of Deg genes. Alternatively, Deg9 could function in processes not related to PSII repair. A methyl viologen sensitive phenotype in the mutants suggests a role in photooxidative stress.



Committee Chair

Vinyard David

Available for download on Sunday, April 04, 2027