The PsbO homolog from Symbiodinium kawagutii (Dinophyceae) characterized using biochemical and molecular methods

A photosystem II component, the PsbO protein is essential for maximum rates of oxygen production during photosynthesis, and has been extensively characterized in plants and cyanobacteria but not in symbiotic dinoflagellates. Its close interaction with D1 protein has important environmental implications since D1 has been identified as the primary site of damage in endosymbiotic dinoflagellates after thermal stress. We identified and biochemically characterized the PsbO homolog from Symbiodinium kawagutii as a 28-kDa protein, and immunolocalized it to chloroplast membranes. Chloroplast association was further confirmed by western blot on photosynthetic membrane preparations. TX-114 phase partitioning, chromatography, and SDS-PAGE for single band separation and partial peptide sequencing yielded peptides identical or with high identity to PsbO from dinoflagellates. Analysis of a cDNA library revealed three genes differing by only one aminoacid residue in the in silico-translated ORFs despite greater differences at nucleotide level in the untranslated, putative regulatory sequences. The consensus full amino acid sequence displayed all the characteristic domains and features of PsbO from other sources, but changes in functionally critical, highly conserved motifs were detected. Our biochemical, molecular, and immunolocalization data led to the conclusion that the 28-kDa protein from S. kawagutii is the PsbO homolog, thereby named SkPsbO. We discuss the implications of critical amino acid substitutions for a putative regulatory role of this protein.