Over-expression of the oxygen-evolving enhancer 1 protein and its consequences on photosystem II accumulation

By transformation with a cloned wild-type oee1 gene, which codes for the oxygen-evolving enhancer 1 (OEE1)protein, we have constructed a strain of Chlamydomonas reinhardtii containing multiple copies of this gene. A transformant (R1-K-50) containing four to five copies of the oee1 gene accumulated oee1 mRNA in approximately threefold excess of the wild type. The OEE1 protein accumulated in proportion to the oee1-mRNA levels in these cells. These data indicate that no apparant feedback mechanism is operating to reduce either transcription or translation of the introduced oee1 genes as a means to regulate OEE1-protein accumulation. The OEE1 protein in R1-K-50 was all of mature size, indicating that the transit peptide had been completely removed, and that all of the protein was located within the thylakoid lumen. Photosystem II reaction-center proteins D1 and D2 accumulated to wild-type levels, but not greater, in these cells, while there was no effect on accumulation of any of the PSII peripheral proteins such as OEE2 or LHCII. The OEE1 protein which accumulated in excess of wild-type levels was not bound to the thylakoid membranes, indicating that a limited number of binding sites for OEE1 exist on the thylakoid membranes. No difference in photosynthetic oxygen evolution was observed between wild-type and Rl-K-50 strains. These data show that whatever mechanisms are used to determine stoichiometry within the PSII complex they are not perturbed by overexpression of the OEE1 protein.