Chloroplast-encoded Polypeptide PsbT Is Involved in the Repair of Primary Electron Acceptor QA of Photosystem II during Photoinhibition in Chlamydomonas reinhardtii

PsbT is a small chloroplast-encoded hydrophobic polypeptide associated with the D1/D2 heterodimer of the photosystem II (PSII) reaction center and is required for the efficient post-translational repair of photodamaged PSII. Here we addressed that role in detail in Chlamydomonas reinhardtii wild type and ΔpsbT cells by analyzing the activities of PSII, the assembly of PSII proteins, and the redox components of PSII during photoinhibition and repair. Strong illumination of cells for 15 min decreased the activities of electron transfer through PSII and QA photoreduction by 50%, and it reduced the amount of atomic manganese by 20%, but it did not affect the steady-state level of PSII proteins, photoreduction of pheophytin (pheoD1), and the amount of bound plastoquinone (QA), indicating that the decrease in PSII activity resulted mainly from inhibition of the electron transfer from pheoD1 to QA. In wild type cells, we observed parallel recovery of electron transfer activity through PSII and QA photoreduction, suggesting that the recovery of QA activity is one of the rate-limiting steps of PSII repair. In ΔpsbT cells, the repairs of electron transfer activity through PSII and of QA photoreduction activity were both impaired, but PSII protein turnover was unaffected. Moreover, about half the QA was lost from the PSII core complex during purification. Since PsbT is intimately associated with the QA-binding region on D2, we propose that this polypeptide enhances the efficient recovery of QA photoreduction by stabilizing the structure of the QA-binding region.