Chloroplast-encoded small polypeptide, PsbT, is required for the efficient recovery of photodamaged photosystem II

Abstract

PsbT is a small hydrophobic PSII subunit that is encoded by chloroplast genome. Although psbT-deletion mutant of Chlamydomonas reinhardtii (?psbT) can grow photoautotrophycally under moderate light condition, the growth of this mutant is severely impaired by strong light illumination. Since the primary target of photoinhibition is PSII, PsbT may protect PSII against excess light and/or may be involved in the repair of photodamaged PSII. In this report, we studied photoinhibition and repair process of PSII in wild-type and ?psbT cells. In the strong light, the steady-state level of PSII proteins was constant in wild-type but slowly decreased in ?psbT. It was also found that O2 evolution was more severely photoinactivated in ?psbT. However, the both strains showed the same rates of photodegradation of PSII proteins and photoinactivation of O2 evolution. Subsequently, we have studied recovery of PSII after partial photoinactivation and found that the recovery was much slower in ?psbT than in wild-type. Since pulse-labeling experiments of chloroplast-encoded proteins revealed that the protein synthesis is not damaged by the photoinhibition treatment, it is concluded that PsbT is involved in the post-translational step(s) of the recovery process of photodamaged PSII. Localization and function of PsbT in PSII core complex is also discussed.