Chloroplast-encoded PsbT is a small hydrophobic polypeptide intimately associated with the photosystem II (PSII) core complex. In order to localize PsbT in the PSII core complex, purified PSII core complex was partially or completely disintegrated into sub-complexes. It was clearly shown that PsbT is bound to the PSII reaction center (PSII-RC), indicating that PsbT is a component of PSII-RC like cytochrome b559 and PsbI polypeptide. It has already been reported that PsbT is not essential for PSII structure and function, although the growth of the psbT-deletion (delta-psbT) mutant is severely impaired in strong light (Monod, et al. 1994). Since the primary target of photoinhibition is PSII, we studied the effects of strong light on the PSII complex and PSII function in wild-type and delta-psbT cells. In strong light, the amount of PSII was constant in wild-type cells, but decreased in delta-psbT cells. In addition, steady-state activity of O2 evolution was reduced in delta-psbT cells more significantly than that in the wild-type cells. However, photodegradation of the PSII protein and photoinactivation of O2-evolving activity occurred in a similar manner in both strains when the recovery of photodamaged PSII was blocked in the presence of a chloroplast protein synthesis inhibitor, chloramphenicol. After partial photoinactivation of PSII by exposure to strong light for 15 min, the recovery of O2-evolving activity of the delta-psbT cells was much slower than that of the wild-type cells. Pulse-labeling experiments with 35S revealed no difference between these strains as regards to synthesis of chloroplast-encoded proteins immediately after photoinhibition and during recovery. It is therefore concluded that PsbT located in the proximity of PSII-RC assists in the efficient repair of photodamaged D1 and/or integration of cofactors associated with D1.

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