Synthesis of reaction center proteins and reactivation of redox components during repair of photosystem II after light-induced inactivation.

The D1 reaction center protein in photosystem II (PSII) has a high turnover rate due to light-induced inactivation of the redox components. We have studied the reactivation kinetics of the redox components of PSII after strong illumination and compared these kinetics with the turnover of the D1 protein and translation kinetics of the plastid-encoded PSII core proteins in Chlamydomonas reinhardtii cells. Repair of PSII was to a large extent dependent on protein translation. During the first hours of repair, D1 translation was highly accelerated as compared to the other PSII core proteins. By addition of protein synthesis inhibitors during the recovery process, it was found that the time from protein synthesis to full reassembly and reactivation of the individual PSII complexes was about 55 +/- 10 min. Inactivation and reactivation of the redox components in PSII were followed by electron spin resonance and electron transport measurements. Combining the data shows that reactivation of the individual components proceeded together or shortly after one another. Thus, no accumulation of any partially active reactivation intermediate occurred. We conclude that the rate-limiting step of the repair cycle of PSII lies in the degradation and synthesis of the PSII reaction center proteins. Once stable synthesis of the PSII core proteins is achieved, reactivation of the redox components occurs very quickly.