PHOTOSYSTEM-II REACTION-CENTER DAMAGE AND REPAIR IN DUNALIELLA-SALINA (GREEN-ALGA) - ANALYSIS UNDER PHYSIOLOGICAL AND IRRADIANCE-STRESS CONDITIONS

Mechanistic aspects of the photosystem II (PSII) damage and repair cycle in chloroplasts were investigated. The D1/32-kD reaction center protein of PSII (known as the psbA chloroplast gene product) undergoes a frequent light-dependent damage and turnover in the thylakoid membrane. In the model organism Dunaliella salina (green alga), growth under a limiting intensity of illumination (100 mumol of photons m-2 s-1; low light) entails damage, degradation, and replacement of D1 every about 7 h. Growth under irradiance-stress conditions (2000 mumol of photons m-2 s-1; high light) entails damage to and replacement of Dl about every 20 min. Thus, the rate of damage and repair of PSII appears to be proportional to the light intensity during plant growth. Low-light-grown cells do not possess the capacity for high rates of repair. Upon transfer of low-light-grown cells to high-light conditions, accelerated damage to reaction center proteins is followed by PSII disassembly and aggregation of neighboring reaction center complexes into an insoluble dimer form. The accumulation of inactive PSII centers that still contain the D1 protein suggests that the rate of Dl degradation is the rate-limiting step in the PSII repair cycle. Under irradiance-stress conditions, chloroplasts gradually acquire a greater capacity for repair. The induction of this phenomenon occurs with a half-time of about 24 h.