Photosystem II damage and repair cycle in chloroplasts: involvement of a chloroplast-localized HSP-70

Organisms of oxygenic photosynthesis are subject to a photo-oxidative damage in which the D1 32 kD reaction center protein of photosystem-II is irreversibly inactivated. Through the process of evolution, organisms of oxygenic photosynthesis have acquired a repair mechanism to rectify this photo-oxidative damage. The repair entails a selective degradation of the photodamaged D1 and replacement with a de novo synthesized protein. Upon exposure of the green alga Dunaliella salina to high irradiance, the rate of photodamage is faster than that of the repair. In consequence, photodamaged reaction centers accumulate in the chloroplast thylakoids (photoinhibition). Under these conditions, in SDS-PAGE, we observed a loss of the D1 protein from the 32 kD electrophoretic position and a concomitant accumulation of a 160 kD D1-containing and cross-linked complex originating from the photodamaged PSII centers. Photoinhibition also entailed enhancement in the level of a chloroplast-localized heat-shock protein (HSP-70B). A gene encoding for the HSP-70B protein in D. salina was cloned and sequenced. Northern blot analysis revealed a transient induction of HSP-70B transcript level upon cell exposure to high irradiance, by about 70-fold, with a peak occurring after about 1 h in high light. Biochemical analyses indicated that a small amount of the HSP-70B protein is tightly associated with thylakoid membranes. Washing with 2 M NaBr or high concentration Tris buffer could not dissociate the HSP-70B protein from the photoinhibited chloroplast thylakoids. Further analyses suggested that HSP-70B might be involved in the repair of PSII from photodamage.