REPAIR OF UV-B INDUCED DAMAGE OF PHOTOSYSTEM-II VIA DE-NOVO SYNTHESISOF THE D1 AND D2 REACTION-CENTER SUBUNITS IN SYNECHOCYSTIS SP. PCC-6803

The repair of ultraviolet-B radiation induced damage to the structure and function of Photosystem II was studied in the cyanobacterium Synec hocystis sp. PCC 6803. UV-B irradiation of intact Synechocystis cells results in the loss of steady-state oxygen evolution, an effect accomp anied by a parallel loss of both D1 and D2 protein subunits of the Pho tosystem II reaction centre. Transfer of the W-irradiated cells to nor mal growth conditions under visible light results in partial recovery of the inhibited oxygen evolving activity and restoration of the lost D1 and D2 proteins. The extent of recovery decreases with increasing d egree of damage: after 50% inhibition, the original activity is comple tely restored within 2 hours. In contrast, after 90-95% inhibition les s than half of the original activity is regained during a 4 hour recov ery period. The translation inhibitor lincomycin completely blocks the recovery process if added after the UV-B treatment, and accelerates t he kinetics of activity loss if added before the onset of W-B irradiat ion. Substantial retardation of recovery and acceleration of activity loss is also observed if the very low intensity short wavelength contr ibution (lambda<290 nm) is not filtered out from the W-B light source. It is concluded that in intact cells W-B induced damage of the Photos ystem II complex can be repaired. This process is the first example of simultaneous D1 and D2 protein repair in Photosystem II, and consider ed to function as an important defence mechanism against detrimental U V-B effects in oxygenic photosynthetic organisms. De novo synthesis of the D1 and D2 reaction centre subunits is a key step of the repair pr ocess, which itself can also be inhibited by ultraviolet light, especi ally by the short wavelength W-C components, or by high doses of UV-B.