EFFECTS OF ULTRAVIOLET-B RADIATION ON PHOTOSYSTEM-II OF THE CYANOBACTERIUM SYNECHOCYSTIS SP PCC-6083

The effects of ultraviolet-B radiation (280-320 nm) on photosystem II of Synechocystis sp. PCC 6803 were investigated at the functional and structural levels. Loss of oxygen-evolving and electron-transport acti vity, measured by various techniques including Clark electrode polarog raphy, fluorescence induction and fluorescence relaxation after a sing le turnover flash, are discussed in terms of two types of damage cause d by ultraviolet-B radiation: (a) depletion of the plastoquinone pool; (b) perturbation and degradation of the D1 protein, with cleavage in the second transmembrane segment. These findings are in full agreement with those obtained, both in vivo and in vitro for higher plants for which a donor-side mechanism involving the water-splitting Mn cluster has been proposed for the main cleavage of the D1 protein. At the stru ctural level, complete disruption of the photosystem II core is docume nted as a conse quence of (or in parallel with) degradation of the D1 protein. From this point of view, ultraviolet-B-induced photoinhibitio n is unlike the visible-induced type and less susceptible to repair by synthesis and reinsertion of new D1 protein.