FUNCTIONAL AND STRUCTURAL-CHANGES OF THE PHOTOSYSTEM-II COMPLEX-INDUCED BY HIGH IRRADIANCE IN CYANOBACTERIAL CELLS

A gradual disintegration of the photosystem II (PSII) complex, initiat ed by a release of the chlorophyll-protein CP43, was identified during low-temperature illumination of Synechococcus cells. This process was slower compared to the decline of the PSII primary charge separation activity, and much slower than the photoinactivation of oxygen evoluti on. All three processes were slowed down in the presence of diuron. Th e results indicate that when the PSII repair was blocked, the inactiva tion of charge separation activity and the release of CP43 preceded th e degradation of the D1 protein. In contrast, a much faster degradatio n of D1 connected to its rapid exchange was triggered by inactivation of oxygen evolution, and no disassembly of PSII was needed. We propose the existence of two different mechanisms of D1 degradation in the ce lls of the thermophilic cyanobacterium Synechococcus elongatus.