Protection of photosynthesis against ultraviolet-B radiation by carotenoids in transformants of the cyanobacterium Synechococcus PCC7942

The cyanobacterium Synechococcus PCC7942 was transformed with various carot enogenic genes, and the resulting transformants either accumulated higher a mounts of beta-carotene and zeaxanthin or showed a shift in the carotenoid pattern toward the formation of zeaxanthin. These transformants were expose d to ultraviolet-B (UV-B) radiation, and the degradation of phycobilins, th e inactivation of photosynthetic oxygen evolution, and the activity of phot osystem II were determined. In the genetically modified cells, the influenc e on destruction of phycobilins was negligible. However, protection of phot osynthetic reactions against UV-B damage was observed and was dependent on the carotenoid concentrations in the different transformants. Furthermore, it was shown that endogenous zeaxanthin is more effective than beta-caroten e. Our results suggest that carotenoids exert their protective function as antioxidants to inactivate UV-B-induced radicals in the photosynthetic memb rane.