Physiological significance of the regulation of photosystem stoichiometry upon high light acclimation of Synechocystis sp PCC 6803

We characterized the photosynthetic properties of the pmgA mutant of Synech ocystis PCC 6803, which cannot change its photosystem stoichiometry under a high-light condition (200 mu mol m(-2) s(-1)), in order to clarify the phy siological significance of the regulation of photosystem stoichiometry. We found that (1) PSII activity was inhibited more in wild-type cells on the f irst day under the high-light conditions than in mutant cells. (2) The grow th of the mutants following the initial imposition of high light was faster than that of wild-type cells. (3) However, growth was severely inhibited i n the mutants after the third day of exposure to high light, (4) The growth inhibition in the mutants under the extended high-light conditions was rev ersed by the addition of sublethal concentrations of 3-(3,4-dichlorophenyl) -1,1-dimethylurea (DCMU), which seemed to mimic photoinhibition of PSII, Th ese results suggest that the main role of adjusting the photosystem stoichi ometry with respect to light intensity is not to maintain efficient photosy nthesis, but to down regulate electron transfer. Failure to down regulate e lectron flow leads to cell death under prolonged exposure to high light in this cyanobacterium.