CHROMATIC REGULATION IN CHLAMYDOMONAS-REINHARDTII - TIME-COURSE OF PHOTOSYSTEM STOICHIOMETRY ADJUSTMENT FOLLOWING A SHIFT IN GROWTH LIGHT QUALITY

Photosystem stoichiometry adjustments in Chlamydomonas reinhardtii wer e induced upon a sudden shift in the light quality during cell growth. Reversible changes in the PSI/PSII ratio were a compensation response to changes in the balance of light absorption by the two photosystems . Quantitations of PSII, Cyt b(6)-f complex and PSI revealed a constan cy in the cellular content of PSII and the Cyt b(6)-f complex, and var iable amounts of PSI in C. reinhardtii. These results strengthen the n otion that PSI is the thylakoid component subject to chromatic regulat ion and responsible for the adjustment and optimization of the PSI/PSI I ratio in the thylakoid of oxygenic photosynthesis. Additional result s, obtained upon the use of protein biosynthesis translation inhibitor s (chloramphenicol and cycloheximide), suggested that a chromatically- induce lowering of the PSI/PSII ratio in C. reinhardtii occurs by supp ression of de novo biosynthesis of PSI components and, therefore, by d ilution of the PSI complex in the thylakoid membrane, rather than by a ctive degradation of assembled PSI in chloroplasts.