Chromatic Regulation in Chlamydomonas reinhardtii: Time Course of Photosystem Stoichiometry Adjustment Following a Shift in Growth Light Quality

Photosystem stoichiometry adjustments in Chlamydomonas reinhardtii were 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 b6-f complex and PSI revealed a constancy in the cellular content of PSII and the Cyt b6-f complex, and variable amounts of PSI in C. reinhardtii. These results strengthen the notion that PSI is the thyla-koid component subject to chromatic regulation and responsible for the adjustment and optimization of the PSI/ PSII ratio in the thylakoid of oxygenic photosynthesis. Additional results, obtained upon the use of protein biosynthesis translation inhibitors (chloramphenicol and cyclohex-imide), suggested that a chromatically-induced lowering of the PSI/PSII ratio in C. reinhardtii occurs by suppression of de novo biosynthesis of PSI components and, therefore, by dilution of the PSI complex in the thylakoid membrane, rather than by active degradation of assembled PSI in chlo-roplasts.