REGULATION OF PHOTOSYSTEM STOICHIOMETRY IN THE PHOTOSYNTHETIC SYSTEM OF THE CYANOPHYTE SYNECHOCYSTIS PCC-6714 IN RESPONSE TO LIGHT-INTENSITY

Light-induced changes in stoichiometry among three thylakoid components, PS I, PS II and Cyt b6-f complexes, were studied with the cyanophyte Synechocystis PCC 6714. Special attention was paid to two aspects of the stoichiometric change; first, a comparison of the patterns of regulation in response to differences in light-intensity with those induced by differences in light-quality, and second, the relationship between regulation of the stoichiometry and the steady state of the electron transport system. Results for the former indicated that (1) the abundance of PS I on a per cell basis was reduced under white light at the intensity as high as that for light-saturation of photosynthesis, but PS I per cell was increased under low light-intensity, (2) PS II and Cyt b6-f complexes remained fairly constant, and (3) changes in the abundance of PS I depended strictly on protein synthesis. The pattern was identical with that of chromatic regulation. For the second problem, the redox steady-states of Cyt f and P700 under white light of various intensities were determined by flash-spectroscopy. Results indicated that (1) Cyt f and P700 in cells grown under low light-intensity [high ratio of PS I to PS II (PS I/PS II)] were markedly oxidized when the cells were exposed to high light-intensity, while they remained in the reduced state under low light-intensity. (2) After a decrease in the abundance of PS I, most of P700 remained in the reduced state even under high light-intensity, while the level of reduced Cyt f remained low. (3) Both Cyt f and P700 in cells of low PS I/PS II were fully reduced under low light-intensity, and Cyt f reduction following the flash was rapid, which indicates that the turnover of PS I limits the overall rate of electron flow. After an increase in the abundance of PS I, the electron transport recovered from the biased state. (4) The redox steady-state of the Cyt b6-f complex correlated well with the regulation of PS I/PS II while the state of the PQ pool did not. Based on these results, a working model of the regulation of assembly of the PS I complex, in which the redox steady-state of the Cyt b6-f complex is closely related to the primary signal, is proposed.