CHANGES IN THE CYTOCHROME-C-OXIDASE ACTIVITY IN RESPONSE TO LIGHT REGIME FOR PHOTOSYNTHESIS OBSERVED WITH THE CYANOPHYTE SYNECHOCYSTIS PCC-6714

Changes in the activity of cytochrome c oxidase (EC 1.9.3.1, Cyt-oxidase) in response to growth conditions were studied with the cyanophyte Synechocystis PCC 6714 in relation to changes in PSI abundance induced by light regime for photosynthesis. The activity was determined with the V-max of mammalian cytochrome c oxidation by isolated membranes. The activity of glucose-6-phosphate (G-6-P):NADP(+) oxidoreductase (EC 1.1.1.49) was also determined supplementarily. Cyt-oxidase activity was enhanced by glucose added to the medium even when cell growth maintained mainly by oxygenic photosynthesis. G-6-P:NADP(+) oxidoreductase was also activated by glucose. The enhanced level of Cyt-oxidase was higher under PSII light, which causes high PSI abundance, than that under PSI light, which causes low PSI abundance. The level was intermediate under hetetrotrophic conditions. Although the activity level was low in cells grown under autotrophic conditions, the level was again lower in cells grown under PSI light than under PSII light. The change of Cyt-oxidase activity in response to light regime occurred in the same direction as that for the variation of PSI abundance. Results suggest that in Synechocystis PCC 6714, the capacity of electron turnover at the two terminal components of thylakoid electron transport system, Cyt-oxidase and PSI, changes in parallel with each other in response to the state of thylakoid electron transport system.