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-oxida se) 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. T he activity of glucose-6-phosphate (G-6-P):NADP(+) oxidoreductase (EC 1.1.1.49) was also determined supplementarily. Cyt-oxidase activity wa s enhanced by glucose added to the medium even when cell growth mainta ined mainly by oxygenic photosynthesis. G-6-P:NADP(+) oxidoreductase w as also activated by glucose. The enhanced level of Cyt-oxidase was hi gher under PSII light, which causes high PSI abundance, than that unde r PSI light, which causes low PSI abundance. The level was intermediat e under hetetrotrophic conditions. Although the activity level was low in cells grown under autotrophic conditions, the level was again lowe r 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 t he two terminal components of thylakoid electron transport system, Cyt -oxidase and PSI, changes in parallel with each other in response to t he state of thylakoid electron transport system.