Interaction between photosynthetic and respiratory electron-transfer chains in the membranes of Anabaena variabilis.

The rate of CO2- and p-benzoquione-dependent photosynthetic O2 evolution by Anabaena variabilis cells remained unaltered and the rate of O2 uptake observed after switching off the light (endogenous respiration) was enhanced by a factor of 6-8 when the O2 concentration was increased from 200 to 400 μM. Photosystem-I-linked O2 uptake and respiration of the cells incubated with ascorbate and N,N,N'N'-tetramethyl-p-phenylenediamine was not appreciable influenced by the O2 concentration. 2-Iodo-6-isopropyl-3-methyl-2',4,4'-trinitrodiphenyl ether, blocking electron transfer at the plastoquinone level, suppressed O2 evolution and had no influence on endogenous respiration. 2-n-Heptyl-4-hydroxyquinoline-N-oxide, an inhibitor of electron transfer between photosystems II and I, as well as the cytochrome-oxidase inhibitors N 3 (-) , CN(-) and NH2OH, caused a 35-50% retardation of endogenous respiration and blocked photosynthetic O2 evolution. The molar ratio of cytochromes b6, f, c-553, aa3 and photosystem-I reaction centers in the isolated membranes equalled approx. 2:1:2:0.7:2. It is inferred that endogenous respiration of A. variabilis cells is inhibited by the light-induced electron flow through both photosystems at the level of the plastoquinone-plastocyanin-oxidoreductase complex.