Evidence for plastoquinol-cytochrome f/b-563 reductase as a common electron donor to P700 and cytochrome oxidase in cyanobacteria

Membranes isolated from $\text{Nostoc}$ sp. strain MAC and $\text{Anacystis}\text{nidulans}$ displayed spectral changes in the cytochrome $\text{f}\text{b}$ region when examined by reduced $\text{minus}$ oxidized or dual wavelength spectrophotometry under physiological conditions. The same changes accompanied both light-induced (photosynthetic) and oxygen-induced (respiratory) electron transport. Physiological reduction of the cytochrome $\text{f}\text{b}$ moiety was abolished after extraction of plastoquinone but reappeared on reconstitution of the depleted membranes with authentic plastoquinone. Moreover, a mutual inhibition of photosynthetic and respiratory activities could be directly demonstrated with the isolated membranes. From the results it is concluded that the membrane-bound plastoquinol-cytochrome $\text{f}\text{b}$ reductase functions as a common electron donor to both P700 and the cytochrome oxidase in cyanobacteria.