Reciprocal formation of plastocyanin and cytochrome c-553 and the influence of cupric ions on photosynthetic electron transport.

The green alga Scenedesmus acutus is able to synthesize plastocyanin and cytochrome c-553. The concentrations of plastocyanin and cytochrome c-553 vary inversely in response to the cupric-ion concentrations of the growth medium (Bohner, H. and Böger, P. (1978) FEBS Lett. 85, 337-339). Both proteins form a homogeneous donor pool to the reaction center of Photosystem I. This donor pool can be varied quantitatively and qualitatively by different growth conditions. These variations have no influence on algal growth or photosynthetic electron transport as measured in vivo by oxygen evolution, fluorescence induction and cytochrome f-553 and c-553 redox reactions using Cu2+ concentrations of less than 10 microM in the culture medium. At higher cupric-ion concentrations, which already retard algal growth, specific sites of the photosynthetic electron-transport chain are affected: the oxidizing side of Photosystem II and the reducing side of Photosystem I.