Some Evidence on the Site of Action of Plastocyanin in the Photosynthetic Electro-Transport Chain between Cytochromes <em>f</em> and P700.

The effect of plastocyanin on the light-induced steady-state changes of cytochrome f in chloroplast particles that were depleted of plastocyanin has been studied. It was observed that the extent of cytochrome f photooxidation in sonic particles in the steady-state during a short exposure to long-wave red light was approximately 30% of that in chloroplasts. The addition of a catalytic amount of plastocyanin to the sonic particles stimulated cytochrome f photooxidation to 80% of its steady-state value in untreated chloroplasts. The reduction of cytochrome f by short-wave red light was not inhibited by sonic treatment of chloroplasts. The rate of cytochrome f photooxidation in the presence of a catalytic amour of detergenttreated chloroplasts was proportional to the amount of plastocyanin added. The enhancement of cytochrome f photooxidation either in sonic panicles or in digitonin, treated particles after an addition of plastocyanin was decreased in the presence of the copperchelating agent sodium diethyldithiocarbamate. Plastocyanin preincubated with sodium diethyldithiocarbamate showed less ability to stimulate cytochrome c photooxidation by detergent-treated chloroplasts. [ABSTRACT FROM AUTHOR]