Functional sits of plastoquinone in photosynthetic electron transport system

Mild extraction of lyophilized chloroplasts with hexane eliminatedHill activity with 2,6-dichlorophenolindophenol (DCIP) as anelectron acceptor, and most of the activity was restored byreconstitution with plastoquinone A. The same extraction didnot affect the activity of Photosystem II, determined by thephotoreduction of DCIP supported with an artificial electrondoneor, 1,5-diphenylcarbazied. The fluorescence yield changesof extracted chloroplasts indicated that the electron transportchain between Photosystems I and II was also blocked. The resultssuggest that plastoquinone functions at both sides of PhotosystemII; at the reductive side it acts as an electron carrier, andat the oxidative side as a structural element of the thylakoidmembrance necessary for a component to be active in the oxygen-evolutionsystem. (Received August 22, 1973; )