ELECTRONS GENERATED BY PHOTOSYSTEM-II ARE UTILIZED BY AN OXIDASE IN THE ABSENCE OF PHOTOSYSTEM-I IN THE CYANOBACTERIUM SYNECHOCYSTIS SP PCC-6803

The reduction and reoxidation kinetics of the first quinone-type electron acceptor in photosystem II, Q(A)(-), were measured by fluorescence in a light-tolerant, photosystem I-less strain of the cyanobacterium Synechocystis sp. PCC 6803. In this strain, which shows excellent amplitudes of variable fluorescence, the rate of Q(A)(-) oxidation after photoreduction of the plastoquinone pool was about half of that in the presence of photosystem I. However, upon addition of 5 mM KCN, Q(A)(-) decay was very slow, and the rate was comparable to that seen in the presence of diuron, which blocks electron transport between Q(A) and Q(B). The KCN-imposed block of Q(A)(-) oxidation was removed efficiently by addition of exogenous quinones that can oxidize the plastoquinone pool. These results indicate that, in the absence of photosystem I, photosystem II-generated electrons are used very effectively by an oxidase located in the thylakoid; this oxidase may be a component of the respiratory chain.