Electrons generated by photosystem II are utilized by an oxidase in the absence of photosystem I in the cyanobacterium Synechocystis sp. PCC 6803

Abstract

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.