Interactions between thylakoid electron transfer complexes. I. In vitro kinetic studies with isolated photosystem II and cytochrome b6-f complexes

Electron transfer from cytochrome f to photosystem I in green algae

The time course of P700(+) reduction and cytochrome f oxidation following a single-turnover flash excitation of photosystem I was measured under various conditions in different strains of green algae. P700(+) was reduced with a half-time of 4 μs. The rate of cytochrome f oxidation was found to depend widely on physiological factors. Reversible transitions are described from a 'slow-oxidation' state (t 1/2=500 μs) to a 'fast-oxidation' state (t 1/2=80 μs). The addition of ionophore strongly favours and stabilizes the 'fast-oxidation' state. We suggest that these transitions reflect either reversible association between the cytochrome bf complex and the reaction center of photosystem I or changes in the mobility of oxidized plastocyanin. The transitions might be under the control of the membrane potential or the intracellular ATP content. The relation of these reversible transitions with the 'light state' transitions, and their possible involvement in a switch from linear to cyclic electron transfer, are discussed.

Quinone interactions with the chloroplast cytochrome b6-f complex

The requirements for reconstitution of electron transfer activity with a plastoquinone (PQ)-depleted cytochrome b6-f complex from spinach have been considered. Full restoration of activity measured as plastocyanin reduction with either duroquinol in the dark or Photosystem II (PSII) in the light requires both PQ-9 and phospholipid. However, a substantial dark activity can be observed with duroquinol and phospholipid in the absence of any added PQ-9. PSII, with its associated PQ molecules, can also donate electrons in the light to the cytochrome complex which has been depleted of plastoquinone. Electron donation by duroquinol in the dark to the PQ-depleted cytochrome complex is stimulated by PSII, and this stimulation is dependent on the presence of the two PQ molecules in the PSII preparation. Measurements of proton translocation with the PQ-depleted complex indicate this quinone is not required for the observed H+/e- ratio of 2. Studies of cytochrome b6 kinetics with the free and liposome-incorporated PQ-depleted complex show this cytochrome undergoes redox reactions similar to those of a control complex which contains PQ. These results indicate the PQ that copurifies with the cytochrome complex is not essential for any of the measured activities. These findings are considered in relation to a quinone binding site(s) in the cytochrome complex which is not specific to PQ but can bind other quinones, such as duroquinol, in a lipid-dependent process.

Interactions between thylakoid electron transfer complexes. II. Modification studies with glutaraldehyde

Photosystem I (PSI) and photosystem II (PSII) complexes have been isolated from stacked spinach thylakoid membranes that had been treated with varying amounts of glutaraldehyde. The concentrations of cytochrome f, Q, and P700 have been determined by spectrophotometric methods. It was found that at low concentrations of glutaraldehyde, the amount of cytochrome f associated with either PSII or PSI increased significantly while the amounts of Q and P700 stayed relatively constant. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis and immunoblotting analyses indicated the presence of cytochrome f and other components of the cytochrome b6-f complex in the PSII and PSI preparations after glutaraldehyde treatment, but no intermolecular cross-linked polypeptides could be detected. Solubilization of the cytochrome b6-f complex was also inhibited after thylakoid membranes were treated with low concentrations of glutaraldehyde. These results are discussed in relation to current models for the organization of the membrane complexes, and relate to the location of the cytochrome b6-f complex in appressed and nonappressed membrane regions of thylakoids.