Interactions between thylakoid electron transfer complexes. I. In vitro kinetic studies with isolated photosystem II...

The cytochrome b6-f complex from spinach thylakoids has been reconstituted with an oxygen-evolving Photosystem II (PSII) preparation isolated from the same source to give oxygenic plastocyanin reductase activity. We observe that (i) mixing of the two complexes in concentrated form prior to dilution with the assay medium is necessary for optimal reconstitution of activity; (ii) incubation for longer times after dilution can also give substantial reconstitution if the two complexes are added separately to the assay mixture; (iii) either monovalent or divalent cations are required for optimum activity in the reconstituted system; (iv) titration of the cytochrome complex with varying amounts of the PSII complex gave a saturation of the plastocyanin reduction activity at a cytochrome complex/PSII ratio of 3-4; (v) kinetic analysis of plastocyanin photoreduction by Photosystem II shows nonlinearity, while first-order reduction kinetics are observed with duroquinol as electron donor; and (vi) as the concentration of plastocyanin is increased, the half-time of the reduction increases. These observations are considered in terms of a functional association between PSII and the cytochrome b6-f complex in this reconstituted system, and the relevance of these observations to the situation in vivo is discussed.