Indirect evidence for structural changes coupled with QB-. formation in photosystem II.

The thermal blockage of QA-. oxidation was analysed in PS II membrane fragments by monitoring flash-induced changes of the relative fluorescence quantum yield as a function of temperature. The results obtained reveal: (a) in dark-adapted samples the fraction of QA-. that is not reoxidised within a time domain of 10 s after the actinic flash increases with lowering the temperature (half-maximum effect at 250-260 K), (b) at low temperatures where QA-. generated in dark-adapted samples remains almost completely reduced, a significant extent of QA-. reoxidation arises when samples are used that were preilluminated at room temperature by one saturating flash followed by rapid freezing before performing the experiment, and (c) the extent of QA-. that is reoxidised at 258 K exhibits a characteristic binary oscillation as a function of the number of preillumination flashes given at room temperature. Based on these data it is inferred that QB and QB-. are located at different equilibrium positions in the QB site. As a consequence the formation of QB-. is coupled with significant structural changes that require sufficient flexibility of the protein matrix. This general feature corresponds with a recently proposed model for the acceptor side reactions of anoxygenic bacteria [Stowell, M.H.B., McPhillips, T.M., Rees, D.C., Soltis, S.M., Abresch, E. and Feher, G., Science 276 (1997) 812-816].