INDIRECT EVIDENCE FOR STRUCTURAL-CHANGES COUPLED WITH Q(B)(-CENTER-DOT) FORMATION IN PHOTOSYSTEM-II

The thermal blockage of Q(A)(-.) oxidation was analysed in PS II membr ane fragments by monitoring flash-induced changes of the relative fluo rescence quantum yield as a function of temperature. The results obtai ned reveal: (a) in dark-adapted samples the fraction of Q(A)(-.) that is not reoxidised within a time domain of 10 s after the actinic flash increases with lowering the temperature (half-maximum effect at 250-2 60 K), (b) at low temperatures where Q(A)(-.) generated in dark-adapte d samples remains almost completely reduced, a significant extent of Q (A)(-.) reoxidation arises mhen samples are used that mere preillumina ted at room temperature by one saturating flash followed by rapid free zing before performing the experiment, and (c) the extent of Q(A)(-.) that is reoxidised at 258 K exhibits a characteristic binary oscillati on as a function of the number of preillumination flashes given at roo m temperature, Based on these data it is inferred that QB and Q(B)(-.) are located at different equilibrium positions in the QB site. As a c onsequence the formation of Q(B)(-.) is coupled with significant struc tural changes that require sufficient flexibility of the protein matri x, 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]. (C) 1998 Federation of European Biochemi cal Societies.