MGCL2-INDUCED REVERSAL OF OXYGEN EVOLUTION DECAY IN PHOTOSYSTEM-II PARTICLES INCUBATED WITH PHOSPHATIDYLGLYCEROL VESICLES AT HIGH LIPID PHOTOSYSTEM-II RATIO

The oxygen-evolving activity of photosystem II (PSII) particles from barley is affected upon incubation with phosphatidylglycerol (PG) vesicles. There is a stimulation of oxygen evolution at a low PG/chlorophyll (Chl) molar ratio, whereas at higher values of this ratio a decay is observed. In addition, the highest oxygen-evolving activity in PSII-PG vesicles is attained immediately after reconstitution, then the activity declines with time. Moreover, the largest oxygen-evolving activity values in the PSII-PG vesicles are observed at MgCl2 concentrations in the incubation medium which are above the aggregation threshold of the reconstituted membranes, i.e. 5-6 mM. The data are explained as the result of neutralization of negative surface charges in PSII-PG vesicles by high salt concentration, e.g. 10 mM Mg2+, which brings about minimization of electrostatic interactions in the PSII-PG vesicle complex. Thus, diffusion of PSII units in the PG vesicle matrix is not restricted by the negative charge in the PO- group of PG. It comes out then that a reasonable mechanism underlying the MgCl2-mediated effects is cooperation between PSII units with concomitant enhancement of oxygen evolution owing to increased probability of collision between PSII units leading to transient, or more or less permanent contacts. This hypothesis provides a novel basis to account for the function of the water-splitting complex in the narrow space (almost-equal-to 60-80 angstrom wide) of the thylakoid lumen.