MASS-SPECTROMETRIC STUDY OF PHOTOSYSTEM-II HETEROGENEITY IN OXYGEN AND NITROGEN-PRODUCTION - EFFECTS OF MAGNESIUM AND OF PHOSPHORYLATION OFPEA THYLAKOIDS

Photosystem II (PS II) is capable of the oxidation of both water and h ydroxylamine with the production of O-2-and N-2-production, respective ly. The resulting changes in the partial pressure of the respective ga ses can be measured by an appropriate mass spectrometric set-up. Analy sis of single turn-over flash saturation curves of O-2- and N-2-produc tion has been performed to determine the relative optical cross sectio ns of the competent PS II units and absolute amounts of their fraction s in pea thylakoids. We studied the changes of these parameters upon M g2+-induced transition of thylakoid membrane from unstacked to stacked configuration and upon protein phosphorylation of the stacked samples . The results showed a 2.5-fold increase of effective antenna size of PS II units competent in either O-2- or N-2-production after addition of 10 mM MgCl2 to cation-depleted thylakoids, which indicates a potent ial capability of both alpha- and beta-units to carry out these altern ative reactions. However, we observed a significant difference in the amounts of PS II units competent in O-2- or N-2-production, with a rat io of 1:4 in unstacked thylakoids, and reciprocal alterations in stack ed ones. This represents an increase by about 20% and a 2-fold decreas e of O-2- and N-2-evolving units, respectively, yielding a ratio of 1: 1.5, which implies a heterogeneity of PS II with respect to these reac tions, the capabilities of alpha- and beta-units being distinct. The p hosphorylation of stacked thylakoids did not essentially influence the antenna size of O-2- and N-2-evolving PS II units but caused opposite and reciprocal changes in their amounts, approximately 30% decrease a nd increase, respectively, to a ratio of 1:3. The relationship of the structure-function heterogeneity in PS II with implications for curren t models of photosynthetic regulation mechanisms is discussed.