PH-DEPENDENT EXTRACTION OF CA2-II MEMBRANES AND THYLAKOID MEMBRANES -INDICATION OF A CA2+-SENSITIVE SITE ON THE ACCEPTOR SIDE OF PHOTOSYSTEM-II( FROM PHOTOSYSTEM)

The action of low pH treatment (pH 3.6) known to release Ca2+ from the oxygen-evolving complex in photosystem II (PSII) membranes and to ind uce Ca2+-reversible inhibition of electron transport at the acceptor s ide of PSII in thylakoid membranes (TM) was compared in PSII membranes and TM. The rate of the inactivation of electron transport by low pH was four times higher in TM than in PSII membranes. Ferricyanide accel erated the inactivation of PSII membranes but decreased it in the case of TM, Low pH treatment also greatly modified the fluorescence induct ion kinetics in both preparations, but significant differences have be en found in the fluorescence induction kinetics of treated TM and PSII membranes. Calcium restored the electron transport activity and fluor escence induction kinetics in PSII membranes and TM, whereas diphenylc arbazide restored these functions only in PSII membranes. The reactiva tion of Ca-depleted PSII membranes was more effective in the dark, whe reas the reactivation of TM required weak light. In the case of PSII m embranes subjected to low pH citrate buffer, maximal reactivation was observed at 60 mM Ca2+ but for TM about 10 mM Ca2+ was required and 60 mM fully inhibited electron transport in TM during reactivation. Thes e results indicate that the Ca-dependent inactivation of the acceptor side of PSII in TM after low pH treatment cannot be explained by the e xtraction of Ca2+ from the oxygen-evolving complex. It is rather sugge sted that the Ca2+ involved in this inhibition is bound to the accepto r side of the photosystem near to the Q(A)-non-heme iron binding site and may participate in the binding of a polypeptide of the PSII light antenna complex to the PSII reaction center.