PHOTOACTIVATION OF THE LATENT WATER-OXIDIZING COMPLEX IN PHOTOSYSTEM-II MEMBRANES ISOLATED FROM DARK-GROWN SPRUCE SEEDLINGS

Photosystem II membranes (D-PSII) were isolated from dark-grown spruce seedlings. All major PSII proteins except the 17- and 23-kDa extrinsi c proteins were present in D-PSII. O-2 evolution and Mn content in D-P SII were negligible, while PSII-donor activity showed a value comparab le to that of NH2OH-treated PSII membranes (NH2OH-L-PSII) from light-g rown seedlings. Light incubation of D-PSII with 1 mM MnCl2 50 mM CaCl2 and 100 mu M DCIP at pH 5.3 resulted in activation of the latent wate r-oxidizing complex. Accomplishment of photoactivation of PSII membran es from dark-grown spruce seedlings clearly indicates that only ligati on of Mn2+ to the ape-water oxidizing complex is required for expressi on of O-2 evolution, and that protein synthesis is not involved in the photoactivation process. There was no essential difference between 'p hotoactivation' of naturally Mn-free PSII membranes and 'photoreactiva tion' of artificially Mn-depleted PSII membranes on kinetics, pH depen dence, Mn2+-concentration dependence. However, kinetics and pH depende nce of photoactivation were appreciably different in spruce PSII membr anes and in PSII membranes of angiosperms such as wheat and spinach.