REDOX DEPENDENCE FOR PHOTOLIGATION OF MANGANESE TO THE APO-WATER-OXIDIZING COMPLEX IN CHLOROPLASTS AND PHOTOSYSTEM-II MEMBRANES

Effects of reducing reagents and redox potentials on photoactivation w ere studied in Mn-depleted chloroplasts and PSII membranes, Exogenous reducing reagents abolished photoactivation in PSII membranes, while t hey stimulated photoactivation in chloroplasts. To determine how reduc ing reagents can have such opposing effects in these preparations, we studied how redox potentials affect photoactivation in the range from 0 mV to +500 mV. In chloroplasts, a modest yield of photoactivation wa s obtained in the redox potential range of +100 and +330 mV at pH 7.5. The yield of photoactivation decreased at redox potentials above +330 mV, and drastically increased below potentials of +100 mV. Nernst plo ts of the data show that an n = 1 redox component with an E-m7.5 Of +3 74 mV, as well as an n = 2 redox component with an E-m7.5 Of +61 mV, i s involved in photoactivation of chloroplasts isolated from dark-grown spruce seedlings. In the case of PSII membranes, photoactivation decr eased sharply on either side of +335 mV at pH 5.5, The n = 1 redox com ponents with E-m5.5 Of +375 and +319 mV may be involved, both of which showed pH dependences of -60 mV/pH unit. DCMU abolished photoactivati on in chloroplasts, but did not affect the dependence of photoactivati on on oxidation-reduction potentials in PSTI membranes. The component with an E-m5.5 of +319 mV involved in photoactivation of PSII membrane s was also observed in the dependence of Mn solubilization on oxidatio n-reduction potentials with PSII membranes lacking extrinsic proteins, suggesting that the reduction of Mn with higher valences to Mn(II) by exogenous reducing reagents reversibly occurs in the intermediates or an active center during photoactivation in PSII membranes. Involvemen t of such redox components in photoactivation in chloroplasts and PSII membranes is discussed.