Effect of exogenous histidine on restoration of electron transfer on the donor side of photosystem II depleted of Mn.

It is shown that restoration of photoinduced electron flow with added Mn2+ (measured by photoreduction of DCPIP and photoinduced change of chlorophyll fluorescence yield) in Mn-depleted Photosystem II (PS II) membrane fragments isolated from spinach chloroplasts, is considerably increased by exogenous histidine (His). The stimulating effect of His is not observed if other electron donors (NH2OH or diphenylcarbazide) are used instead of Mn2+. His added alone does not induce electron transfer in Mn-depleted PS II preparations. Investigation of pH dependence of the stimulating effect of 2 mM His shows that the effect is observed only at pH > 5.0, it gives a 50% activation around pH 6.0 and saturates at pH 7.0-7.5. Nearly 200 microM His is required for a 50 effect at pH 7.0. It is suggested that the added His can be involved in stimulation of electron transfer on the donor side of PS II through direct interaction of Mn2+ with deprotonated form(s) of His resulting in formation of Mn-His complexes capable of efficient electron donation to PS II (though it is not excluded that His serves as a base that takes part in proton exchange coupled with redox reactions on the donor side of PS II or as an electron donor to the oxidized Mn).