LOCALIZATION IN PHOTOSYSTEM-II OF THE HISTIDINE RESIDUE PUTATIVELY RESPONSIBLE FOR THERMOLUMINESCENCE-AT-BAND AS PROBED BY TRYPSIN ACCESSIBILITY

Effects of trypsin digestion on the capability of the thermoluminescence A(T)-band that arises from charge recombination between Q(A)- and an oxidized histidine in PS II (Ono and Inoue (1991) FEBS Lett. 278, 183-186) were studied. The following results have been obtained. (i) Trypsin digestion markedly inhibited the capability of A(T)-band only when applied at higher pH levels, showing a clear threshold pH at 7.25. (ii) This pH dependence agreed with that of the inhibition of O2 evolution, but disagreed with the pH-independent modification of the acceptor side of PS II, the impairment of Q(A) to Q(B) electron transport. (iii) Trypsin digestion at pH 7.75 did not affect the capability of the EPR Signal II(f) arising from oxidized secondary electron donor of PS II, even though it totally abolished the capabilities of both A(T)-band and O2 evolution. (iv) The susceptibility to trypsin of the A-band capability retained a pronounced pH dependence even after depletion of Mn, although the dependence curve was shifted to lower pH levels. (v) Trypsin digestion inhibited the capability of Mn2+-photooxidation by Mn-depleted PS II, showing a pH dependence similar to those found for inhibitions of A(T)-band capability and O2 evolution. It was inferred that the histidine residue putatively responsible for the A(T)-band capability is localized in the domain of a PS II protein(s) that provides ligands for the Mn-cluster, and this domain becomes exposed through pH-dependent structural rearrangement of the O2-evolving enzyme to be attacked by trypsin.