MUTATION OF A PUTATIVE LIGAND TO THE NONHEME IRON IN PHOTOSYSTEM-II -IMPLICATIONS FOR Q(A) REACTIVITY, ELECTRON-TRANSFER, AND HERBICIDE BINDING

In Photosystem II (PS II), a non-heme iron is present near the electro n-accepting quinones Q(A) and Q(B). A putative ligand to this non-heme iron, His-268 in the D2 protein, was mutated to Gin in the cyanobacte rium Synechocystis sp. PCC 6803. The resulting mutant H268Q has lost p hotoautotrophic capacity and shows large alterations in the properties both of Q(A) and of the Q(B) pocket. In the mutant, the quantum effic iency of Q(A) reduction is decreased by approximately 50-fold, electro n transfer from Q(A) to the plastoquinone pool is blocked, Q(A) appare ntly can be displaced by exogenous quinones, and the stability of redu ced QA is increased by more than an order of magnitude. Also the affin ity of the PS II-directed herbicide diuron to the PS II complex is dec reased to undetectable levels. We suggest that these mutation-induced changes in the properties of the acceptor side of PS II are caused by a functional loss of the non-heme iron. This would imply that the non- heme iron in PS II plays a functionally more important role than obser ved in reaction centers from purple bacteria, and has drastic effects on the properties of Q(A). Moreover, the results obtained on the D2 mu tant H268Q illustrate that the D2 protein can have a pronounced influe nce on the properties of the Q(B)/herbicide-binding environment, which is associated mostly with the D1 protein. Thus, the non-heme iron in PS II appears to both affect Q(A) reactivity and alter the properties of the Q(B) pocket.