The redox potential of Q(A) in Photosystem II (PSII) from Thermosynechococcus elongatus was titrated monitoring chlorophyll fluorescence. A high potential form (E-m = +60 +/- 25 mV) was found in the absence of Mn4Ca, the active site for water oxidation. The low potential form (E-m = -60 +/- 48 mV), which is difficult to measure in conventional titration experiments, could be "locked in" by cross-linking the active enzyme. This indicates that the presence of Mn4Ca is relayed to the quinone site by significant structural changes in the protein. The presence of high and low potential forms agrees with what has been seen in plants, algae from our lab and in T. elongatus (Shibamoto et al., Biochemistry 48 (2009) 10682-10684). In the latter work, the potentials of Q(A) were shifted to lower potentials compared to other measurements. The redox potential of Q(A) in Mn-depleted PSII from spinach was titrated in the presence of redox mediators and the midpoint potential was shifted by 80 mV towards a more negative value compared to titrations without mediators. The lower values of the midpoint potential of the (Q(A)/Q(A)(-)) redox couple in the literature could be due to a perturbation due to a specific mediator. (C) 2011 Elsevier B.V. All rights reserved.