Photosystern II (PSII) is a photosynthetic reaction center that oxidizes water and reduces bound plastoquinone. PSII electron transfer is mediated by two redox-active tyrosine residues. One of these residues, tyrosine D (YD), has been assigned as Tyr160 of the D2 polypeptide by site-directed mutagenesis and isotopic labeling. Previous spectroscopic evidence has established that His189 in the D2 subunit forms a hydrogen bond with YD center dot and donates a proton to YD center dot when the radical is reduced. However, the mechanism of this reaction has not been elucidated. In this report, EPR spectroscopy and (H2O)-H-2 solvent exchange were used to investigate the pL dependence of the YD center dot reduction rate. The kinetic isotope effect (KIE), induced by solvent exchange, was also measured as a function of pL. Under the conditions employed, the reduction of YD center dot is attributed to recombination with the QA(-) plastoquinone acceptor of PSII. The kinetic data were fit with a biexponential function. The majority, slow phase exhibited a pL-dependent rate constant, with a minimum at pL 7.5. Solvent exchange gave significant KIE at values between pL 5.5 and 8.0. In particular, at high pL (>= 7.5), the values of the KIE were determined to be 2.1 +/- 0.6 and 2.4 +/- 0.5. These values are consistent with a coupled electron and proton reaction, which occurs with a single kinetic step at pL values >= 7.5. The lower KIE values and the rate acceleration observed at low pL may be consistent with a change of mechanism in which the protonation of YD center dot occurs first, followed by rate-limiting electron transfer. The more modest acceleration in rate at high pL values is attributed to a small, pL-induced change in the distance between YD center dot and Q(A)(-)