Kinetic and spectroscopic properties of the Yz• radical in Ca2+- and Cl--depleted photosystem II preparations

Depletion of Ca2+ and/or Cl- ions from PSII membranes blocks the electron-transfer reactions that precede O-2 evolution on the oxidizing side of the enzyme. Illumination of these inhibited preparations at 273 K generates a paramagnetic species that is detectable by low-temperature (T < 20 K) EPR as a signal in the g = 2 region, 90-230 G wide, depending on the treatment that PSII has undergone. This signal has recently been assigned to Y-Z(.) in magnetic interaction with the manganese cluster in its S-2 State [Gilchrist et al. (1995) Proc. Natl. Acad. Sci. U.S.A. 92, 9545-9549]. This view, however, is not universal, owing, in part, to the fact that its spectroscopic properties depend on the preparation and the experimental conditions used for its study and, in part, to uncertainties as to the room temperature behavior of Yi in inhibited preparations. Here, we report time-resolved and conventional EPR data showing that, at room temperature and at 273 K, Y-Z(.) can be accumulated in its 20 G form in high yields in both Ca2+-depleted and acetate-inhibited preparations, and that the kinetics of its decay match the decay kinetics of the low-temperature signal generated in corresponding samples. The properties of the Y-Z(.) signal, however, are shown to depend on the polypeptide content, the temperature, and the electron donors and accepters present in the sample under examination. Our results support assignment of the EPR signal in inhibited preparations to S-2 Y-Z(.) and demonstrate a protective role of the 17 and 23 kDa extrinsic polypeptides for the manganese cluster against externally added reductants.