Low-Temperature Electron Transfer in Photosystem II:  A Tyrosyl Radical and Semiquinone Charge Pair

The states induced by illumination at 7 K in the oxygen-evolving enzyme (PSII) from Thermosynechococcus elongatus were studied by EPR. In the S(0) and S(1) redox states, two g approximately 2 EPR signals, a split signal and a g = 2.03 signal, respectively, were generated by illumination with visible light. These signals were comparable to those already reported in plant PSII in terms of their g value, shape, and stability at low temperatures. We report that the formation and decay of these signals correlate with EPR signals from the semiquinone of the first quinone electron acceptor, Q(A)(-). The light-induced EPR signals from oxidized side-path electron donors (Cyt b(559), Car, and Chl(Z)) were also measured, and from these and the signals from Q(A)(-), estimates were made of the proportion of centers involved in the formation of the g approximately 2 signals (approximately 50% in S(0) and 40% in S(1)). Comparisons with the signals generated in plant PSII indicated approximately similar yields for the S(0) split signal. A single laser flash at 7 K induced more than 75% of the maximum split and g = 2.03 EPR signal observed by continuous illumination, with no detectable oxidation of side-path donors. The matching electron acceptor side reactions, the high quantum yield, and the relatively large proportion of centers involved support earlier suggestions that the state being monitored is Tyr(Z)(*)Q(A)(-), with the g approximately 2 EPR signals arising from Tyr(Z)(*) interacting magnetically with the Mn complex. The current picture of the photochemical reactions occurring in PSII at low temperatures is reassessed.