Does histidine 332 of the D1 polypeptide ligate the manganese cluster in photosystem II? An electron spin echo envelope modulation study

The tetranuclear manganese cluster in photosystem II is ligated by one or m ore histidine residues, as shown by an electron spin echo envelope modulati on (ESEEM) study conducted with [N-15]histidine-labeled photosystem II part icles isolated from the cyanobacterium Synechocystis sp, strain PCC 6803 [T ang, X.-S., Diner, B. A., Larsen, B. S., Gilchrist, M. L., Jr., Lorigan, G. A., and Britt, R. D. (1994) Proc. Natl. Acad. Sci. U.S.A. 91, 704-708]. On e of these residues may be His332 of the D1 polypeptide. Photosystem IT par ticles isolated from the Synechocystis mutant D1-H332E exhibit an altered S -2 State multiline EPR signal that has more hyperfine lines and narrower sp littings than the corresponding signal in wild-type PSII particles [Debus, R. J., Campbell, K. A., Peloquin, J. M., Pham, D. P., and Britt, R. D. (200 0) Biochemistry 39, 470-478]. These D1-H332E PSII particles are also unable to advance beyond an altered S2Yz. state, and the quantum yield for formin g the St state is very low, corresponding to an 8000-fold slowing of the ra te of Mn oxidation by Y-z(.). These observations are consistent with His332 being close to the Mn cluster and modulating the redox properties of both the Mn cluster and tyrosine YZ. To determine if D1-His332 ligates the Mn cl uster, we have conducted an ESEEM study of D1-H332E PSII particles. The his tidyl nitrogen modulation observed near 5 MHz in ESEEM spectra of the St st ate multiline EPR signal of wild-type PSII particles is substantially dimin ished in D1-H332E PSII particles. This result is consistent with ligation o f the Mn cluster by D1-His332. However, alternate explanations are possible . These are presented and discussed.