Plant Physiol, October 2001, Vol. 127, pp. 633-644

Photosystem II Peripheral Accessory Chlorophyll Mutants in Chlamydomonas reinhardtii. Biochemical Characterization and Sensitivity to Photo-Inhibition^1,2

School of Biological Sciences, University of Exeter, Exeter EX4 4PS, United Kingdom (S.V.R.); Departments of Plant Biology (J.W., R.T.S.) and Chemistry (J.W., H.G.J., T.L.G.), Ohio State University, Columbus, Ohio 43210; Department of Biology, North Dakota State University, Fargo, North Dakota 58105 (R.S.H.); The Institute of Low Temperature Science, Hokkaido University, N19 W9 Sapporo 060-0819, Japan (J.M.); and Departments of Biophysics and Computational Biology and Microbiology, University of Illinois, Urbana, Illinois 61801 (A.C.)

In addition to the four chlorophylls (Chls) involved in primary charge separation, the photosystem II (PSII) reaction center polypeptides, D1 and D2, coordinate a pair of symmetry-related, peripheral accessory Chls. These Chls are axially coordinated by the D1-H118 and D2-H117 residues and are in close association with the proximal Chl antennae proteins, CP43 and CP47. To gain insight into the function(s) of each of the peripheral Chls, we generated site-specific mutations of the amino acid residues that coordinate these Chls and characterized their energy and electron transfer properties. Our results demonstrate that D1-H118 and D2-H117 mutants differ with respect to: (a) their relative numbers of functional PSII complexes, (b) their relative ability to stabilize charge-separated states, (c) light-harvesting efficiency, and (d) their sensitivity to photo-inhibition. The D2-H117N and D2-H117Q mutants had reduced levels of functional PSII complexes and oxygen evolution capacity as well as reduced light-harvesting efficiencies relative to wild-type cells. In contrast, the D1-H118Q mutant was capable of near wild-type rates of oxygen evolution at saturating light intensities. The D1-H118Q mutant also was substantially more resistant to photo-inhibition than wild type. This reduced sensitivity to photo-inhibition is presumably associated with a reduced light-harvesting efficiency in this mutant. Finally, it is noted that the PSII peripheral accessory Chls have similarities to a to a pair of Chls also present in the PSI reaction center complex.