Photosystem II peripheral accessory chlorophyll mutants in Chlamydomonas reinhardtii. Biochemical characterization and sensitivity to photo-inhibition

In addition to the four chlorophylls (Chls) involved in primary charge sepa ration, the photosystem II (PSII) reaction center polypeptides, D1 and D2, coordinate a pair of symmetry-related, peripheral accessory Chls. These Chl s are axially coordinated by the D1-H118 and D2-H117 residues and are in cl ose association with the proximal Chl antennae proteins, CP43 and CP47. To gain insight into the function(s) of each of the peripheral Chls, we genera ted site-specific mutations of the amino acid residues that coordinate thes e Chls and characterized their energy and electron transfer properties. Our results demonstrate that D1-H118 and D2-H117 mutants differ with respect t o: (a) their relative numbers of functional PSII complexes, (b) their relat ive ability to stabilize charge-separated states, (c) light-harvesting effi ciency, and (d) their sensitivity to photo-inhibition. The D2-H117N and D2- H117Q mutants had reduced levels of functional PSII complexes and oxygen ev olution capacity as well as reduced light-harvesting efficiencies relative to wild-type cells. In contrast, the D1-H118Q mutant was capable of near wi ld-type rates of oxygen evolution at saturating light intensities. The D1-H 118Q mutant also was substantially more resistant to photo-inhibition than wild type. This reduced sensitivity to photo-inhibition is presumably assoc iated with a reduced light-harvesting efficiency in this mutant. Finally, i t is noted that the PSII peripheral accessory Chls have similarities to a t o a pair of Chls also present in the PSI reaction center complex.