SPECTROSCOPIC CHARACTERIZATION OF TYROSINE-Z IN HISTIDINE-190 MUTANTSOF THE D1 PROTEIN IN PHOTOSYSTEM-II (PSII) IN CHLAMYDOMONAS-REINHARDTII - IMPLICATIONS FOR THE STRUCTURAL MODEL OF THE DONOR SIDE OF PSII

EPR spectra attributed to the redox active tyrosine residues on the ox idizing side of photosystem II (Tyr(Z) and Tyr(D)) have almost identic al line shapes, although the tyrosyl radicals differ in stability and redox characteristics. Strongly modified spectra of oxidized Tyr(D) in site-directed mutants in a histidine residue, H189 on the D2 reaction center protein in the cyanobacterium Synechocystis 6803, support a st ructural model where H189 interacts closely, probably via a hydrogen b ond, to Tyr(D) (Tommos, C., Davidsson, L., Svensson, B., Madsen, C., V ermaas, W., and Styring, S. (1993) Biochemistry 32, 5436-5441). To det ermine whether Tyr(Z) and the corresponding histidine on the D1 protei n (D1-H190) interacts similarly, we have generated His-Phe (H190F) and His-Tyr (H190Y) mutations in the C2 symmetry related H190 residue on the D1 reaction center protein by site-directed mutagenesis in Chlamyd omonas reinhardtii. The H190F and H190Y mutants assemble photosystem I I reaction centers capable of primary photochemistry but unable to oxi dize water. We have obtained kinetic spectra of a flash-induced transi ent EPR signal that we assign to oxidized Tyr(Z) in the D1-H190 mutant s. The spectra are identical in line width (18-20 G) and hyperfine str ucture to the wild-type spectrum from oxidized Tyr(Z) and exhibit deca y kinetics (t(1/2) approximate to 500 ms) typical for the Tyr(Z) radic al in managenese-depleted photosystem II membranes, However, both Tyr( Z) and Tyr(D) were oxidized with reduced (10-15%) quantum yield in the se mutants, indicating that the kinetics of electron donation to P-680 (+), were significantly modified as a result of the mutation. Thus, th e altered kinetics of Tyr(Z) in the mutants suggest that there is an i nteraction between TyrZ and His-190 on the D1 protein. However, unlike the situation on the D2 side, the presence of a hydrogen bond between Tyr(Z) and H190 on the D1 protein is improbable.