Search

Intact cells of herbicide-resistant mutants of Chlamydomonas reinhanhit, allcrcd in single amino :tcids in the DI protein, w'crc analyzed for chlorophyll a (Chl a) fluorescence decay in terms of electron transfer at the O~xOl~ complex of Photosystcm II. The DI mutants Ar-207 (F255Y), Br-202 (L275F) and Dr-18 (V2191) had an ahnost unimpaired fl~rv, ard electron transfer (QAQ~ ~ -'-' QAQI~ ~ ~) and an almost unchanged ratio (0.1-0.2) of the slow to thc f~st PSII centers, compared to that of the wild type. Thus, the Ou binding pocket was minimally modified. However, the mutants Ar-204 (G256D) and DCMU-4 ($264A) had an altered forward electron transfer, an altered apparent cquilibrum fi)r QAQ~ ~ QaOn reaction, and an abnormally high rati¢, (0.6-0.8) of the slow to the fast PSII centers. These results suggest that thcsc, o ntutations modified the Ql~ binding pocket significantly. Addition of 100 or 200 mM formate led to an increase in the lifetime of the forward (QA tO QB) electron transfer ie the wild type, and the five mutants, with the largest change in the $264A, and the smallest in the L275F mutant. Formate treatment also led to a decrease in the apparent equilibrium for QAQB ~ QAQB reaction and an increase in the fraction of the slow centers. All effects were reversed by 20 mM bicarbonate. We suggest that the amino acid $21~4. but not L275, F255 and V219, plays an important role in the binding and the function of plastoquinone and bicarbonate in PSII. The role of G256 remains to be further examined since glycine was changed to a significantly different amhto acid. aspartic ~cid.