Abstract
Bicarbonate (HCO3 (-)) bindingregulateselectron flow between the primary (Q(A)) and secondary (Q(B)) plastoquinone electron acceptors of Photosystem II (PS II).Lys264 of the D2 subunit of PS II contributes to a hydrogen-bond networkthat stabilizes HCO3 (-) ligation to thenon-heme iron in the Q(A)-Fe-Q(B) complex. Usingthe cyanobacterium Synechocystis sp. PCC 6803, alanineand glutamate were introduced to create the K264A and K264E mutants.Photoautotrophic growth was slowed in K264E cells but not in the K264Astrain. Both mutants accumulated an unassembled CP43 precomplex aswell as the CP43-lacking RC47 assembly intermediate, indicating weakenedbinding of the CP43 precomplex to RC47. Assembly was impeded morein K264E cells than in the K264A strain, but K264A cells were moresusceptible to high-light-induced photodamage when assayed using PSII-specific electron acceptors. Furthermore, an impaired repair mechanismwas observed in the K264A mutant in protein labeling experiments.Unexpectedly, unlike the K264A strain, the K264E mutant displayedinhibited oxygen evolution following high-light exposure when HCO3 (-) was added to support whole chain electrontransport. In both mutants, the decay of chlorophyll fluorescencewas slowed, indicating impaired electron transfer between Q(A) and Q(B). Furthermore, the fluorescence decay kineticsin the K264E strain were insensitive to addition of either formateor HCO3 (-), whereas HCO3 (-)-reversible formate-induced inhibition in the K264Amutant was observed. Exchange of plastoquinol with the membrane plastoquinonepool at the Q(B)-binding site was also retarded in both mutants.Hence, D2-Lys264 possesses key roles in both assembly and activityof PS II.