Abstract
The non-heme iron of Photosystem II (PSII), found between the primary (QA) and secondary (QB) plastoquinone electron acceptors, is coordinated by histidine residues from the D1 and D2 reaction center proteins and a bicarbonate ion. The D2-Val247 and D2-Ser254 residues of the DE hydrophilic loop of D2 are located in the QA-binding region and interact with the PsbT subunit. Substitutions at these residues caused reductions in photoautotrophic growth, increased sensitivity to high light, and weakened bicarbonate binding. In addition, the core antenna CP43 protein accumulated in these mutants and electron transfer from QA- to QB was perturbed. In several mutants, bicarbonate addition only partially restored electron transfer, consistent with disruption of a hydrogen-bonding network linking D2-Ser254 to D2-Asn250 and PsbT. These results demonstrate that D2-Val247 and D2-Ser254 stabilize the quinone-iron-bicarbonate acceptor complex of PSII and influence the stable association of CP43 with the photosystem.