The contribution of CP43:Phe345 to the oxygen-evolving complex of Photosystem II is essential for stable water splitting

Water splitting by Photosystem II (PS II) in photosynthesis is catalyzed by a Mn₄CaO₅ oxygen-evolving complex (OEC). The reaction center D1 protein provides most of the ligands to the OEC but its complete assembly requires the association of the CP43 core antenna subunit which provides CP43:Glu341 as an essential ligand. In addition, CP43:Arg344 contributes key interactions to the OEC environment. These residues are in a hydrophilic loop (Loop E) that must gain access to the pocket housing the OEC in the membrane. We have investigated the role of the nearby and conserved CP43:Phe345 residue in the stabilization and function of the OEC. Substitutions removing the aromatic character of the side chain inactivated PS II activity by either preventing assembly (in the case of proline) or resulted in an inactive assembled photosystem (in the case of lysine). Substitution by alanine or by aromatic residues (histidine, tyrosine and tryptophan) supported photoautotrophic growth, although electron transfer between the OEC and the PS II reaction center was impaired. Additionally, the F345H mutant appeared to have fewer active PS II centers than control cells. Furthermore, the F345Y mutant, and to some extent the F345W strain, exhibited a redirection of light energy from Photosystem I to PS II as a potential response to correct for the reduced flow of electrons into the electron transport chain of these mutants. Collectively, these data show the hydrophobic character of CP43:Phe345 is required for the assembly and optimal activity of the OEC in PS II.