Abstract
Photosystem II (PS II) assembly is a stepwise process involving preassembly complexes or modules focused around four core PS II proteins. The current model of PS II assembly in cyanobacteria is derived from studies involving the deletion of one or more of these core subunits. Such deletions may destabilize other PS II assembly intermediates, making constructing a clear picture of the intermediate events difficult. Information on plastoquinone exchange pathways operating within PS II is also unclear and relies heavily on computer -aided simulations. Deletion of PsbX in [S. Biswas, J.J. Eaton -Rye, Biochim. Biophys. Acta - Bioenerg. 1863 (2022) 148519] suggested modified Q B binding in PS II lacking this subunit. This study has indicated the phenotype of the Delta PsbX mutant arose by disrupting a conserved hydrogen bond between PsbX and the D2 (PsbD) protein. We mutated two conserved arginine residues (D2:Arg24 and D2:Arg26) to further understand the observations made with the Delta PsbX mutant. Mutating Arg24 disrupted the interaction between PsbX and D2, replicating the high -light sensitivity and altered fluorescence decay kinetics observed in the Delta PsbX strain. The Arg26 residue, on the other hand, was more important for either PS II assembly or for stabilizing the fully assembled complex. The effects of mutating both arginine residues to alanine or aspartate were severe enough to render the corresponding double mutants non-photoautotrophic. Our study furthers our knowledge of the amino -acid interactions stabilizing plastoquinone-exchange pathways while providing a platform to study PS II assembly and repair without the actual deletion of any proteins.