New tetragonal form of reaction centers from Rhodobacter sphaeroides and the involvement of a manganese ion at a crystal contact point

Crystals have been obtained of wild-type reaction centers from Rhodobacter sphaeroides using manganese chloride as a precipitating agent. The crystals belong to the tetragonal space group P4₂22, with unit-cell parameters a = b = 207.8, c = 107.5 Å. The crystal structure has been determined to a resolution limit of 4.6 Å using a previously determined structure of the reaction center as a molecular-replacement model. The calculated electron-density maps show the presence of a manganese ion at one of the crystal contact points bridging two symmetry-related histidine residues, suggesting that the metal plays a key role in facilitating the crystallization of the protein in this form.