The core complex ofRhodobacter sphaeroidesis formed by the association of the light-harvesting antenna 1 (LH1) and the reaction center (RC). The PufX protein is essential for photosynthetic growth; it is locatedwithin the core in a 1 : 1 stoichiometry with the RC. PufX is required for a fast ubiquinol exchange between the QBsite of the RC and the Qo site of the cytochromebc1complex.