In the bacterium R. sphaeroides, the polypeptide PufX is indispensable for photosynthetic growth. Its deletion is known to have important consequences on the organization of the photosynthetic apparatus. In the wildtype strain, complexes between the reaction center (RC) and the antenna (light-harvesting complex 1 (LH1)) are associated in dimers, and LH1 does not fully encircle the RC. In the absence of PufX, the complexes become monomeric, and the LH1 ring closes around the RC. We analyzed the

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... unctional consequences of PufX deletion. Some effects can be ascribed to the monomerization of the RC⅐LH1 complexes: the number of RCs that share a common antenna for excitation transfer or a common quinone pool become smaller. We examined the kinetic effects of the closed LH1 ring on quinone turnover: diffusion across LH1 entails a delay of ϳ1 ms, and the barrier appears to be located directly against the quinone-binding (secondary quinone acceptor (Q B )) pocket. The diffusion of ubiquinol from the RC to the cytochrome bc 1 complex is ϳ2-fold slower in the mutant, suggesting an increased distance between the two complexes. The properties of the Q B pocket (binding of inhibitors, stabilization of Q B ؊ , and rate of Q B -H 2 formation) appear to be modified in the mutant. Another specificity of PufX ؊ is the accumulation of closed centers in the Q A ؊ (where Q A is the primary quinone acceptor) state as the secondary acceptor pool becomes reduced, which is probably the origin of photosynthetic incompetence. We suggest that this is related to the Q B pocket alterations. The malfunction of the reaction center is probably due to a faulty association with LH1 that is prevented in the PufX-containing structure.