Functional and structural analysis of the photosynthetic apparatus of Rhodobacter veldkampii

In the widely studied purple bacterium Rhodobacter sphaeroides, a small transmembrane protein, named PufX, is required for photosynthetic growth and is involved in the supramolecular dimeric organization of the core complex. We performed a structural and functional analysis of the photosynthetic apparatus of Rhodobacter veldkampii, a related species which evolved independently. Time-resolved optical spectroscopy of R. veldkampii chromatophores showed that the reaction center shares with R. sphaeroides spectral and redox properties and interacts with a cytochrome bc(1) complex through a Q-cycle mechanism. Kinetic analysis of flash-induced cytochrome b(561) reduction indicated a fast delivery of the reduced quinol produced by the reaction center to the cytochrome bc(1) complex. A core complex, along with two light-harvesting LH2 complexes significantly different in size, was purified and analyzed by sedimentation, size exclusion chromatography, mass spectroscopy, and electron microscopy. A PufX subunit identified by MALDI-TOF was found to be associated with the core complex. However, as shown by sedimentation and single-particle analysis by electron microscopy, the core complex is monomeric, suggesting that in R. veldkampii, PufX is involved in the photosynthetic growth but is unable to induce the dimerization of the core complex.