The reaction center-LH1 antenna complex of Rhodobacter sphaeroides contains one PufX molecule which is involved in dimerization of this complex.

The PufX membrane protein is essential for photosynthetic growth of Rhodobacter sphaeroides wild-type cells. PufX is associated with the reaction center-light harvesting 1 (RC-LH1) core complex and plays a key role in lateral ubiquinone/ubiquinol transfer. We have determined the PufX/RC stoichiometry by quantitative Western blot analysis and RC photobleaching. Independent of copy number effects and growth conditions, one PufX molecule per RC was observed in native membranes as well as in detergent-solubilized RC-LH1 complexes which had been purified over sucrose gradients. Surprisingly, two gradient bands with significantly different sedimentation coefficients were found to have a similar subunit composition, as judged by absorption spectroscopy and protein gel electrophoresis. Gel filtration chromatography and electron microscopy revealed that these membrane complexes represent a monomeric and a dimeric form of the RC-LH1 complex. Since PufX is strictly required for the isolation of dimeric core complexes, we suggest that PufX has a central structural role in forming dimeric RC-LH1 complexes, thus allowing efficient ubiquinone/ubiquinol exchange through the LH1 ring surrounding the RC.     

Functional consequences of the organization of the photosynthetic apparatus in Rhodobacter sphaeroides: II. A study of PufX- membranes

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 wild-type 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 functional consequences of PufX deletion. Some effects can be ascribed to the monomerization of the RC.LH1 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 approximately 1 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 bc1 complex is approximately 2-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)-H2 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.     

Variation in supramolecular organisation of the photosynthetic membrane of Rhodobacter sphaeroides induced by alteration of PufX

In purple bacteria of the genus Rhodobacter (Rba.), an LH1 antenna complex surrounds the photochemical reaction centre (RC) with a PufX protein preventing the LH1 complex from completely encircling the RC. In membranes of Rba. sphaeroides, RC–LH1 complexes associate as dimers which in turn assemble into longer range ordered arrays. The present work uses linear dichroism (LD) and dark-minus-light difference LD (ΔLD) to probe the organisation of genetically altered RC–LH1 complexes in intact membranes. The data support previous proposals that Rba. capsulatus, and Rba. sphaeroides heterologously expressing the PufX protein from Rba. capsulatus, produce monomeric core complexes in membranes that lack long-range order. Similarly, Rba. sphaeroides with a point mutation in the Gly 51 residue of PufX, which is located on the membrane-periplasm interface, assembles mainly non-ordered RC–LH1 complexes that are most likely monomeric. All the Rba. sphaeroides membranes in their ΔLD spectra exhibited a spectral fingerprint of small degree of organisation implying the possibility of ordering influence of LH1, and leading to an important conclusion that PufX itself has no influence on ordering RC–LH1 complexes, as long-range order appears to be induced only through its role of configuring RC–LH1 complexes into dimers.     

The 8.5A projection structure of the core RC-LH1-PufX dimer of Rhodobacter sphaeroides

Two-dimensional crystals of dimeric photosynthetic reaction centre-LH1-PufX complexes have been analysed by cryoelectron microscopy. The 8.5A resolution projection map extends previous analyses of complexes within native membranes to reveal the alpha-helical structure of two reaction centres and 28 LH1 alphabeta subunits within the dimer. For the first time, we have achieved sufficient resolution to suggest a possible location for the PufX transmembrane helix, the orientation of the RC and the arrangement of helices within the surrounding LH1 complex. Whereas low-resolution projections have shown an apparent break in the LH1, our current map reveals a diffuse density within this region, possibly reflecting high mobility. Within this region the separation between beta14 of one monomer and beta2 of the other monomer is approximately 6A larger than the average beta-beta spacing within LH1; we propose that this is sufficient for exchange of quinol at the RC Q(B) site. We have determined the position and orientation of the RC within the dimer, which places its Q(B) site adjacent to the putative PufX, with access to the point in LH1 that appears most easily breached. PufX appears to occupy a strategic position between the mobile alphabeta14 subunit and the Q(B) site, suggesting how the structure, possibly coupled with a flexible ring, plays a role in optimizing quinone exchange during photosynthesis.     

Structural basis for the PufX-mediated dimerization of bacterial photosynthetic core complexes

In Rhodobacter (Rba.) sphaeroides, the subunit PufX is involved in the dimeric organization of the core complex. Here, we report the 3D reconstruction at 12 A by cryoelectron microscopy of the core complex of Rba. veldkampii, a complex of approximately 300 kDa without symmetry. The core complex is monomeric and constituted by a light-harvesting complex 1 (LH1) ring surrounding a uniquely oriented reaction center (RC). The LH1 consists of 15 resolved alpha/beta heterodimers and is interrupted. Within the opening, PufX polypeptide is assigned at a position facing the Q(B) site of the RC. This core complex is different from a dissociated dimer of the core complex of Rba. sphaeroides revealing that PufX in Rba. veldkampii is unable to dimerize. The absence in PufX of Rba. veldkampii of a G(31)XXXG(35) dimerization motif highlights the transmembrane interactions between PufX subunits involved in the dimerization of the core complexes of Rhodobacter species.     

Monomeric RC-LH1 core complexes retard LH2 assembly and intracytoplasmic membrane formation in PufX-minus mutants of Rhodobacter sphaeroides

In the model photosynthetic bacterium Rhodobacter sphaeroides domains of light-harvesting 2 (LH2) complexes surround and interconnect dimeric reaction centre-light-harvesting 1-PufX (RC-LH1-PufX) 'core' complexes, forming extensive networks for energy transfer and trapping. These complexes are housed in spherical intracytoplasmic membranes (ICMs), which are assembled in a stepwise process where biosynthesis of core complexes tends to dominate the early stages of membrane invagination. The kinetics of LH2 assembly were measured in PufX mutants that assemble monomeric core complexes, as a consequence of either a twelve-residue N-terminal truncation of PufX (PufXΔ12) or the complete removal of PufX (PufX(-)). Lower rates of LH2 assembly and retarded maturation of membrane invagination were observed for the larger and less curved ICM from the PufX(-) mutant, consistent with the proposition that local membrane curvature, initiated by arrays of bent RC-LH1-PufX dimers, creates a favourable environment for stable assembly of LH2 complexes. Transmission electron microscopy and high-resolution atomic force microscopy were used to examine ICM morphology and membrane protein organisation in these mutants. Some partitioning of core and LH2 complexes was observed in PufX(-) membranes, resulting in locally ordered clusters of monomeric RC-LH1 complexes. The distribution of core and LH2 complexes in the three types of membrane examined is consistent with previous models of membrane curvature and domain formation (Frese et al., 2008), which demonstrated that a combination of crowding and asymmetries in sizes and shapes of membrane protein complexes drives membrane organisation.     

Structure of the dimeric RC–LH1–PufX complex from Rhodobaca bogoriensis investigated by electron microscopy

Electron microscopy and single-particle averaging were performed on isolated reaction centre (RC)—antenna complexes (RC–LH1–PufX complexes) of Rhodobaca bogoriensis strain LBB1, with the aim of establishing the LH1 antenna conformation, and, in particular, the structural role of the PufX protein. Projection maps of dimeric complexes were obtained at 13 Å resolution and show the positions of the 2 × 14 LH1 α- and β-subunits. This new dimeric complex displays two open, C-shaped LH1 aggregates of 13 αβ polypeptides partially surrounding the RCs plus two LH1 units forming the dimer interface in the centre. Between the interface and the two half rings are two openings on each side. Next to the openings, there are four additional densities present per dimer, considered to be occupied by four copies of PufX. The position of the RC in our model was verified by comparison with RC–LH1–PufX complexes in membranes. Our model differs from previously proposed configurations for Rhodobacter species in which the LH1 ribbon is continuous in the shape of an S, and the stoichiometry is of one PufX per RC.     

Native architecture of the photosynthetic membrane from Rhodobacter veldkampii

The photosynthetic membrane in purple bacteria contains several pigment–protein complexes that assure light capture and establishment of the chemiosmotic gradient. The bioenergetic tasks of the photosynthetic membrane require the strong interaction between these various complexes. In the present work, we acquired the first images of the native outer membrane architecture and the supramolecular organization of the photosynthetic apparatus in vesicular chromatophores of Rhodobacter (Rb.) veldkampii. Mixed with LH2 (light-harvesting complex 2) rings, the PufX-containing LH1–RC (light-harvesting complex 1 – reaction center) core complexes appear as C-shaped monomers, with random orientations in the photosynthetic membrane. Within the LH1 fence surrounding the RC, a remarkable gap that is probably occupied (or partially occupied) by PufX is visualized. Sequence alignment revealed that one specific region in PufX may be essential for PufX-induced core dimerization. In this region of ten amino acids in length all Rhodobacter species had five conserved amino acids, with the exception of Rb. veldkampii. Our findings provide direct evidence that the presence of PufX in Rb. veldkampii does not directly govern the dimerization of LH1–RC core complexes in the native membrane. It is indicated, furthermore, that the high membrane curvature of Rb. veldkampii chromatophores (Rb. veldkampii features equally small vesicular chromatophores alike Rb. sphaeroides) is not due to membrane bending induced by dimeric RC–LH1–PufX cores, as it has been proposed in Rb. sphaeroides.     

Protein-Induced Membrane Curvature Investigated through Molecular Dynamics Flexible Fitting

In the photosynthetic purple bacterium Rhodobacter (Rba.) sphaeroides, light is absorbed by membrane-bound light-harvesting (LH) proteins LH1 and LH2. LH1 directly surrounds the reaction center (RC) and, together with PufX, forms a dimeric (RC-LH1-PufX)₂ protein complex. In LH2-deficient Rba. sphaeroides mutants, RC-LH1-PufX dimers aggregate into tubular vesicles with a radius of ∼250-550 Å, making RC-LH1-PufX one of the few integral membrane proteins known to actively induce membrane curvature. Recently, a three-dimensional electron microscopy density map showed that the Rba. sphaeroides RC-LH1-PufX dimer exhibits a prominent bend at its dimerizing interface. To investigate the curvature properties of this highly bent protein, we employed molecular dynamics simulations to fit an all-atom structural model of the RC-LH1-PufX dimer within the electron microscopy density map. The simulations reveal how the dimer produces a membrane with high local curvature, even though the location of PufX cannot yet be determined uniquely. The resulting membrane curvature agrees well with the size of RC-LH1-PufX tubular vesicles, and demonstrates how the local curvature properties of the RC-LH1-PufX dimer propagate to form the observed long-range organization of the Rba. sphaeroides tubular vesicles.     

Cross-species investigation of the functions of the Rhodobacter PufX polypeptide and the composition of the RC-LH1 core complex

In well-characterised species of the Rhodobacter (Rba.) genus of purple photosynthetic bacteria it is known that the photochemical reaction centre (RC) is intimately-associated with an encircling LH1 antenna pigment protein, and this LH1 antenna is prevented from completely surrounding the RC by a single copy of the PufX protein. In Rba. veldkampii only monomeric RC-LH1 complexes are assembled in the photosynthetic membrane, whereas in Rba. sphaeroides and Rba. blasticus a dimeric form is also assembled in which two RCs are surrounded by an S-shaped LH1 antenna. The present work established that dimeric RC-LH1 complexes can also be isolated from Rba. azotoformans and Rba. changlensis, but not from Rba. capsulatus or Rba. vinaykumarii. The compositions of the monomers and dimers isolated from these four species of Rhodobacter were similar to those of the well-characterised RC-LH1 complexes present in Rba. sphaeroides. Pigment proteins were also isolated from strains of Rba. sphaeroides expressing chimeric RC-LH1 complexes. Replacement of either the Rba. sphaeroides LH1 antenna or PufX with its counterpart from Rba. capsulatus led to a loss of the dimeric form of the RC-LH1 complex, but the monomeric form had a largely unaltered composition, even in strains in which the expression level of LH1 relative to the RC was reduced. The chimeric RC-LH1 complexes were also functional, supporting bacterial growth under photosynthetic conditions. The findings help to tease apart the different functions of PufX in different species of Rhodobacter, and a specific protein structural arrangement that allows PufX to fulfil these three functions is proposed.     

PucC and LhaA direct efficient assembly of the light‐harvesting complexes in Rhodobacter sphaeroides

The mature architecture of the photosynthetic membrane of the purple phototroph Rhodobacter sphaeroides has been characterised to a level where an atomic‐level membrane model is available, but the roles of the putative assembly proteins LhaA and PucC in establishing this architecture are unknown. Here we investigate the assembly of light‐harvesting LH2 and reaction centre‐light‐harvesting1‐PufX (RC‐LH1‐PufX) photosystem complexes using spectroscopy, pull‐downs, native gel electrophoresis, quantitative mass spectrometry and fluorescence lifetime microscopy to characterise a series of lhaA and pucC mutants. LhaA and PucC are important for specific assembly of LH1 or LH2 complexes, respectively, but they are not essential; the few LH1 subunits found in ΔlhaA mutants assemble to form normal RC‐LH1‐PufX core complexes showing that, once initiated, LH1 assembly round the RC is cooperative and proceeds to completion. LhaA and PucC form oligomers at sites of initiation of membrane invagination; LhaA associates with RCs, bacteriochlorophyll synthase (BchG), the protein translocase subunit YajC and the YidC membrane protein insertase. These associations within membrane nanodomains likely maximise interactions between pigments newly arriving from BchG and nascent proteins within the SecYEG‐SecDF‐YajC‐YidC assembly machinery, thereby co‐ordinating pigment delivery, the co‐translational insertion of LH polypeptides and their folding and assembly to form photosynthetic complexes.     

The PuhB protein of Rhodobacter capsulatus functions in photosynthetic reaction center assembly with a secondary effect on light-harvesting complex 1

The core of the photosynthetic apparatus of purple photosynthetic bacteria such as Rhodobacter capsulatus consists of a reaction center (RC) intimately associated with light-harvesting complex 1 (LH1) and the PufX polypeptide. The abundance of the RC and LH1 components was previously shown to depend on the product of the puhB gene (formerly known as orf214). We report here that disruption of puhB diminishes RC assembly, with an indirect effect on LH1 assembly, and reduces the amount of PufX. Under semiaerobic growth conditions, the core complex was present at a reduced level in puhB mutants. After transfer of semiaerobically grown cultures to photosynthetic (anaerobic illuminated) conditions, the RC/LH1 complex became only slightly more abundant, and the amount of PufX increased as cells began photosynthetic growth. We discovered that the photosynthetic growth of puhB disruption strains of R. capsulatus starts after a long lag period, which is due to physiological adaptation rather than secondary mutations. Using a hybrid protein expression system, we determined that the three predicted transmembrane segments of PuhB are capable of spanning a cell membrane and that the second transmembrane segment could mediate self-association of PuhB. We discuss the possible function of PuhB as a dimeric RC assembly factor.     

Carotenoid to bacteriochlorophyll energy transfer in the RC–LH1–PufX complex from <Emphasis Type="Italic">Rhodobacter sphaeroides</Emphasis> containing the extended conjugation keto-carotenoid diketospirilloxanthin

RC–LH1–PufX complexes from a genetically modified strain of Rhodobacter sphaeroides that accumulates carotenoids with very long conjugation were studied by ultrafast transient absorption spectroscopy. The complexes predominantly bind the carotenoid diketospirilloxanthin, constituting about 75% of the total carotenoids, which has 13 conjugated C=C bonds, and the conjugation is further extended to two terminal keto groups. Excitation of diketospirilloxanthin in the RC–LH1–PufX complex demonstrates fully functional energy transfer from diketospirilloxanthin to BChl a in the LH1 antenna. As for other purple bacterial LH complexes having carotenoids with long conjugation, the main energy transfer route is via the S₂–Q_x pathway. However, in contrast to LH2 complexes binding diketospirilloxanthin, in RC–LH1–PufX we observe an additional, minor energy transfer pathway associated with the S₁ state of diketospirilloxanthin. By comparing the spectral properties of the S₁ state of diketospirilloxanthin in solution, in LH2, and in RC–LH1–PufX, we propose that the carotenoid-binding site in RC–LH1–PufX activates the ICT state of diketospirilloxanthin, resulting in the opening of a minor S₁/ICT-mediated energy transfer channel.     

Cross-species investigation of the functions of the Rhodobacter PufX polypeptide and the composition of the RC–LH1 core complex

In well-characterised species of the Rhodobacter (Rba.) genus of purple photosynthetic bacteria it is known that the photochemical reaction centre (RC) is intimately-associated with an encircling LH1 antenna pigment protein, and this LH1 antenna is prevented from completely surrounding the RC by a single copy of the PufX protein. In Rba. veldkampii only monomeric RC–LH1 complexes are assembled in the photosynthetic membrane, whereas in Rba. sphaeroides and Rba. blasticus a dimeric form is also assembled in which two RCs are surrounded by an S-shaped LH1 antenna. The present work established that dimeric RC–LH1 complexes can also be isolated from Rba. azotoformans and Rba. changlensis, but not from Rba. capsulatus or Rba. vinaykumarii. The compositions of the monomers and dimers isolated from these four species of Rhodobacter were similar to those of the well-characterised RC–LH1 complexes present in Rba. sphaeroides. Pigment proteins were also isolated from strains of Rba. sphaeroides expressing chimeric RC–LH1 complexes. Replacement of either the Rba. sphaeroides LH1 antenna or PufX with its counterpart from Rba. capsulatus led to a loss of the dimeric form of the RC–LH1 complex, but the monomeric form had a largely unaltered composition, even in strains in which the expression level of LH1 relative to the RC was reduced. The chimeric RC–LH1 complexes were also functional, supporting bacterial growth under photosynthetic conditions. The findings help to tease apart the different functions of PufX in different species of Rhodobacter, and a specific protein structural arrangement that allows PufX to fulfil these three functions is proposed.     

Integration of energy and electron transfer processes in the photosynthetic membrane of Rhodobacter sphaeroides

Photosynthesis converts absorbed solar energy to a protonmotive force, which drives ATP synthesis. The membrane network of chlorophyll–protein complexes responsible for light absorption, photochemistry and quinol (QH₂) production has been mapped in the purple phototrophic bacterium Rhodobacter (Rba.) sphaeroides using atomic force microscopy (AFM), but the membrane location of the cytochrome bc₁ (cytbc₁) complexes that oxidise QH₂ to quinone (Q) to generate a protonmotive force is unknown. We labelled cytbc₁ complexes with gold nanobeads, each attached by a Histidine₁₀ (His₁₀)-tag to the C-terminus of cytc₁. Electron microscopy (EM) of negatively stained chromatophore vesicles showed that the majority of the cytbc₁ complexes occur as dimers in the membrane. The cytbc₁ complexes appeared to be adjacent to reaction centre light-harvesting 1-PufX (RC–LH1–PufX) complexes, consistent with AFM topographs of a gold-labelled membrane. His-tagged cytbc₁ complexes were retrieved from chromatophores partially solubilised by detergent; RC–LH1–PufX complexes tended to co-purify with cytbc₁ whereas LH2 complexes became detached, consistent with clusters of cytbc₁ complexes close to RC–LH1–PufX arrays, but not with a fixed, stoichiometric cytbc₁–RC–LH1–PufX supercomplex. This information was combined with a quantitative mass spectrometry (MS) analysis of the RC, cytbc₁, ATP synthase, cytaa₃ and cytcbb₃ membrane protein complexes, to construct an atomic-level model of a chromatophore vesicle comprising 67 LH2 complexes, 11 LH1–RC–PufX dimers & 2 RC–LH1–PufX monomers, 4 cytbc₁ dimers and 2 ATP synthases. Simulation of the interconnected energy, electron and proton transfer processes showed a half-maximal ATP turnover rate for a light intensity equivalent to only 1% of bright sunlight. Thus, the photosystem architecture of the chromatophore is optimised for growth at low light intensities.     

Overexpression of Rhodobacter sphaeroides PufX-bearing maltose-binding protein and its effect on the stability of reconstituted light-harvesting core antenna complex

The PufX protein, encoded by the pufX gene of Rhodobacter sphaeroides, plays a key role in the organization and function of the core antenna (LH1)-reaction centre (RC) complex, which collects photons and triggers primary photochemical reactions. We synthesized a PufX/maltose-binding protein (MBP) fusion protein to study the effect of the PufX protein on the reconstitution of B820 subunit-type and LH1-type complexes. The fusion protein was synthesized using an Escherichia coli expression system and purified by affinity chromatography. Reconstitution experiments demonstrated that the MBP-PufX protein destabilizes the subunit-type complex (20°C), consistent with previous reports. Interestingly, however, the preformed LH1-type complex was stable in the presence of MBP-PufX. The MBP-PufX protein did not influence the preformed LH1-type complexes (4°C). The LH1-type complex containing MBP-PufX showed a unique temperature-dependent structural transformation that was irreversible. The predominant form of the complex at 4°C was the LH1-type. When shifted to 20°C, subunit-type complexes became predominant. Upon subsequent cooling back to 4°C, instead of re-forming the LH1-type complexes, the predominant form remained the subunit-type complexes. In contrast, reversible transformation of LH1 (4°C) and subunit-type complexes (20°C) occurs in the absence of PufX. These results are consistent with the suggestion that MBP-PufX interacts with the LH1α- polypeptide in the subunit (α/β)-type complex (at 20°C), preventing oligomerization of the subunit to form LH1-type complexes.     

The solution structure of the PufX polypeptide from Rhodobacter sphaeroides

PufX organises the photosynthetic reaction centre–light harvesting complex 1 (RC–LH1) core complex of Rhodobacter sphaeroides and facilitates quinol/quinone exchange between the RC and cytochrome bc₁ complexes. The structure of PufX in organic solvent reveals two hydrophobic helices flanked by unstructured termini and connected by a helical bend. The proposed location of basic residues and tryptophans at the membrane interface orients the C-terminal helix along the membrane normal, with the GXXXG motifs in positions unsuitable as direct drivers of dimerisation of the RC–LH1 complex. The N-terminal helix is predicted to extend 40 Å along the membrane interface.     

Dimerisation of the Rhodobacter sphaeroides RC-LH1 photosynthetic complex is not facilitated by a GxxxG motif in the PufX polypeptide

In purple photosynthetic bacteria the initial steps of light energy transduction take place in an RC-LH1 complex formed by the photochemical reaction centre (RC) and the LH1 light harvesting pigment-protein. In Rhodobacter sphaeroides, the RC-LH1 complex assembles in a dimeric form in which two RCs are surrounded by an S-shaped LH1 antenna. There is currently debate over the detailed architecture of this dimeric RC-LH1 complex, with particular emphasis on the location and precise function of a minor polypeptide component termed PufX. It has been hypothesised that the membrane-spanning helical region of PufX contains a GxxxG dimerisation motif that facilitates the formation of a dimer of PufX at the interface of the RC-LH1 dimer, and more specifically that the formation of this PufX dimer seeds assembly of the remaining RC-LH1 dimer (J. Busselez et al., 2007). In the present work this hypothesis was tested by site directed mutagenesis of the glycine residues proposed to form the GxxxG motif. Mutation of these glycines to leucine did not decrease the propensity of the RC-LH1 complex to assemble in a dimeric form, as would be expected from experimental studies of the effect of mutation on GxxxG motifs in other membrane proteins. Indeed increased yields of dimer were seen in two of the glycine-to-leucine mutants constructed. It is concluded that the PufX from Rhodobacter sphaeroides does not contain a genuine GxxxG helix dimerisation motif.     

Investigation of Rhodobacter capsulatus PufX interactions in the core complex of the photosynthetic apparatus

The photosynthetic apparatus of purple bacteria in the genus Rhodobacter includes a core complex consisting of the reaction centre (RC), light-harvesting complex 1 (LH1), and the PufX protein. PufX modulates LH1 structure and facilitates photosynthetic quinone/quinol exchange. We deleted RC/LH1 genes in pufX ⁺ and pufX ⁺⁺ (merodiploid) strains of Rhodobacter capsulatus, which reduced PufX levels regardless of pufX gene copy number and location. Photosynthetic growth of RC-only strains and independent assembly kinetics of the RC and LH1 were unaffected by pufX merodiploidy, but the absorption spectra of strains expressing the RC plus either LH1 α or β indicated that PufX may influence bacteriochlorophyll binding environments. Significant self-association of the PufX transmembrane segment was detected in a hybrid protein expression system, consistent with a role of PufX in core complex dimerization, as proposed for other Rhodobacter species. Our results indicate that in R. capsulatus PufX has the potential to be a central, homodimeric core complex component, and its cellular level is increased by interactions with the RC and LH1.