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 (QH ₂) 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 cytbc ₁ 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.