Cross-section of the R. sphaeroides ICM, illustrating the function of the major components of this bioenergetic membrane. The figure depicts the orientation of ICM components relative to the cytoplasmic and periplasmic compartments. Abbreviations: Bchl, bacteriochlorophyll; Bchl2, bacteriochlorophyll special pair; ADP, adenosine diphosphate; ATP, adenosine triphosphate; Pi, phosphate; e^-, electron; Q, ubiquinone; cyt c₂, cytochrome c₂; B800-850 & B875, light harvesting complexes, respectively.

Supramolecular organization of ICM pigment-protein complexes. Sealed ICM vesicles (green) contain a high-density array of ordered pigment-protein complexes. In this array, each RC complex is surrounded by a series of B875 light harvesting complexes and are proposed to be coupled or linked with another B875-RC complex by the PufX protein. B875-RC pairs are arranged in a lattice, enabling tight ordered packing of ICM pigment-protein complexes. B800-850 light harvesting complexes are proposed to be arranged in pools two to three complexes wide between the B875-RC pairs. It is proposed that this ordered arrangement of pigment-protein complexes allows efficient energy transfer from B800-850 pigments to B875 pigments and to the RC special pair where photochemistry occurs.

Cellular depiction of the R. sphaeroides ICM that houses the photosynthetic apparatus. This image shows the ICM (red spheres) protruding from the cytoplasmic membrane (purple) into the cytoplasm of this Gram-negative bacterium (outer membrane is green). Left panel: long-axis cross section of a cell. Right panel: short-axis cross section of a cell. These images were generated by digital reconstruction (Harold Trease, Pacific Northwest National Laboratory, US Department of Energy) of sequential thin-section electron micrographs of photosynthetically grown cells (Samuel Kaplan, University of Texas Medical School at Houston) and are reproduced here with permission.

Homeostatic control of gene expression by the PrrBA pathway. The left panel depicts the state of PrrA-dependent gene expression in the presence of O₂. Under these conditions, the aerobic electron transport chain (ETC) is operative, the steady-state level of phosphorylated PrrA (PrrA-Pi) is low, and so there is a low basal level transcription of target genes (e.g. PS genes and electron sinks). The right panel depicts the state of PrrA-dependent gene expression under low O₂ or anaerobic conditions. In this case, the lack of aerobic electron transport chain activity increases the steady state level of PrrA-Pi, which activates transcription of photosynthesis genes and gene products that recycle excess reducing power under anaerobic conditions.