Identification of the components of a putative cytochrome bc1 complex in Rhodopseudomonas viridis

Photosynthetic system in Blastochloris viridis revisited

The bacterium Blastochloris viridis carries one of the simplest photosynthetic systems, which includes a single light-harvesting complex that surrounds the reaction center, membrane soluble quinones, and a soluble periplasmic protein cytochrome c(2) that shuttle between the reaction center and the bc(1) complex and act as electron carriers, as well as the ATP synthase. The close arrangement of the photosynthetic membranes in Bl. viridis, along with the extremely tight arrangement of the photosystems within these membranes, raises a fundamental question about the diffusion of the electron carriers. To address this issue, we analyzed the structure and response of the Bl. viridis photosynthetic system to various light conditions, by using a combination of electron microscopy, whole-cell cryotomography, and spectroscopic methods. We demonstrate that in response to high light intensities, the ratio of both cytochrome c(2) and bc(1) complexes to the reaction centers is increased. The shorter membrane stacks, along with the notion that the bc(1) complex is located at the highly curved edges of these stacks, result in a smaller average distance between the reaction centers and the bc(1) complexes, leading to shorter pathways of cytochrome c(2) between the two complexes. Under anaerobic conditions, the slow diffusion rate is further mitigated by keeping most of the quinone pool reduced, resulting in a concentration gradient of quinols that allows for a constant supply of theses electron carriers to the bc(1) complex.

Structural analysis of photosynthetic membranes by cryo-electron tomography of intact Rhodopseudomonas viridis cells

During the photosynthetic process, highly organized membranal assemblies convert light into biochemical energy with high efficiency. We have used whole-mount cryo-electron tomography to study the intracellular architecture of the photosynthetic membranes of the anaerobic purple photosynthetic bacterium Rhodopseudomonas viridis, as well as the organization of the photosynthetic units within the membranes. Three-dimensional reconstruction demonstrates a continuity of the plasma membrane with the photosynthetic membranes that form tunnel-like structures with an average diameter of 31 nm+/-8 nm at the connection sites. The spacing between the photosynthetic membranes at their cytoplasmic faces was found to be 11 nm, thus enforcing a highly close packaging of the photosynthetic membranes. Analysis of successive tomographic slices allowed for derivation of the spacing between adjacent photosynthetic core complexes from a single-layered photosynthetic membrane, in situ. This analysis suggests that most, if not all, photosynthetic membranes in R. viridis are characterized by a similar two-dimensional hexagonal lattice organization.

In vitro and in vivo electron transfer to the triheme cytochrome subunit bound to the photosynthetic reaction center complex in the purple bacterium Rhodovulum sulfidophilum

The cytochrome subunit bound to the photosynthetic reaction center (RC) complex in Rhodovulum sulfidophilum lacks one heme-binding motif (CXXCH) out of four motifs found in other purple bacteria resulting in the absence of the most distal heme from the RC-core complex (S. Masuda et al., J. Biol. Chem. 274 (1999) 10795). Cytochrome c(2), which acts as the electron donor to the RC was purified, and its gene was cloned and sequenced. The redox midpoint potential of cytochrome c(2) was determined to be E(m)=357 mV. The photo-oxidation and re-reduction of purified cytochrome c(2) were observed in the presence of membrane preparations. Flash-induced photo-oxidation and re-reduction of the RC-bound cytochrome were also observed in intact cells. Despite the unusual nature of the RC-bound cytochrome subunit, the cyclic electron transfer system in Rdv. sulfidophilum was shown to be similar to those in other purple bacteria.

The interaction between cytochrome c2 and the cytochrome bc1 complex in the photosynthetic purple bacteria Rhodobacter capsulatus and Rhodopseudomonas viridis

The rates of electron transfer from a ubiquinol analogue to cytochrome c2 catalyzed by the cytochrome bc1 complexes of Rhodobacter capsulatus and Rhodopseudomonas viridis were measured as a function of ionic strength. The effects of ionic strength on the kinetic parameters for the reactions are consistent with a role for electrostatic complex formation between cytochrome c2 and the cytochrome bc1 complex in the electron-transfer pathways in both photosynthetic purple non-sulfur bacteria. Additional support for a docking model in which positively charged lysines on cytochrome c2 interact with negatively charged groups on the Rb. capsulatus cytochrome bc1 complex was obtained from kinetic experiments using Rb. capsulatus cytochrome c2 and equine cytochrome c in which specific lysine residues were altered by site-directed mutagenesis and chemical modification, respectively. Equine cytochrome c, which is a poor electron donor to the reaction center of Rps. viridis, is an effective electron acceptor for the Rps. viridis cytochrome bc1 complex. Chemical modification of lysine residues on Rps. viridis cytochrome c2 has a substantially greater effect on the reduction of the Rps. viridis reaction center by ferrocytochrome c2 than on the oxidation of the Rps. viridis cytochrome bc1 complex by ferricytochrome c2. These data suggest that the docking site for Rps. viridis cytochrome c2 on the Rps. viridis reaction center tetraheme subunit differs in structure from the docking site for the cytochrome on the Rps. viridis cytochrome bc1 complex to a significant extent. In this respect, Rps. viridis differs from photosynthetic purple non-sulfur bacteria in which the reaction center does not contain a tetraheme subunit, where the binding sites for cytochrome c2 on the reaction center and the cytochrome bc1 complex appear to be quite similar.

Cytochrome bc1 complexes of microorganisms

The cytochrome bc1 complex is the most widely occurring electron transfer complex capable of energy transduction. Cytochrome bc1 complexes are found in the plasma membranes of phylogenetically diverse photosynthetic and respiring bacteria, and in the inner mitochondrial membrane of all eucaryotic cells. In all of these species the bc1 complex transfers electrons from a low-potential quinol to a higher-potential c-type cytochrome and links this electron transfer to proton translocation. Most bacteria also possess alternative pathways of quinol oxidation capable of circumventing the bc1 complex, but these pathways generally lack the energy-transducing, protontranslocating activity of the bc1 complex. All cytochrome bc1 complexes contain three electron transfer proteins which contain four redox prosthetic groups. These are cytochrome b, which contains two b heme groups that differ in their optical and thermodynamic properties; cytochrome c1, which contains a covalently bound c-type heme; and a 2Fe-2S iron-sulfur protein. The mechanism which links proton translocation to electron transfer through these proteins is the proton motive Q cycle, and this mechanism appears to be universal to all bc1 complexes. Experimentation is currently focused on understanding selected structure-function relationships prerequisite for these redox proteins to participate in the Q-cycle mechanism. The cytochrome bc1 complexes of mitochondria differ from those of bacteria, in that the former contain six to eight supernumerary polypeptides, in addition to the three redox proteins common to bacteria and mitochondria. These extra polypeptides are encoded in the nucleus and do not contain redox prosthetic groups. The functions of the supernumerary polypeptides of the mitochondrial bc1 complexes are generally not known and are being actively explored by genetically manipulating these proteins in Saccharomyces cerevisiae.

Crystallization and preliminary X-ray diffraction study of ferrocytochrome c2 from Rhodopseudomonas viridis

Crystals of ferrocytochrome c2 from a non-sulphur purple photosynthetic bacterium, Rhodopseudomonas viridis, have been grown from ammonium sulphate solution at pH 8.5 by the sitting-drop vapour-diffusion procedure. The crystals belong to the trigonal system, space group P3(1)21 (or its enantiomorph P3(2)21) with unit-cell dimensions of a = b = 75.8 A and c = 40.1 A, and diffract to at least 2.0 A resolution. Assuming that an asymmetric unit contains one protein molecule (approx. 12,300 Mr), the solvent content of the crystal is approximately 54.5% (v/v).

Isolation of cytochrome bc 1 complexes from the photosynthetic bacteria Rhodopseudomonas viridis and Rhodospirillum rubrum

Cytochrome bc 1 complexes have been isolated from wild type Rhodopseudomonas viridis and Rhodospirillum rubrum and purified by affinity chromatography on cytochrome c-Sepharose 4B. Both complexes are largely free of bacteriochlorophyll and carotenoids and contain cytochromes b and c 1 in a 2:1 molar ratio. For the Rps. viridis complex, evidence has been obtained for two spectrally distinct b-cytochromes. The R. rubrum complex contains a Rieske iron-sulfur protein (present in approximately 1:1 molar ratio to cytochrome c 1) and catalyzes an antimycin A- and myxothiazol-sensitive electron transfer from duroquinol to equine cytochrome c or R. rubrum cytochrome c 2. Although an attempt to prepare a cytochrome bc 1 complex from the gliding green bacterium Chloroflexus aurantiacus was not successful, membranes isolated from phototrophically grown Cfl. aurantiacus were shown to contain a Rieske iron-sulfur protein and protoheme (the prosthetic group of b-type cytochromes).