Electron transfer from high-potential iron-sulfur protein and low-potential cytochrome c-551 to the primary donor of Rubrivivax gelatinosus reaction center mutationally devoid of the bound cytochrome subunit

Properties and structure of a low-potential, penta-heme cytochrome c552 from a thermophilic purple sulfur photosynthetic bacterium Thermochromatium tepidum

The thermophilic purple sulfur bacterium Thermochromatium tepidum possesses four main water-soluble redox proteins involved in the electron transfer behavior. Crystal structures have been reported for three of them: a high potential iron-sulfur protein, cytochrome c', and one of two low-potential cytochrome c₅₅₂ (which is a flavocytochrome c) have been determined. In this study, we purified another low-potential cytochrome c₅₅₂ (LPC), determined its N-terminal amino acid sequence and the whole gene sequence, characterized it with absorption and electron paramagnetic spectroscopy, and solved its high-resolution crystal structure. This novel cytochrome was found to contain five c-type hemes. The overall fold of LPC consists of two distinct domains, one is the five heme-containing domain and the other one is an Ig-like domain. This provides a representative example for the structures of multiheme cytochromes containing an odd number of hemes, although the structures of multiheme cytochromes with an even number of hemes are frequently seen in the PDB database. Comparison of the sequence and structure of LPC with other proteins in the databases revealed several characteristic features which may be important for its functioning. Based on the results obtained, we discuss the possible intracellular function of this LPC in Tch. tepidum.

Photo-induced electron transfer in intact cells of Rubrivivax gelatinosus mutants deleted in the RC-bound tetraheme cytochrome: insight into evolution of photosynthetic electron transport

Deletion of two of the major electron carriers, the reaction center-bound tetrahemic cytochrome and the HiPIP, involved in the light-induced cyclic electron transfer pathway of the purple photosynthetic bacterium, Rubrivivax gelatinosus, significantly impairs its anaerobic photosynthetic growth. Analysis on the light-induced absorption changes of the intact cells of the mutants shows, however, a relatively efficient photo-induced cyclic electron transfer. For the single mutant lacking the reaction center-bound cytochrome, we present evidence that the electron carrier connecting the reaction center and the cytochrome bc(1) complex is the High Potential Iron-sulfur Protein. In the double mutant lacking both the reaction center-bound cytochrome and the High Potential Iron-sulfur Protein, this connection is achieved by the high potential cytochrome c(8). Under anaerobic conditions, the halftime of re-reduction of the photo-oxidized primary donor by these electron donors is 3 to 4 times faster than the back reaction between P(+) and the reduced primary quinone acceptor. This explains the photosynthetic growth of these two mutants. The results are discussed in terms of evolution of the type II RCs and their secondary electron donors.

The cytochrome c₈ involved in the nitrite reduction pathway acts also as electron donor to the photosynthetic reaction center in Rubrivivax gelatinosus

The purple photosynthetic bacterium Rubrivivax gelatinosus has, at least, four periplasmic electron carriers, i.e., HiPIP, two cytochromes c₈with low- and high-midpoint potentials, and cytochrome c₄ as electron donors to the photochemical reaction center. The quadruple mutant lacking all four electron carrier proteins showed extremely slow photosynthetic growth. During the long-term cultivation of this mutant under photosynthetic conditions, a suppressor strain recovering the wild-type growth level appeared. In the cells of the suppressor strain, we found significant accumulation of a soluble c-type cytochrome that has not been detected in wild-type cells. This cytochrome c has a redox midpoint potential of about +280 mV and could function as an electron donor to the photochemical reaction center in vitro. The amino acid sequence of this cytochrome c was 65% identical to that of the high-potential cytochrome c₈of this bacterium. The gene for this cytochrome c was identified as nirM on the basis of its location in the newly identified nir operon, which includes a gene coding cytochrome cd₁-type nitrite reductase. Phylogenetic analysis and the well-conserved nir operon gene arrangement suggest that the origin of the three cytochromes c₈ in this bacterium is NirM. The two other cytochromes c₈, of high and low potentials, proposed to be generated by gene duplication from NirM, have evolved to function in distinct pathways.

Cytochrome c4 can be involved in the photosynthetic electron transfer system in the purple bacterium Rubrivivax gelatinosus

Three periplasmic electron carriers, HiPIP and two cytochromes c8 with low- and high-midpoint potentials, are present in the purple photosynthetic bacterium Rubrivivax gelatinosus. Comparison of the growth rates of mutants lacking one, two, or all three electron carrier proteins showed that HiPIP is the main electron donor to the photochemical reaction center and that high-potential cytochrome c8 plays a subsidiary role in the electron donation in photosynthetically growing cells. However, the triple deletion mutant was still capable of photosynthetic growth, indicating that another electron donor could be present. A new soluble cytochrome c, which can reduce the photooxidized reaction center in vitro, was purified. Based on amino acid sequence comparisons to known cytochromes, this cytochrome was identified as a diheme cytochrome c of the family of cytochromes c4. The quadruple mutant lacking this cytochrome and three other electron carriers showed about three times slower growth than the triple mutant under photosynthetic growth conditions. In conclusion, cytochrome c4 can function as a physiological electron carrier in the photosynthetic electron transport chain in R. gelatinosus.

High potential iron-sulfur proteins and their role as soluble electron carriers in bacterial photosynthesis: tale of a discovery

This review is an attempt to retrace the chronicle of the discovery of the role of high-potential iron-sulfur proteins (HiPIPs) as electron carriers in the photosynthetic chain of bacteria. Data and facts are presented through the magnifying lenses of the authors, using their best judgment to filter and elaborate on the many facets of the research carried out on this class of proteins over the years. The tale is divided into four main periods: the seeds, the blooming, the ripening, and the harvest, representing the times from the discovery of these proteins to the most recent advancements in the understanding of the relationship between their structure and their function.

Chimeric photosynthetic reaction center complex of purple bacteria composed of the core subunits of Rubrivivax gelatinosus and the cytochrome subunit of Blastochloris viridis

A gene coding for the photosynthetic reaction center-bound cytochrome subunit, pufC, of Blastochloris viridis, which belongs to the alpha-purple bacteria, was introduced into Rubrivivax gelatinosus, which belongs to the beta-purple bacteria. The cytochrome subunit of B. viridis was synthesized in the R. gelatinosus cells, in which the native pufC gene was knocked out, and formed a chimeric reaction center (RC) complex together with other subunits of R. gelatinosus. The transformant was able to grow photosynthetically. Rapid photo-oxidization of the hemes in the cytochrome subunit was observed in the membrane of the transformant. The soluble electron carrier, cytochrome c(2), isolated from B. viridis was a good electron donor to the chimeric RC. The redox midpoint potentials and the redox difference spectra of four hemes in the cytochrome subunit of the chimeric RC were almost identical with those in the B. viridis RC. The cytochrome subunit of B. viridis seems to retain its structure and function in the R. gelatinosus cell. The chimeric RC and its mutagenesis system should be useful for further studies about the cytochrome subunit of B. viridis.

Natural resistance to inhibitors of the ubiquinol cytochrome c oxidoreductase of Rubrivivax gelatinosus: sequence and functional analysis of the cytochrome bc(1) complex

Biochemical analyses of Rubrivivax gelatinosus membranes have revealed that the cytochrome bc(1) complex is highly resistant to classical inhibitors including myxothiazol, stigmatellin, and antimycin. This is the first report of a strain exhibiting resistance to inhibitors of both catalytic Q(0) and Q(i) sites. Because the resistance to cytochrome bc(1) inhibitors is primarily related to the cytochrome b primary structure, the petABC operon encoding the subunits of the cytochrome bc(1) complex of Rubrivivax gelatinosus was sequenced. In addition to homologies to the corresponding proteins from other organisms, the deduced amino acid sequence of the cytochrome b polypeptide shows (i) an E303V substitution in the highly conserved PEWY loop involved in quinol/stigmatellin binding, (ii) other substitutions that could be involved in resistance to cytochrome bc(1) inhibitors, and (iii) 14 residues instead of 13 between the histidines in helix IV that likely serve as the second axial ligand to the b(H) and b(L) hemes, respectively. These characteristics imply different functional properties of the cytochrome bc(1) complex of this bacterium. The consequences of these structural features for the resistance to inhibitors and for the properties of R. gelatinosus cytochrome bc(1) are discussed with reference to the structure and function of the cytochrome bc(1) complexes from other organisms.

Different mechanisms of the binding of soluble electron donors to the photosynthetic reaction center of Rubrivivax gelatinosus and Blastochloris viridis

The tetraheme cytochrome subunits of the photosynthetic reaction centers (RCs) in two species of purple bacteria, Rubrivivax gelatinosus and Blastochloris (Rhodopseudomonas) viridis, were compared in terms of their capabilities to bind different electron-donor proteins. The wild-type RCs from both species and mutated forms of R. gelatinosus RCs (with amino acid substitutions introduced to the binding domain for electron-donor proteins) were tested for their reactivity with soluble cytochromes and high potential iron-sulfur protein. Cytochromes from both species were good electron donors to the B. viridis RC and the R. gelatinosus RC. The reactivity in the R. gelatinosus RC showed a clear dependence on the polarity of the charges introduced to the binding domain, indicating the importance of the electrostatic interactions. In contrast, high potential iron-sulfur protein, presumed to operate according to the hydrophobic mechanism of binding, reacted significantly only with the R. gelatinosus RC. Evolutionary substitution of amino acids in a region of the binding domain on the cytochrome subunit surface probably caused the change in the principal mode of protein-protein interactions in the electron-transfer chains.