The distribution of soluble metallo-redox proteins in purple phototrophic bacteria

Genome Sequences of Marichromatium gracile HOL-1 and Its Purple Photosynthetic Coisolate, Afifella sp. H1R

We sequenced the genomes of both the purple sulfur gammaproteobacterium Marichromatium gracile HOL-1 and another purple photosynthetic organism, strain H1R, that was originally isolated as an unidentified contaminant. Through genome sequencing, we have now identified organism H1R as a species of Afifella. A whole-genome-based phylogenetic analysis of both species is provided.

Electron Accepting Units of the Diheme Cytochrome c TsdA, a Bifunctional Thiosulfate Dehydrogenase/Tetrathionate Reductase

The enzymes of the thiosulfate dehydrogenase (TsdA) family are wide-spread diheme c-type cytochromes. Here, redox carriers were studied mediating the flow of electrons arising from thiosulfate oxidation into respiratory or photosynthetic electron chains. In a number of organisms, including Thiomonas intermedia and Sideroxydans lithotrophicus, the tsdA gene is immediately preceded by tsdB encoding for another diheme cytochrome. Spectrophotometric experiments in combination with enzymatic assays in solution showed that TsdB acts as an effective electron acceptor of TsdA in vitro when TsdA and TsdB originate from the same source organism. Although TsdA covers a range from -300 to +150 mV, TsdB is redox active between -100 and +300 mV, thus enabling electron transfer between these hemoproteins. The three-dimensional structure of the TsdB-TsdA fusion protein from the purple sulfur bacterium Marichromatium purpuratum was solved by X-ray crystallography to 2.75 Å resolution providing insights into internal electron transfer. In the oxidized state, this tetraheme cytochrome c contains three hemes with axial His/Met ligation, whereas heme 3 exhibits the His/Cys coordination typical for TsdA active sites. Interestingly, thiosulfate is covalently bound to Cys³³⁰ on heme 3. In several bacteria, including Allochromatium vinosum, TsdB is not present, precluding a general and essential role for electron flow. Both AvTsdA and the MpTsdBA fusion react efficiently in vitro with high potential iron-sulfur protein from A. vinosum (E_m +350 mV). High potential iron-sulfur protein not only acts as direct electron donor to the reaction center in anoxygenic phototrophs but can also be involved in aerobic respiratory chains.

Complete genome sequence of Allochromatium vinosum DSM 180(T)

Allochromatium vinosum formerly Chromatium vinosum is a mesophilic purple sulfur bacterium belonging to the family Chromatiaceae in the bacterial class Gammaproteobacteria. The genus Allochromatium contains currently five species. All members were isolated from freshwater, brackish water or marine habitats and are predominately obligate phototrophs. Here we describe the features of the organism, together with the complete genome sequence and annotation. This is the first completed genome sequence of a member of the Chromatiaceae within the purple sulfur bacteria thriving in globally occurring habitats. The 3,669,074 bp genome with its 3,302 protein-coding and 64 RNA genes was sequenced within the Joint Genome Institute Community Sequencing Program.

Study of the high-potential iron sulfur protein in Halorhodospira halophila confirms that it is distinct from cytochrome c as electron carrier

The role of high-potential iron sulfur protein (HiPIP) in donating electrons to the photosynthetic reaction center in the halophilic gamma-proteobacterium Halorhodospira halophila was studied by EPR and time-resolved optical spectroscopy. A tight complex between HiPIP and the reaction center was observed. The EPR spectrum of HiPIP in this complex was drastically different from that of the purified protein and provides an analytical tool for the detection and characterization of the complexed form in samples ranging from whole cells to partially purified protein. The bound HiPIP was identified as iso-HiPIP II. Its Em value at pH 7 in the form bound to the reaction center was approximately 100 mV higher (+140 +/- 20 mV) than that of the purified protein. EPR on oriented samples showed HiPIP II to be bound in a well defined geometry, indicating the presence of specific protein-protein interactions at the docking site. At moderately reducing conditions, the bound HiPIP II donates electrons to the cytochrome subunit bound to the reaction center with a half-time of < or =11 micros. This donation reaction was analyzed by using Marcus's outer-sphere electron-transfer theory and compared with those observed in other HiPIP-containing purple bacteria. The results indicate substantial differences between the HiPIP- and the cytochrome c2-mediated re-reduction of the reaction center.

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.

Structural and functional studies on the tetraheme cytochrome subunit and its electron donor proteins: the possible docking mechanisms during the electron transfer reaction

The photosynthetic reaction centers (RCs) classified as the group II possess a peripheral cytochrome (Cyt) subunit, which serves as the electron mediator to the special-pair. In the cycle of the photosynthetic electron transfer reactions, the Cyt subunit accepts electrons from soluble electron carrier proteins, and re-reduces the photo-oxidized special-pair of the bacteriochlorophyll. Physiologically, high-potential cytochromes such as the cytochrome c2 and the high-potential iron-sulfur protein (HiPIP) function as the electron donors to the Cyt subunit. Most of the Cyt subunits possess four heme c groups, and it was unclear which heme group first accepts the electron from the electron donor. The most distal heme to the special-pair, the heme-1, has a lower redox potential than the electron donors, which makes it difficult to understand the electron transfer mechanism mediated by the Cyt subunit. Extensive mutagenesis combined with kinetic studies has made a great contribution to our understanding of the molecular interaction mechanisms, and has demonstrated the importance of the region close to the heme-1 in the electron transfer. Moreover, crystallographic studies have elucidated two high-resolution three-dimensional structures for the RCs containing the Cyt subunit, the Blastochloris viridis and Thermochromatium tepidum RCs, as well as the structures of their electron donors. An examination of the structural data also suggested that the binding sites for both the cytochrome c2 and the HiPIP are located adjacent to the solvent-accessible edge of the heme-1. In addition, it is also indicated by the structural and biochemical data that the cytochrome c2 and the HiPIP dock with the Cyt subunit by c2 is recognized through electrostatic interactions while hydrophobic interactions are important in the HiPIP docking.

HiPIP in Rubrivivax gelatinosus is firmly associated to the membrane in a conformation efficient for electron transfer towards the photosynthetic reaction centre

High potential iron-sulfur protein (HiPIP), a small soluble redox protein, has been shown to serve in vivo as electron donor to the photosynthetic reaction centre (RC) in Rubrivivax gelatinosus [Biochemistry 34 (1995) 11736]. The results of time-resolved optical spectroscopy on membrane-fragments from this organism indicates that the photooxidized RC is re-reduced by HiPIP even in the absence of the soluble fraction. This implies that a significant fraction of HiPIP can firmly bind to the membrane in a conformation able to interact with the RCs. Salt treatment of the membrane-fragments abolishes these re-reduction kinetics, demonstrating the presence of HiPIP on the membrane due to association with the RC rather than due to simple trapping in hypothetical chromatophores. The existence of such a functional complex in membranes is confirmed and its structure further examined by electron paramagnetic resonance (EPR) performed on membrane-fragments. Orientation-dependent EPR spectra of HiPIP were recorded on partially ordered membranes, oxidized either chemically or photochemically. Whereas hardly any preferential orientation of the HiPIP was seen in the chemically oxidised sample, a subpopulation of HiPIP showing specific orientations could be photooxidised. This fraction arises from the electron transfer complex between HiPIP and the RC.

An Alternative to the Accepted Phylogeny of Purple Bacteria Based on 16S rRNA: Analyses of the Amino Acid Sequences of Cytochromes C2 and C556 from Rhodobacter (Rhodovulum) sulfidophilus

It is becoming increasingly apparent from complete genome sequences that 16S rRNA data, as currently interpreted, does not provide an unambiguous picture of bacterial phylogeny. In contrast, we have found that analysis of insertions and deletions in the amino acid sequences of cytochrome c2 has some advantages in establishing relationships and that this approach may have broad utility in acquiring a better understanding of bacterial relationships. The amino acid sequences of cytochromes c2 and c556 have been determined in whole or in part from four strains of Rhodobacter sulfidophilus. The cytochrome c2 contains three- and eight-residue insertions as well as a single-residue deletion in common with the large cytochromes c2 but in contrast to the small cytochromes c2 and mitochondrial cytochromes. In addition, the Rb. sulfidophilus protein shares a rare six- to seven-residue insertion with other Rhodobacter cytochromes c2. The cytochrome c556 is a low-spin class II cytochrome c homologous to the greater family of cytochromes c', which are usually high-spin. The similarity of cytochrome c556 to other species of class II cytochromes is consistent with the relationships deduced from comparisons of cytochromes c2. Thus, our results do not support placement of Rb. sulfidophilus in a separate genus, Rhodovulum, which was proposed primarily on the basis of 16S rRNA sequences. Instead, the Rhodobacter cytochromes c2 are distinct from those of other genera and species of purple bacteria and show a different pattern of relationships among species than reported for 16S rRNA.

Primary structure characterization of a Rhodocyclus tenuis diheme cytochrome c reveals the existence of two different classes of low-potential diheme cytochromes c in purple phototropic bacteria

The complete amino acid sequence of a 26-kDa low redox potential cytochrome c-551 from Rhodocyclus tenuis was determined by a combination of Edman degradation and mass spectrometry. There are 240 residues including two heme binding sites at positions 41, 44, 128, and 132. There is no evidence for gene doubling. The only known homolog of Rc. tenuis cytochrome c-551 is the diheme cytochrome c-552 from Pseudomonas stutzeri which contains 268 residues and heme binding sites at nearly identical positions. There is 44% overall identity between the Rc. tenuis and Ps. stutzeri cytochromes with 10 internal insertions and deletions. The Ps. stutzeri cytochrome is part of a denitrification gene cluster, whereas Rc. tenuis is incapable of denitrification, suggesting different functional roles for the cytochromes. Histidines at positions 45 and 133 are the fifth heme ligands and conserved histidines at positions 29, 209, and 218 and conserved methionines at positions 114 and 139 are potential sixth heme ligands. There is no obvious homology to the low-potential diheme cytochromes characterized from other purple bacterial species such as Rhodobacter sphaeroides. There are therefore at least two classes of low-potential diheme cytochromes c found in phototrophic bacteria. There is no more than 11% helical secondary structure in Rc. tenuis cytochrome c-551 suggesting that there is no relationship to class I or class II c-type cytochromes.

A membrane-bound flavocytochrome c-sulfide dehydrogenase from the purple phototrophic sulfur bacterium Ectothiorhodospira vacuolata

The amino acid sequence of Ectothiorhodospira vacuolata cytochrome c-552, isolated from membranes with n-butanol, shows that it is a protein of 77 amino acid residues with a molecular mass of 9,041 Da. It is closely related to the cytochrome subunit of Chlorobium limicola f. sp. thiosulfatophilum flavocytochrome c-sulfide dehydrogenase (FCSD), having 49% identity. These data allowed isolation of a 5.5-kb subgenomic clone which contains the cytochrome gene and an adjacent flavoprotein gene as in other species which have an FCSD. The cytochrome subunit has a signal peptide with a normal cleavage site, but the flavoprotein subunit has a signal sequence which suggests that the mature protein has an N-terminal cysteine, characteristic of a diacyl glycerol-modified lipoprotein. The membrane localization of FCSD was confirmed by Western blotting with antibodies raised against Chromatium vinosum FCSD. When aligned according to the three-dimensional structure of Chromatium FCSD, all but one of the side chains near the flavin are conserved. These include the Cys 42 flavin adenine dinucleotide binding site; the Cys 161-Cys 337 disulfide; Glu 167, which modulates the reactivity with sulfite; and aromatic residues which may function as charge transfer acceptors from the flavin-sulfite adduct (C. vinosum numbering). The genetic context of FCSD is different from that in other species in that flanking genes are not conserved. The transcript is only large enough to encode the two FCSD subunits. Furthermore, Northern hybridization showed that the production of E. vacuolata FCSD mRNA is regulated by sulfide. All cultures that contained sulfide in the medium had elevated levels of FCSD RNA compared with cells grown on organics (acetate, malate, or succinate) or thiosulfate alone, consistent with the role of FCSD in sulfide oxidation.

Amino acid sequences of two high-potential iron-sulfur proteins (HiPIPs) from the moderately halophilic purple phototrophic bacterium, Rhodospirillum salinarum

The amino acid sequences of two very different high-potential iron-sulfur protein (HiPIP) isozymes have been determined from the moderately halophilic purple phototrophic bacterium, Rhodospirillum salinarum. Iso-1 HiPIP, which is monomeric and contains 57 amino acid residues, is most similar to the Thiobacillus ferrooxidans iron-oxidizing enzyme (45% identity and a 6-residue deletion). On the other hand, iso-2 HiPIP, which is isolated as an oligomer, contains a peptide chain with 54 amino acid residues. It is the smallest reported to date and is only 31% identical to iso-1 HiPIP. A massive deletion of 17 residues is found at the N-terminus, such that only 2 residues remain prior to the first cysteine. Iso-2 HiPIP also has a 12-residue insertion and a 5-residue deletion. Prior to this study, there were only 2 absolutely conserved residues (Tyr 19 and Gly 75, Chromatium numbering) in addition to the 4 iron-sulfur cluster binding cysteine residues among the 13 HiPIPs sequenced to date. We found that Tyr 19 is absent in iso-2 HiPIP along with the entire N-terminal loop. Moreover, Gly 75 is substituted in both R. salinarum HiPIPs. These characteristics make the R. salinarum HiPIPs, and especially iso-2, the most divergent yet characterized.

On the role of high-potential iron-sulfur proteins and cytochromes in the respiratory chain of two facultative phototrophs

The capability of high potential iron-sulfur proteins (HiPIPs) and soluble cytochromes to shuttle electrons between the bc1 complex and the terminal oxidase in aerobically grown cells of Rhodoferax fermentans and Rhodospirillum salinarum, two facultative phototrophs, was evaluated. In Rs. salinarum, HiPIP and a c-type cytochrome (alpha-band at 550 nm, Em,7=+290 mV) are both involved in the electron transfer step from the bc1 complex to the terminal oxidase. Kinetic studies indicate that cytochrome c550 is more efficient than HiPIP in oxidizing the bc1 complex, and that HiPIP is a more efficient reductant of the terminal oxidase as compared to cytochrome c550. Rs. salinarum cells contain an additional c-type cytochrome (asymmetric alpha-band at 556 nm, Em,7=+180 mV) which is able to reduce the terminal oxidase, but unable to oxidize the bc1 complex. c-type cytochromes could not be isolated from Rf. fermentans, in which HiPIP, the most abundant soluble electron carrier, is reduced by the bc1 complex (zero-order kinetics) and oxidized by the terminal oxidase (first-order kinetics), respectively. These data, taken together, indicate for the first time that HiPIPs play a significant role in bacterial respiratory electron transfer.

Evidence for a novel type of iron cluster in the respiratory chain of the archaeon Sulfolobus metallicus

A new type of metal centre was detected in the membranes of the thermoacidophilic archaeon Sulfolobus metallicus. This centre has an S = 1/2 ground state in the oxidised form, yielding an axial EPR signal with g values at 2.035 (g(parallel)) and 1.97 (g(perpendicular)), optimally detected at 4.6-10 K; in the reduced form it is EPR silent (even spin). These magnetic properties point to a spin-coupled iron cluster, with a minimum of two iron ions. The centre has a high reduction potential of +350 mV, at pH 6.5. The physiological role of this novel centre was probed through a general characterisation of S. metallicus respiratory chain: this archaeon contains NADH and succinate dehydrogenase activities, and cytochromes b562, a586 and a600 on the oxygen reductase system. Since it is reduced in the presence of succinate, and taking into account its high reduction potential, this centre is proposed to be a functional analogue of the Rieske centres.

Cloning and sequencing of the gene encoding the high potential iron-sulfur protein (HiPIP) from the purple sulfur bacterium Chromatium vinosum

The gene encoding the high potential iron-sulfur protein (HiPIP) of Chromatium vinosum strain D (DSM 180T) was cloned from an EcoRI-HindIII digest of genomic DNA. A nucleotide sequence of 648 bp length was determined which contained the coding region and putative promoter and termination sites. The gene codes for a 122 residue 12761 Da protein. The C-terminal 85 residues are those of the previously biochemically determined sequence, whereas the N-terminal 37 residues constitute a leader peptide which shows characteristics of the double arginine signal sequences of complex cofactor containing periplasmic proteins.

The primary structure of Rhodoferax fermentans high-potential iron-sulfur protein, an electron donor to the photosynthetic reaction center

The complete amino acid sequence of Rhodoferax fermentans high-potential iron-sulfur protein (Hipip), which is known to be an efficient electron donor to the photosynthetic reaction center, has been determined using both N-terminal and C-terminal analyses. The sequence contains 75 residues, with 11 positive charges, 10 negative charges, and one histidine residue. The molecular mass of apo-Hipip, determined by electrospray ionization mass spectrometry, is 7849.64 Da. Multiple sequence alignment, based both on primary and tertiary structure information, reveals conservation of Tyr19 and Gly75 (Chromatium vinosum numbering) in addition to the four [Fe4S4]-bound cysteines. The Hipip from Rf. fermentans is most similar (57% similarity) to the Hipip from Rubrivivax gelatinosus, a photosynthetic bacterium belonging to the beta-1 subgroup of the proteobacteria.

Kinetics of photo-induced electron transfer from high-potential iron-sulfur protein to the photosynthetic reaction center of the purple phototroph Rhodoferax fermentans

The kinetics of photo-induced electrontransfer from high-potential iron-sulfur protein (HiPIP) to the photosynthetic reaction center (RC) of the purple phototroph Rhodoferarfermentans were studied. The rapid photooxidation of heme c-556 belonging to RC is followed, in the presence of HiPIP, by a slower reduction having a second-order rate constant of 4.8 x 10(7) M(-1) x s(-1). The limiting value of kobs at high HiPIP concentration is 95 s(-1). The amplitude of this slow process decreases with increasing HiPIP concentration. The amplitude of a faster phase, observed at 556 and 425 nm and involving heme c-556 reduction, increases proportionately. The rate constant of this fast phase, determined at 425 and 556 nm, is approximately 3 x 10(5) s(-1). This value is not dependent on HiPIP concentration, indicating that it is related to a first-order process. These observations are interpreted as evidence for the formation of a HiPIP-RC complex prior to the excitation flash, having a dissociation constant of -2.5 microM. The fast phase is absent at high ionic strength, indicating that the complex involves mainly electrostatic interactions. The ionic strength dependence of kobs for the slow phase yields a second-order rate constant at infinite ionic strength of 5.4 x 10(6) M(-1) x s(-1) and an electrostatic interaction energy of -2.1 kcal/mol (1 cal = 4.184 J). We conclude that Rhodoferar fermentans HiPIP is a very effective electron donor to the photosynthetic RC.

A high-potential soluble cytochrome c-551 from the purple phototrophic bacterium Chromatium vinosum is homologous to cytochrome c8 from denitrifying pseudomonads

A minor cytochrome c-551 component of Chromatium vinosum was previously found to efficiently couple electron transfer between the cytochrome bc1 complex and the photosynthetic reaction center. We have now determined the amino acid sequence of this cytochrome c-551 and find that it is homologous to cytochrome c8 (formerly called Pseudomonas cytochrome c-551). It is most similar to Methylophilus methylotrophus, Rhodocyclus tenuis, and Azotobacter vinelandii cytochromes c8 (respectively, 57%, 52% and 51%). The C. vinosum cytochrome c8 has a single residue insertion relative to Pseudomonas and Azotobacter cytochromes c8. It has fewer charged residues than its homologs and is essentially neutral, which may explain why it is less soluble than the others. The cytochromes c8 are only very distantly related to the cytochromes c2 found in other species of purple bacteria which are much larger in size and which usually mediate electron transfer between the cytochrome bc1 complex and the reaction center. The photosynthetic pathway in Chromatium thus appears to be radically different from that in purple non-sulfur bacteria.

HiPiP oxido-reductase activity in membranes from aerobically grown cells of the facultative phototroph Rhodoferax fermentans

The role of the periplasmically located, water-soluble, HiPIP (high-potential iron-sulfur protein) in the respiratory chain of the facultative phototroph Rhodoferax fermentans has been examined. The oxidized HiPIP is reduced by succinate-dependent respiration via the bc1 complex, this reaction being inhibited by myxothiazol and/or stigmatellin. The reduced HiPIP can be oxidized by the membrane-bound cytochrome oxidase, this reaction being inhibited by 0.1 mM cyanide. We conclude that aerobically grown Rf. fermentans contains a redox chain in which HiPIP mediates electron transfer between the bc1 complex and the cb-type cytochrome oxidase.

Reversible super-reduction of the cubane [4Fe-4S](3+;2+;1+) in the high-potential iron-sulfur protein under non-denaturing conditions. EPR spectroscopic and electrochemical studies

The reversible 2 x 1 e- reduction of the cubane cluster from oxidized to reduced to super-reduced states ([4Fe-4S]3+<-->[4Fe-4S]2+<-->[4Fe-4S]1+) was studied in high-potential iron-sulfur proteins (HiPIPs). Super-reduction to the 1+ state was not observed in any of the seven HiPIPs tested during cyclic voltammetry (down to -0.95 V). However, equilibration at low potential (pH 7.5) of Rhodopila globiformis HiPIP yields a transient peak around -0.47 V due to the oxidation of super-reduced HiPIP adsorbed at the electrode. The peak area depends on the equilibration potential according to a one-electron Nernst curve with a half-wave potential at -0.91 V. Reduction of R. globiformis HiPIP with titanium (III)citrate at pH 9.5 is very slow [pseudo-first-order half-life of 23 min with a 100-fold excess Ti(III)] but is reversible, and the EPR spectrum with g values of 2.04 and 1.92 is similar to that of reduced [4Fe-4S]1+ ferredoxins. Chemical or electrochemical reoxidation of the super-reduced form resulted in an EPR spectrum with g parallel = 2.12 and g perpendicular = 2.03, i.e. identical to that of oxidized HiPIP. From the equilibrium concentration of super-reduced HiPIP at a low concentration of Ti(III), a reduction potential of -0.64 V can be estimated. Super-reduction of the large HiPIP (iso-2) from Rhodospirillum salinarum is also possible with Ti(III)(gz = 2.05) but the super-reduced state is unstable. No super-reduction with Ti(III) was observed for the other HiPIPs. The difference between the electrochemically observed reduction potential and oxidation potential is explained by a fast and reversible conformational change upon super-reduction. The rate of super-reduction with Ti(III) is limited by the small amount (0.1%) of HiPIP in the 2+ state with the super-reduced conformation.

1H and 13C NMR assignments and structural aspects of a ferrocytochrome c-551 from the purple phototrophic bacterium Ectothiorhodospira halophila

Two-dimensional nuclear magnetic resonance was used to assign the 1H and 13C resonances of ferrocytochrome c-551 from Ectothiorhodospira halophila, a halophilic phototrophic purple bacterium. This 78-residue protein belongs to a small subgroup of class I cytochromes c together with the analogous cytochromes c-551 from E. halochloris and E. abdelmalekii. A nearly complete assignment of 13C resonances was obtained at natural abundance using a gradient-enhanced 1H-detected heteronuclear single quantum coherence experiment (HSQC). This was found to be extremely useful for the unambigous assignment of side chain protons. The secondary structure of the protein was determined from analyses of short- and medium-range nuclear Overhauser enhancements (NOE), amide proton exchange and 13C alpha chemical shifts. Three helices could be identified which are well conserved among the class I cytochromes c. There is some evidence for two other regions of less well defined helical structure. From a preliminary analysis of long-range NOE it is shown that in the E. halophila cytochrome c-551 the general cytochrome c fold is well conserved, including the three conserved helices (residues 2-8, 41-50, 63-76), the regions around the heme ligands (Cys14-Ser15-Ser16-Cys17-His18, Met55) and the omega loop (residues 18-28). In addition, three variable segments of the protein are discussed in detail, one of those including a cis-proline, a feature so far unique in the cytochrome c family. Structural alignments of the E. halophila cytochrome c-551 with two other Pseudomonas cytochrome c5 homologs (Azotobacter vinelandii cytochrome c5 and Chlorobium limicola cytochrome c-555) are provided which are based on sequence similarities and secondary structure alignments.

The high potential iron-sulfur protein (HiPIP) from Rhodoferax fermentans is competent in photosynthetic electron transfer

The functional role of the High Potential Iron-sulfur Protein (HiPIP) from the photosynthetic bacterium Rhodoferax fermentans was investigated. We demonstrated that the HiPIP increased the rate of light-induced oxygen reduction mediated by the photosynthetic reaction center (RC); this stimulation reached half-saturation at [HiPIP]/[RC] ca. 15. The capability of the HiPIP in delivering electrons to the reaction center of Rhodoferax fermentans was demonstrated through kinetic spectrophotometry of cytochrome c-556 oxidation in the presence or in the absence of HiPIP. It is concluded that the HiPIP is competent in the photosynthetic electron transfer chain of Rhodoferax fermentans.

A membrane-bound HIPIP type center in the thermohalophile Rhodothermus marinus

A HIPIP-type center was discovered in intact membranes of the thermohalophilic aerobe Rhodothermus marinus. In both the membrane-bound state and after detergent solubilization and partial purification, this center exhibits an almost axial EPR spectrum, with g-values at 2.13 and 2.03, similar to those of soluble HIPIP proteins isolated from purple bacteria. It has a high reduction potential, of 260 mV at pH 7.5. Rhodothermus HIPIP is involved in the main membrane-bound electron-transfer pathway, being reduced by NADH or succinate only in the presence of cyanide. The possible physiological function of this novel HIPIP-type center is discussed.

Iron metabolism in Rhodobacter capsulatus. Characterisation of bacterioferritin and formation of non-haem iron particles in intact cells

The water-soluble cytochrome b557 from the photosynthetic bacterium Rhodobacter capsulatus was purified and shown to have the properties of the iron-storage protein bacterioferritin. The molecular mass of R. capsulatus bacterioferritin is 428 kDa and it is composed of a single type of 18-kDa subunit. The N-terminal amino acid sequence of the bacterioferritin subunit shows 70% identity to the sequence of bacterioferritin subunits from Escherichia coli, Nitrobacter winogradskyi, Azotobacter vinelandii and Synechocystis PCC 6803. The absorbance spectrum of reduced bacterioferritin shows absorbance maxima at 557 nm (alpha band), 526 nm (beta band) and 417 nm (Soret band) from the six haem groups/molecule. Antibody assays reveal that bacterioferritin is located in the cytoplasm of R. capsulatus, and its levels stay relatively constant during batch growth under aerobic conditions when the iron concentration in the medium is kept constant. Iron deficiency leads to a decrease in bacterioferritin and iron overload leads to an increase. Bacterioferritin from R. capsulatus has an amorphous iron-oxide core with a high phosphate content (900-1000 Fe atoms and approximately 600 phosphates/bacterioferritin molecule). Mössbauer spectroscopy indicates that in both aerobically and anaerobically (phototrophically) grown cells bacterioferritin with an Fe3+ core is formed, suggesting that iron-core formation in vivo may not always require molecular oxygen.

Amino acid sequences of cytochromes c2 and c' from the moderately halophilic purple phototrophic bacterium Rhodospirillum salexigens

Rhodospirillum salexigens is a moderately halophilic purple phototrophic bacterium which grows optimally in 8% NaCl. The amino acid sequences of the two principal soluble cytochromes c have been determined. One of these is a cytochrome c2, similar in size to mitochondrial cytochrome c. While clearly of the same sequence class as mitochondrial cytochrome c and the proteins from several other Gram-negative bacteria, it does not show particular affinity to any already known sequence in terms of the percentage sequence identity. The other protein is a cytochrome c', but is also a divergent member of this widespread group. The lack of appreciable sequence identity to other species is probably due to a limit of divergence which has been reached for the majority of purple bacterial species. However, the numbers of insertions and deletions and their locations in cytochromes c2 and c' suggest that R salexigens may be related to Rhodospirillum molischianum. Like other electron transport proteins from halophiles, both of these cytochromes are notable for their high content of arginine as compared with lysine and both are acidic. However, they do not show any particular sequence homology to electron transport proteins that have been characterized from the extremely halophilic phototrophes of the genus Ectothiorhodospira. Thus, it appears that adaptation to halophilic habitats has independently occurred more than once in purple bacteria.

Evolution of photosynthetic reaction centers and light harvesting chlorophyll proteins

It is proposed that there is a single evolutionary origin for photosynthetic reaction centers and also for most light-harvesting chlorophyll proteins. It is generally accepted that the purple bacterial reaction center (quinone-reducing photosystem) and the plant and cyanobacterial PSII (oxygen-evolving photosystem) are homologous. It is also apparent that the green sulfur bacterial reaction center is homologous to cyanobacterial PSI (the pyridine nucleotide reducing photosystem). However, it is less obvious that PSI is related to the purple bacterial reaction center. It is herein proposed that PSI represents a gene fusion of the precursors of small light harvesting bacteriochlorophyll proteins from purple bacteria and purple bacterial reaction centers. Furthermore, it is proposed that reaction centers evolved from the membrane-spanning cytochrome b of the cytochrome bc1 complex and that most membrane-spanning cytochromes may have a common origin.

Interaction between cytochrome c and the photosynthetic reaction center of purple bacteria: behaviour at low temperature

In purple photosynthetic bacteria the electron donor to the special pair, after its oxidation by a light-induced reaction, is a c-type cytochrome: either a soluble monoheme cytochrome which forms a transitory complex with the reaction center, or a tetraheme cytochrome which remains permanently bound to the reaction center. The effects of low temperatures on electron transfer in the complex are presented and discussed. They provide estimates for the reorganization energy. The most prominent effect of low temperature is that a dominant fast phase of electron transfer becomes impossible at a temperature of around 250 K (monoheme cytochrome) or located between 250 K and 80 K according to the redox state (tetraheme cytochrome). This inhibition is attributed to a freezing-like transition of pools of water molecules which blocks structural changes of the protein which are normally associated with the cytochrome oxidation.

Structure and function of the tetraheme cytochrome associated to the reaction center of Roseobacter denitrificans

We have characterized the tetrahemic RC bound cytochrome isolated from the quasi-photosynthetic bacterium Roseobacter denitrificans in terms of absorption spectrum, redox property and orientation with respect to the membrane plane. The heme, designated H1, which possesses the highest redox midpoint potential (+290 mV), absorbs at 555 nm. Its plane makes an angle of 40 degrees with the membrane plane. The second high potential heme, H2 (+240 mV), peaks at 554 nm and makes a tilt of 55 degrees with the membrane. The two low potential hemes, L1 and L2, present a similar and rather high redox midpoint potential (+90 mV). They absorb at 553 nm and 550 nm. One of these hemes is oriented at 40 degrees while the other makes an angle of 90 degrees with the membrane plane. The soluble cytochrome c551 completes the cyclic electron transfer between the RC and the bc1 complex. Both the oxidation and the re-reduction of cytochrome c551 are diffusible processes. Under semi-aerobic conditions, one of the low potential hemes is photo-oxidized under illumination but only extremely slowly re-reduced. This explains the requirement of high aerobic conditions for growth of Roseobacter denitrificans cells in the light.

One- and two-dimensional NMR characterization of oxidized and reduced cytochrome c' from Rhodocyclus gelatinosus

1D and 2D NMR spectra of both the reduced and oxidized forms of cytochrome c' from Rhodocyclus gelatinosus have been recorded. The analysis of the pH dependence of the 1H NMR spectrum of the ferric form has been performed, and two main ionizing groups have been identified. By comparison of the pH dependence of the available spectra of cytochromes c', an ambiguity remaining from previous studies on related cytochromes c' has been solved. By means of 2D spectra, an assignment of all the paramagnetically shifted signals is proposed both for the ferrous and for the ferric forms.

Evolution of cytochromes and photosynthesis

The following is an outline of the direction of research into the evolutionary origins of photosynthesis as revealed by the study of cytochromes c. Determination of the numbers of kinds of cytochromes, their structures, their functional roles, and their distribution are the principal kinds of data being collected and analyzed. A hypothesis on the origin of photosynthesis is presented.