Cytochromes of the non-thiosulfate-utilizing green sulfur bacterium Chlorobium limicola

Flavocytochrome c-553 of the non-thiosulfateutilizing green sulfur bacterium Chlorobium limicola strain 6330 was partially purified by ion exchange column chromatography and ammonium sulfate fractionation (highest purity index obtained: A ₂₈₀/A _{417 red}=0.96). It is autoxidizable and located in the soluble fraction. This hemoprotein contains a flavin component and one heme per molecule. The dithionite reduced spectrum reveals the typical maxima of a c-type cytochrome: =553,5 nm; =523 nm; =417 nm, while the oxidized form shows a -band at 410 nm and two shoulders at 440 nm and 480 nm indicating the flavin component. The flavocytochrome is a basic protein with an isoelectric point at pH 9.0 (± 0.5), a redox potential of 65 mV, a molecular weight of 56,000. It participates in sulfide oxidation and shows neither adenylylsulfate reductase nor sulfite reductase activity. C. limicola further contains a soluble cytochrome c-555 (highest purity index obtained: A ₂₈₀/A _{412 ox}=0.13; isoelectric point between pH 9.5 and 10) and the non-heme iron-containing proteins rubredoxin and ferredoxin, but lacks cytochrome c-551. Besides these soluble electron transfer proteins a membrane-bound c-type cytochrome (=554,5 nm) can be detected spectrophotometrically.Non-common abbreviations HIPIP high-potential iron sulfur protein - APS adenylylsulfate     

Cytochromes of the green sulfur bacterium Chlorobium vibrioforme f. thiosulfatophilum. Purification,characterization and sulfur metabolism

Three cytochromes of the thiosulfate-utilizing green sulfur bacterium Chlorobium vibrioforme f. thiosulfatophilum were highly purified by ion exchange column chromatography and ammonium sulfate fractionation. All three cytochromes are located in the soluble fraction. Cytochrome c-551 (highest purity index obtained: A₂₈₀/A₄₁₆=0.39) shows maxima at 551 nm (-band), 521 nm (-band), and 416 nm (-band) for the reduced form. This cytochrome is an acidic protein with a molecular weight of 32,000, a redox potential of 150 mV, and an isoelectric point at pH 6.0. Cytochrome c-553 (highest purity index obtained: A₂₈₀/A₄₁₇=0.8) is also an acidic protein with maxima at 553,5 nm, 523,5 nm and 417 nm for the reduced form, a molecular weight of 63,000, a redox potential of 90 mV, an isoelectric point at pH 6.3, and it contains FAD as flavin component. It is autoxidizable and participates in sulfide oxidation, but cannot catalyze the reverse reaction. The cytochrome c-555 (highest purity index obtained: A₂₈₀/A₄₁₈=0.16) is a small basic protein with maxima at 555 nm, 523 nm and 418 nm (reduced form), a molecular weight of 12,500, an isoelectric point between pH 10 and 10.5, and a redox potential of 155 mV. The ratio of the cytochrome contents to each other is constant and does not change when the organism has only thiosulfate or sulfide as the main electron donor in the medium.The soluble fraction further contains the non-heme ironcontaining proteins rubredoxin and ferredoxin. The anaerobic sulfide oxidation in a growing culture of Chlorobium vibrioforme f. thiosulfatophilum is accompanied by a rapid formation of thiosulfate, which is only utilized when sulfide is no longer available, while the elemental sulfur concentration increases constantly until thiosulfate is consumed.Non-common abbreviations C Chlorobium - SDS sodium dodecylsulfate - HIPIP high-potential-iron-sulfur-protein     

Some properties and occurrence of cytochrome c-552 in the aerobic photosynthetic bacterium Roseobacter denitrificans

Characteristics and occurrence of cytochrome c-552 from an aerobic photosynthetic bacterium, Roseobacter denitrificans, were described.Relative molecular mass of the cytrochrome was 13.5 kDa on sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) and 15,000 by gel filtration. This cytochrome was a acidic protein having a pI of 5.6 and E_m was +215 mV at pH 7.0. Absorption peaks were at 278, 408 and 524 nm in the oxidized form and 416, 523 and 552 nm in the reduced form.Amino acid composition and N-terminal amino acid sequence of cytochrome c-552 determined for 24 residues had low similarities to those of cytochrome c-551 of this bacterium, which is homologous to cytochrome c ₂, although the physico-chemical properties of these two cytochromes were similar to each other.Cytochrome c-552 was maximally synthesized in the light under aerobic conditions but not in the dark. The synthesis also occurred in the presence of alternative acceptors such as trimethylamine N-oxide (TMAO) and nitrate under anaerobic conditions. Our results suggest that cytochrome c-552 is involved in TMAO respiration and denitrification in R. denitrificans, although the effect of light remains to be solved.Abbreviations E_m Midpoint redox potential - PAGE Polyacrylamide ge electrophoresis - SDS-PAGE Sodium dodecyl sulfate polyacrylamide gel electrophoresis - TMAO Trimethylamine N-oxide     

Sulfur oxidation in mutants of the photosynthetic green sulfur bacterium Chlorobium tepidum devoid of cytochrome c-554 and SoxB

A mutant devoid of cytochrome c-554 (CT0075) in Chlorobium tepidum (syn. Chlorobaculum tepidum) exhibited a decreased growth rate but normal growth yield when compared to the wild type. From quantitative determinations of sulfur compounds in media, the mutant was found to oxidize thiosulfate more slowly than the wild type but completely to sulfate as the wild type. This indicates that cytochrome c-554 would increase the rate of thiosulfate oxidation by serving as an efficient electron carrier but is not indispensable for thiosulfate oxidation itself. On the other hand, mutants in which a portion of the soxB gene (CT1021) was replaced with the aacC1 cassette did not grow at all in a medium containing only thiosulfate as an electron source. They exhibited partial growth yields in media containing only sulfide when compared to the wild type. This indicates that SoxB is not only essential for thiosulfate oxidation but also responsible for sulfide oxidation. An alternative electron carrier or electron transfer path would thus be operating between the Sox system and the reaction center in the mutant devoid of cytochrome c-554. Cytochrome c-554 might function in any other pathway(s) as well as the thiosulfate oxidation one, since even green sulfur bacteria that cannot oxidize thiosulfate contain a cycA gene encoding this electron carrier.     

Cytochromes in Thiobacillus tepidarius and the respiratory chain involved in the oxidation of thiosulphate and tetrathionate

Thiobacillus tepidarius was shown to contain cytochrome(s) c with absorption maxima at 421, 522 and 552 nm in room temperature reduced minus oxidized difference spectra, present at 1.1–1.2 nmol per mg dry wt and present in both membrane and soluble fractions of the cell. The membrane-bound cytochrome c (1.75 nmol per mg membrane protein) had a midpoint potential (E_m, pH 7.0) of 337 mV, while the soluble fractions appeared to contain cytochrome(s) c with E_m (pH 7.0) values of about 270 and 360 mV. The organism also contained three distinct membrane-bound b-type cytochromes (totalling 0.33 nmol per mg membrane protein), each with absorption maxima in reduced minus oxidized difference spectra at about 428, 532 and 561 nm. The E_m (pH 7.0) values for the three cytochromes b were 8 mV (47.8% of total), 182 mV (13.7%) and 322 mV (38.5%). No a- or d-type cytochromes were detectable spectrophotometrically in the intact organism or its membrane and soluble fractions. Evidence is presented for both CO-binding and CO-unreactive cytochromes b or o, and CO-binding cytochrome(s) c. From redox effects observed with CO it is proposed that a cytochrome c donates electrons to a cytochrome b, and that a high potential cytochrome b or o may be acting as the terminal oxidase in substrate oxidation. This may be the 445 nm pigment, a photodissociable CO-binding membrane haemoprotein. Substrate oxidation was relatively insensitive to CO-inhibition, but strongly inhibited by cyanide and azide. Thiosulphate oxidation couples directly to cytochrome c reduction, but tetrathionate oxidation is linked (probably via ubiquinone Q-8) to reduction of a cytochrome b of lower potential than the cytochrome c. The nature of possible electron transport pathways in Thiobacillus tepidarius is discussed. One speculative sequence is: c^– b₈ b₁₈₂ c₂₇₀ c₃₃₇ b₃₂₂/c₃₆₀ O₂ Abbreviations E_m midpoint electrode potential - E _inf0 ^sup pH 7, standard electrode potential at pH 7.0 - Q-8 coenzyme Q-8 (ubiquinone-40)     

The major soluble cytochromes of the obligately aerobic sulfur bacterium,Thiobacillus neapolitanus

Four cytochromes were isolated from soluble extracts of the aerobic sulfur bacterium, Thiobacillus neapolitanus. The two most abundant proteins were purified to homogeneity and thoroughly characterized. Cytochrome c-554 (547) is a monomeric, small molecular weight protein which is unusual in having two well-resolved alpha peaks in UV-visible absorption spectra. The redox potential is 208 mV. Native cytochrome c-549 is oligometric, but has a subunit size of about 26.000. The yield of this protein could be improved dramatically by washing membranes with 30% ammonium sulfate, but the material solubilized by this method had a larger native molecular weight than that in the initial 0.1 M Tris-Cl extract and behaved differently on chromatography. The properties of cytochrome c-549 including subunit size and UV-visible absorption spectra are similar to mitochondrial cytochrome c ₁ and chloroplast cytochrome f, which suggests that it may be a modified form of the predominant membrane cytochrome. Based on cytochrome content, it is suggested that T. neapolitanus is not closely related to other thiobacilli.Dedicated to Prof. Dr. G. Drews on the occasion of his sixtieth birthday     

Highly efficient integration of foreign DNA into the genome of the green sulfur bacterium,Chlorobium vibrioforme by homologous recombination

Highly efficient and reproducible transformation ofChlorobium vibrioforme with plasmid DNA has been achieved by electroporation. Specific parameters have been optimized for the electrotransformation procedure. The method was developed using a construct containing a full copy of thepscC gene encoding the cytochromec ₅₅₁ subunit of the photosynthetic reaction center complex and theaadA gene encoding streptomycin resistance as selectable marker. Southern blotting analysis showed that the tested colonies were true transformants with the plasmid integrated into the genome by single homologous recombination. No transformants were obtained using the vector without thepscC gene showing that this vector does not replicate inC. vibrioforme. Thus transformation is possible only by homologous recombination. When using constructs designed to inactivate thepscC gene by insertion no transformants were obtained, indicating that the gene is indispensable for growth. The vector pVS2 carrying genes for erythromycin and chloramphenicol resistance was shown to replicate inC. vibrioforme. The two transformations shown here, provide an important genetical tool in the further analysis of structure and function of the photosynthetic apparatus in green sulfur bacteria.     

Utilization of sulfide and elemental sulfur by Ectothiorhodospira halochloris

Ectothiorhodospira halochloris grows photoheterotrophically with a variety of sulfur sources. During sulfide oxidation to elemental sulfur considerable amounts of polysulfides may be accumulated transiently. When grown on elemental sulfur no sulfate was produced by oxidation, but sulfide and polysulfide were formed by reduction. Only one soluble cytochrome c-551 was isolated and purified. It was a small acidic hemeprotein with a molecular weight of 6,300, an isoelectric point of 3.1 and a redox potential of-11 mV at pH 7.0. It showed three absorption maxima in the reduced state (=551 nm; =523 nm; =417 nm). The addition of various c-type cytochromes to a suspension of spheroplasts stimulated the velocity of sulfide oxidation. This stimulation was best with the small acidic cytochromes from E. halochloris or Ectothiorhodospira abdelmalekii. Sulfide oxidation was stopped by several uncoupling agents, ionophores and electron transport inhibitors. Antimycin A, rotenone and cyanide had no effect on sulfide oxidation.Dedicated to Prof. Dr. H. G. Schlegel on the occasion of his 60th birthday     

Evidence for a dual role of cytochrome c-553 and plastocyanin in photosynthesis and respiration of the cyanobacterium,Anabaena variabilis

The cytochrome oxidase activity (oxygen uptake in the dark) of a membrane preparation from Anabaena variabilis was found to be stimulated by cytochrome c-553 and plastocyanin obtained from this alga. Cytochrome c from horse heart was as active as cytochrome c-553, whereas little or no stimulation of oxygen uptake was obtained with cytochromes c ₂ from two Rhodospirillaceae, the plastidic cytochrome c-552 from Euglena, and plastocyanin from spinach. Cytochrome c-553 (A. variabilis) stimulated photosystem 1 activity in the same preparation much more than cytochrome c (horse heart). The results indicate that cytochrome c-553 and plastocyanin, besides their established function as electron donors of photosystem 1, participate in respiratory electron transport as reductants of a terminal oxidase. Photooxidation and dark oxidation show a different donor specificity.Abbreviations Chl chlorophyll a - TMPD N,N,N,N-tetramethyl-p-phenylenediamine     

Purification,primary structure,and evolution of cytochrome c-550 from the magnetic bacterium,Magnetospirillum magnetotacticum

Cytochrome c-550 was purified from Magnetospirillum magnetotacticum to an electrophoretically homogeneous state, and some of its properties were determined. The cytochrome showed absorption peaks at 528 and 409 nm in the oxidized form, and at 550, 521, and 414 nm in the reduced form. Its midpoint redox potential at pH 7.0 was determined to be +289 mV. The primary structure of cytochrome c-550 was determined. Cytochrome c is composed of 97 amino acid residues, and its molecular weight was calculated to be 10,873, including heme c. Its primary structure is very similar to those of Rhodospirillum fulvum and Rhodospirillum molischianum cytochromes c ₂, suggesting that M. magnetotacticum is phylogenetically related to photosynthetic bacteria.     

ATP-linked citrate lyase activity in the green sulfur bacterium Chlorobium limicola forma thiosulfatophilum

Cell-free extracts of the green sulfur bacterium Chlorobium limicola forma thiosulfatophilum strains 1C and L have been shown to cleave citrate with the formation of oxaloacetate and acetyl-CoA. This capacity was found in autotrophically grown cells as well as in the cells grown on media with acetate or L-glutamate. Citrate lyase activity in cell-free extracts is only measurable in the presence of citrate, adenosine-5-triphosphate, coenzyme A and Mg²⁺ or Mn²⁺. It is concluded on the basis of the obtained data that C. limicola f. thiosulfatophilum contains adenosine-5-triphosphate-linked citrate lyase (E.C.4.1.3.8). In contrast to green bacteria in the purple bacteria Ectothiorhodospira shaposhnikovii, Rhodospirillum rubrum and Thiocapsa roseopersicina citrate lyase activity was not found.     

The lipopolysaccharide of the green sulfur bacterium Chlorobium vibrioforme f. thiosulfatophilum

The cell wall lipopolysaccharide of the green sulfur bacterium Chlorobium vibrioforme f. thiosulfatophilum was obtained by the phenol-chloroform-petroleum ether and the hot phenol-water methods, respectively. It contained mannose, glucose, galacturonic acid, glucosamine, glycine, and small amounts of rhamnose, galactose and glucuronic acid. In addition to d-glycero-d-mannoheptose, the corespecific constituents 2-keto-3-deoxyoctonate and l-glycero-d-mannoheptose were found. Polyacrylamide gel-electrophoresis in the presence of sodium deoxycholate gave no indication for the presence of O-specific repeating units. Degradation of the lipopolysaccharide required 10% acetic acid (100° C, 2 h). The lipid A moiety contained the total of glucosamine of the lipopolysaccharide as well as small amounts of 2,3-diamino-2,3-dideoxy-glucose. It was phosphate-free. The fatty acid spectrum comprised 3-OH-14:0, 3-OH-16:0, and iso-3-OH-18:0 besides little 12:0, 14:0 and 16:0. Hydroxylaminolysis and sodium methylate treatment revealed all of the three hydroxy fatty acids to be amidebound.Abbreviations DOC sodium deoxycholate - PAGE polyacrylamide gel-electrophoresis     

Kinetics of electron transfer between soluble cytochrome c-554 and purified reaction center complex from the green sulfur bacterium Chlorobium tepidum

Soluble cytochrome c-554 (M _r 10 kDa) is purified from the green sulfur bacterium Chlorobium tepidum. Its midpoint redox potential is determined to be +148 mV from redox titration at pH 7.0. The kinetics of cytochrome c-554 oxidation by a purified reaction center complex from the same organism were studied by flash absorption spectroscopy at room temperature, and the results indicate that the reaction partner of cytochrome c-554 is cytochrome c-551 bound to the reaction center rather than the primary donor P840. The second-order rate constant for the electron donation from cytochrome c-554 to cytochrome c-551 was estimated to be 1.7×10⁷ M^–1 s^–1. The reaction rate was not significantly influenced by the ionic strength of the reaction medium.This revised version was published online in October 2005 with corrections to the Cover Date.     

An isolated reaction center complex from the green sulfur bacterium Chlorobium vibrioforme can photoreduce ferredoxin at high rates

Chlorosome-depleted membranes and a reaction center complex with well-defined subunit composition were prepared from the green sulfur bacterium Chlorobium vibrioforme under anaerobic conditions. The reaction center complex contains a 15-kDa polypeptide with the N-terminal amino acid sequence MEPQLSRPETASNQVR/. This sequence is nearly identical to the N-terminus of the pscD gene product from Chlorobium limicola (Hager-Braun et al. (1995) Biochemistry 34: 9617–9624). In the presence of ferredoxin and ferredoxin:NADP⁺ oxidoreductase, the membranes and the isolated reaction center complex photoreduced NADP⁺ at rates of 333 and 110 mol (mg bacteriochlorophyll a)^–1 h^–1, respectively. This shows that the isolated reaction center complex contains all the components essential for steady state electron transport. Midpoint potentials at pH 7.0 of 160 mV for cytochrome c ₅₅₁ and of 245 mV for P840 were determined by redox titration. Antibodies against cytochrome c ₅₅₁ inhibit NADP⁺ reduction while antibodies against the bacteriochlorophyll a-binding Fenna-Matthews-Olson protein do not.Abbreviations FMO protein Fenna-Matthews-Olson protein - TMBZ 3,3,5,5-tetramethylbenzidine     

Influence of growth phase and carbon source on the content of rubredoxin in Acinetobacter calcoaceticus

The rubredoxin content of Acinetobacter calcoaceticus in dependence on the carbon source (acetate, n-alkanes, succinate, L-malate) and on the growth phase was studied by means of a radioimmunoassay. The method used was specific for rubredoxin from Acinetobacter calcoaceticus. The formation of rubredoxin increased with time up to the end of the logarithmic phase when n-alkanes were the sole carbon source. After growth of Acinetobacter calcoaceticus on non-hydrocarbon substrates, rubredoxin was not detected.     

Chlorosomes of green sulfur bacteria: Pigment composition and energy transfer

The pigment composition and energy transfer pathways in isolated chlorosomes ofChlorobium phaeovibrioides andChlorobium vibrioforme were studied by means of high performance liquid chromatography (HPLC) and picosecond absorbance difference spectroscopy. Analysis of pigment extracts of the chlorosomes revealed that they contain small amounts of bacteriochlorophyll (BChl)a esterified with phytol, whereas the BChlsc, d ande are predominantly esterified with farnesol. The chlorosomal BChla content inC. phaeovibrioides andC. vibrioforme was found to be 1.5% and 0.9%, respectively. The time resolved absorbance difference spectra showed a bleaching shifted to longer wavelengths as compared to the Q_y absorption maxima and in chlorosomes ofC. vibrioforme also an absorbance increase at shorter wavelengths was observed. These spectral features were ascribed to excitation of oligomers of BChle and BChlc/d, respectively. One-color and two-color pump-probe kinetics ofC. phaeovibrioides showed rapid energy transfer to long-wavelength absorbing BChle oligomers, followed by trapping of excitations by BChla with a time constant of about 60 ps. Time resolved anisotropy measurements inC. vibrioforme showed randomization of excitations among BChla molecules with a time constant of about 20 ps, indicating that BChla in the baseplate is organized in clusters. One-color and two-color pump-probe measurements inC. vibrioforme showed rapid energy transfer from short-wavelength to long-wavelength absorbing oligomers with a time constant of about 11 ps. Trapping of excitations by BChla in this species could not be resolved unambiguously due to annihilation processes in the BChla clusters, but may occur with time constants of 15, 70 and 200 ps.     

Purification and characterisation of periplasmic C-type cytochromes from Desulfovibrio desulfuricans (NCIMB 8372)

Periplasmic extract from Desulfovibrio desulfuricans (NCIMB 8372) was found to contain two different c-type cytochromes. One is tetraheme cytochrome c₃ and the other is monoheme cytochrome c₅₅₃. Cytochrome c₃ could be purified by a procedure involving only one chromatographic step, whereas cytochrome c₅₅₃ required several such steps. Cytochrome c₃ was found to have a relative molecular mass of 14300 and an isoionic point higher than 9. Analysis of the redox potentials indicated one heme at -260 mV and three hemes around -330 mV. Cytochrome c₅₅₃ had a relative molecular mass of 7200, an isoionic point higher than 9 and a redox potential of 0 mV.     

Study of the chlorosomal antenna of the green mesophilic filamentous bacterium Oscillochloris trichoides

Whole cells, chlorosome-membrane complexes and isolated chlorosomes of the green mesophilic filamentous bacterium Oscillochloris trichoides, representing a new family of the green bacteria Oscillochloridaceae, were studied by optical spectroscopy and electron microscopy. It was shown that the main light-harvesting pigment in the chlorosome is BChl c. The presence of BChl a in chlorosomes was visualized only by pigment extraction and fluorescence spectroscopy at 77 K. The molar ratio BChl c: BChl a in chlorosomes was found to vary from 70:1 to 110:1 depending on light intensity used for cell growth. Micrographs of negatively and positively stained chlorosomes as well as of ultrathin sections of the cells were obtained and used for morphometric measurements of chlorosomes. Our results indicated that Osc. trichoides chlorosomes resemble, in part, those from Chlorobiaceae species, namely, in some spectral features of their absorption, fluorescence, CD spectra, pigment content as well as the morphometric characteristics. Additionally, it was shown that similar to Chlorobiaceae species, the light-harvesting chlorosome antenna of Osc. trichoides exhibited a highly redox-dependent BChl c fluorescence. At the same time, the membrane B805–860 BChl a antenna of Osc. trichoides is close to the membrane B808–866 BChl a antenna of Chloroflexaceae species. This revised version was published online in June 2006 with corrections to the Cover Date.     

Spectroscopic studies of bound cytochrome c and an iron-sulfur center in a purified reaction center complex from the green sulfur bacterium Chlorobium tepidum

Flash-induced optical kinetics at room temperature of cytochrome (Cyt) c ₅₅₁ and an Fe-S center (CF_A/CF_B) bound to a purified reaction center (RC) complex from the green sulfur photosynthetic bacterium Chlorobium tepidum were studied. At 551 nm, the flash-induced absorbance change decayed with a t _1/2 of several hundred ms, and the decay was accelerated by 1-methoxy-5-methylphenazinium methyl sulfate (mPMS). In the blue region, the absorbance change was composed of mPMS-dependent (Cyt) and mPMS-independent component (CF_A/CF_B) which decayed with a t _1/2 of 400–650 ms. Decay of the latter was effectively accelerated by benzyl viologen (Em –360 mV) and methyl viologen (–440 mV), and less effectively by triquat (–540 mV). The difference spectrum of Cyt c had negative peaks at 551, 520 and 420 nm, with a positive rise at 440 to 500 nm. The difference spectrum of CF_A/CF_B resembled P430 of PSI, and had a broad negative peak at 430435 nm.Abbreviations (B)Chl (bacterio)chlorophyll - Cyt cytochrome - F_A, F_B and F_X iron-sulfur center A, B and X of Photosystem I - CF_A, CF_B and CF_X F_A-,F_B- and F_X-like Fe-S center of Chlorobium - mPMS 1-methoxy-5-methylphenazinium methyl sulfate - PSI Photosystem I - RC reaction center     

Isolation and characterization of the membrane-bound cytochrome c-554 from the thermophilic green photosynthetic bacterium Chloroflexus aurantiacus

The membrane-bound photooxidizable cytochrome c-554 from Chloroflexus aurantiacus has been purified. The purified protein runs as a single heme staining band on SDS-PAGE with an apparent molecular mass of 43 000 daltons. An extinction coefficient of 28 ± 1 mM^–1 cm^–1 per heme at 554 nm was found for the dithionite-reduced protein. The potentiometric titration of the hemes takes place over an extended range, showing clearly that the protein does not contain a single heme in a well-defined site. The titration can be fit to a Nernst curve with midpoint potentials at 0, +120, +220 and +300 mV vs the standard hydrogen electrode. Pyridine hemochrome analysis combined with a Lowry protein assay and the SDS-PAGE molecular weight indicates that there are a minimum of three, and probably four hemes per peptide. Amino acid analysis shows 5 histidine residues and 29% hydrophobic residues in the protein. This cytochrome appears to be functionally similar to the bound cytochrome from Rhodopseudomonas viridis. Both cytochrome c-554 from C. aurantiacus and the four-heme cytochrome c-558-553 from R. viridis appear to act as direct electron donors to the special bacteriochlorophyll pair of the photosynthetic reaction center. They have a similar content of hydrophobic amino acids, but differ in isoelectric point, thermodynamic characteristics, spectral properties, and in their ability to be photooxidized at low temperature.Abbreviations LDAO lauryl dimethyl amine-N-oxide - SDS-PAGE sodium dodecyl sulfate polyacrylamide gel electrophoresis - mV millivolt - E_m.8 midpoint potential at pH 8.0 - ODV optical density x volume in ml