Photosynthesis Research 43:107-112, 1995. (~) 1995 Kluwer Academic Publishers. Printed h~ the Netherlands.
Regular paper
Spectroscopic studies of bound cytochrome c and an iron-sulfur center in a purified reaction center complex from the green sulfur bacterium
Chlorobium tepidum N o r i a k i K u s u m o t o 1, K a z u h i t o I n o u e 2 & H i d e h i r o S a k u r a i 1
1Department of Biology, School of Education, Waseda University, Tokyo 169-50, Japan; 2Department of Biological Sciences, Faculty of Science, Kanagawa University, 2946 Tsuchiya, Hiratsuka, Kanagawa 259-12, Japan Received 17 October 1994; accepted in revised form 7 December 1994
Key words: cytochrome c, iron-sulfur center, electron transfer, reaction center, flash photolysis spectroscopy, Chlorobium tepidum
Abstract Flash-induced optical kinetics at room temperature ofcytochrome (Cyt) CSsl and an Fe-S center (CFA/CFB) 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 tl/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 (CFA/CFB) which decayed with a h/2 of ,-~400-650 ms. Decay of the latter was effectively accelerated by benzyl viologen (Em - 3 6 0 mV) and methyl viologen ( - 4 4 0 mV), and less effectively by triquat ( - 5 4 0 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 CFA/CFB resembled P430 of PSI, and had a broad negative peak at 430,,~435 nm.
Abbreviations: (B)Chl - (bacterio)chlorophyll; Cyt - cytochrome; FA, FB and Fx - iron-sulfur center A, B and X of Photosystem I; CFA, CFB and CFx - FA-,FB- and Fx-like Fe-S center of Chlorobium; mPMS - l-methoxy-5methylphenazinium methyl sulfate, PSI - Photosystem I; RC - reaction center Introduction There is growing evidence which indicates that the RC of green sulfur bacteria and heliobacteria have similarities, on the acceptor side, to P S I of plants and cyanobacteria (for review of green sulfur bacteria, see Olson (1980), Mathis (1990), Blankenship (1992), Golbeck (1993); for heliobacteria, see Prince et al. (1985), Nitschke et al. (1990b), Trost et al. (1992), Kleinherenbrink et al. (1994) and the references cited therein). The RC of these bacteria can reduce ferredoxin without the requirement of a proton-motive force, and binds Chl-like pigments as the primary electron acceptor (van de Meent et al. 1992) and several Fe-S
centers as early acceptors (Jennings and Evans 1977; Swarthoff et al. 1981a; Prince et al. 1985; Nitschke et al. 1990a,b; Kusumoto et al. 1992; Oh-oka et al. 1993; Kusumoto et al. 1994; Kj~er et al. 1994). Moreover, the deduced amino acid sequences of the RC protein of C. limicola (Btittner et al. 1992) and Heliobacillus mobilis (Liebl et al. 1993) and of Fe-S protein (Illinger et al. 1993) of C. limicola show distinct similarity to those of PS I. In contrast with PS I, the RC of green sulfur bacteria binds Cyt c on the donor side (Hurt and Hauska 1984; Okkels et al. 1992; Oh-oka et al. 1993; Kusumoto et al. 1994). Although functioning of Fe-S centers in RCs of green sulfur bacteria was repeatedly observed by EPR spectroscopy, only
108 a continuous-light induced optical difference spectrum of one of them was reported (Swarthoffet al. 198 la). In Heliobacteria, Kleinherenbrink et al. (1994) recently reported the difference spectrum of Fx from the analysis of a 20-ms decay component in membrane preparations treated with urea which destroyed functional FA and FB. The flash-induced optical difference spectra of Fe-S centers, a counterpart known in P S I as P430 (Hiyama and Ke 197 la), and their reoxidation kinetics have not been determined yet. Until recently, only few preliminary reports had been presented regarding the kinetics of electron transfer by bound Cyt in a purified PC complex. Kinetic studies of these components were hampered by the lack of photochemically efficient RC preparations. In the RC preparation of Swarthoff et al. (1981b) from P aestuarii, the efficiency of photooxidation of Cyt c553 was low. In the RC preparation of Oh-oka et al. from C. limicola (Oh-oka et al. 1993), only 5% of the bound Cyt was oxidizable by flash. With such limited RC preparations, it is difficult to judge if the kinetic properties obtained are related to main pathway or side pathway of electron transfer. By taking great care to maintain anaerobic conditions throughout the course of preparation, we recently succeeded in preparing an RC complex from the thermophilic green sulfur bacterium Chlorobium tepidum which could photooxidize ,,~60% of the bound Cyt c551 with a single flash excitation, as well as with continuous illumination at room temperature. The RC complex contained about 50-70 BChl molecules and 1.21.7 photooxidizable Cyt per P840, three bound Fe-S centers (CFA, CFB, CFx; C for Chlorobium), and was composed of 5--6 peptides (Kusumoto et al. 1994). Since a preliminary investigation indicated the feasibility of this RC preparation for flash kinetic optical studies, in this study we measured reduction kinetics of photooxidized Cyt c551 and oxidation kinetics of a photoreduced Fe-S center, and obtained their optical oxidation-reduction difference spectra.
Materials and methods
Chlorobium tepidum cells were grown (Wahlund et al. 1991) and the RC complex was purified (Kusumoto et al. 1994) as described. Briefly, with N2 gas, cells were disrupted using a cell disintegrator (BioNebulizer, prototype developed by Togasaki, Surzycki and Kitayama at Indiana University). Chromatophore membranes were obtained by centrifugation, and the RC complexes were solubilized from the membranes with Triton
X-100. The RC complexes were purified by sucrose density gradient centrifugation, DEAE chromatography and hydroxyapatite chromatography. All the operations were conducted under anaerobic conditions. The purified RC preparation could be stored frozen at - 8 0 °C for at least 3 months without appreciable loss of activity. Xenon flash-induced absorbance changes in a purified RC complex were measured by a single beam spectrophotometer (Inoue et al. 1986) at ,-,25 °C as previously described (Kusumoto et al. 1994). The RC complex, when kept under anaerobic conditions, was stable and good for repeated measurement lasting for more than 10 h without appreciable loss of activity. In order to estimate the maximal absorbance change in the blue region, data were computer analyzed using MS-Excel (Microsoft, version 5.0). Triquat and mPMS (Hisada and Yagi 1977) were generous gifts from Dr T. Erabi of Tottori University and from Prof. T. Yagi of Shizuoka University, respectively.
Results and discussion
In a previous study, Kusumoto et al. (1994) reported flash kinetic optical measurements of a purified RC complex from C. tepidum, with 2,6-dichlorophenolindophenol as an electron donor, showing decreased absorbance in the blue region which recovered biphasically. One phase of the change appeared to be contributed by bound Cyt c551 and the other by another component decaying with different kinetics at room temperature. In order to determine difference spectra of the two kinetically different components, the effects of concentrations of mPMS (Hisada and Yagi 1977), a photostable analog of 5-methylphenazinium methyl sulfate (phenazine methosulfate), on the decay kinetics after flash excitation of a purified RC complex at room temperature, were studied at 430, 435, and 551 nm (Fig. 1). At 551 nm in the absence of mPMS, the light-induced absorbance change decayed with kinetics approximating a single exponentially decaying component with a tl/2 of-,,200 ms. The tl/2 usually ranged from -,-150 to 400 ms, and might be ascribed to reductants (ascorbate, dithiothreitol etc.) present in the reaction mixture. The decay was accelerated by mPMS with kinetics which are approximately described by the following equation:
109
[mPMS] (/zM) 0
430nm 435nm , , 100ms :.,,~,~>, 500 ms) of charge recombination between Fe-Sand Cyt c551+ in C. tepidum RC. Although the reacting oxidized component differs between the two systems, we may conclude that the spectrum (b) in Fig. 2 represents the difference spectrum of (CFA/CFB)- because the q/2 of charge recombination was very long and CFB was very efficiently reduced at cryogenic temperature (Kusumoto et al. 1994).
Acknowledgement This work was supported in part by a Grant-in-aid for Scientific Research from the Ministry of Education, Science and Culture, Japan (06640852) to H.S.
References Blankenship RE (1992) Origin and early evolution of photosynthesis. Photosynth Res 33:91-111 Biittner M, Xie D-L, Nelson H, Pinther W, Hanska G and Nelson N (1992) Photosynthetic reaction center genes in green sulfur bacteria and in Photosystem 1 are related. Proc Natl Acad Sci USA 89:8135-8139 Golbeck JH (1993) Shared thematic elements in photochemical reaction centers. Proc Natl Acad Sci USA 90:1642-1646 Hisada R and Yagi T (1977) l-methoxy-5-methylphenazinium methyl sulfate. J Biochem 82:1469-1473 Hiyama T and Ke B (197 la) A new photosynthetic pigment,'P430': Its possible role as the primary electron acceptor of Photosystem I. Proc Natl Acad Sci USA 68:1010-1013 Hiyama T and Ke B (197 lb) A further study of P430: a possible primary electron acceptor of Photosystem I. Arch Biochem Biophys 147:99-108 Hurt EC and Hauska G (1984) Purification of membrane-bound cytochromes and a photoactive P840 protein complex of the green sulfur bacterim Chlorobium limicola f. thiosulfatophilum. FEBS Lett 168:149-154 Illinger N, Xie D-L, Hauska G and Nelson N (1993) Identification of the subunit carrying FeS-centers A and B in the P840-reaction center preparation of Chlorobium limicola. Photosynth Res 38: lll-ll4 Inoue K, Sakurai H and Hiyama T (1986) Photoinhibition sites of Photosystem I in isolated chloroplasts. Plant Cell Physiol 27: 961-968 Jennings JV and Evans MCW (1977) The irreversible photoreduction of a low potential component at low temperatures in a preparation of the green photosynthetic bacterium Chlorobium thiosulphatophilum. FEBS Lett 75:33-36 Kjaer B, Jung Y-S, Yu L. Golbeck JH and Scheller HV (1994) Ironsulfur centers in the photosynthetic reaction center complex from Chlorobium vibrioforme. Differences from and similarities to the iron-sulfur centers in Photosystem I. Photosynth Res 41: 105-114 Kleinherenbrink FAM, Chiou H-C, LoBrutto R and Blankenship RE (1994) Spectroscopic evidence for the presence of an iron-sulfur center similar to F x of Photosystem I in Heliobacillus mobilis. Photosynth Res 41:115-123 Kusumoto N, Inoue K, Nasu H, Takano H and Sakurai H (1992) Preparation of reaction center particles containing photoreducible and dithionite-reducible Fe-S centers from a green sulfur bacterium, Chlorobium tepidum. In: Murata N (ed) Research in Photosynthesis, Vol I, pp 389-392. Kluwer Academic Publishers, Dordrecht
112 Kusumoto N, Inoue K, Nasu H and Sakurai H (1994) Preparation of a photoactive reaction center complex containing photoreducible Fe-S centers and photooxidizable cytochrome c from the green sulfur bacterium Chlorobium tepidum. Plant Cell Physio135: 1725 Liebl U, Mockensturm-Wilson M, Trost JT, Brune DC, Blankenship RE and Vermaas W (1993) Single core polypeptide in the reaction center of the photosynthetic bacterium Heliobacillus mobilis: Structual implications and relations to other photosystems. Proc Natl Acad Sci USA 90:7124-7128 Mathis P (1990) Compared structure of plant and bacterial photosynthetic reaction centers. Evolutionary implications. Biochim Biophys Acta 1018:163-167 Nitschke W, Feiler U and Rutherford AW (1990a) Photosynthetic reaction center of green sulfur bacteria studied by EPR. Biochemistry 29:3834-3842 Nitschke W, S6tif P, Liebl U, Feiler U and Rutherford AW (1990b) Reaction center photochemistry of Heliobacterium chlorum. Biochemistry 29:11079-11088 Oh-oka H, Kakutani S, Matsubara H, Malkin R and Itoh S (1993) Isolation of the photoactive reaction center complex that contains three types of Fe-S centers and a cytochrome c subunit from the green sulfur bacterium Chlorobium limicola f. thiosulfatophilum, strain Larsen. Plant Cell Physiol 34:93-101 Okkels JS, Kj~erB, Hansson O, Svendsen I, MOilerBL and Scheller HV (1992) A membrane-bound monoheme cytochrome c551 of a novel type is the immediate electron donor to P840 of the Chlorobium vibrioforme photosynthetic reaction center complex. J Biol Chem 267:21139-21145
Okumura N, Shimada K and Matsuura K (1994) Photo-oxidation of membrane-bound and soluble cytochrome c in the green sulfur bacterium Chlorobium tepidum. Photosynth Res 41:125-134 Olson JM (1980) Chlorophyll organization in green photosynthetic bacteria. Biochim Biophys Acta 594:33-51 Olson JM, Philipson KD and Saner K (1973) Circular dichroism and absorption spectra of bacteriochlorophyll-protein and reaction center complexes from Chlorobium thiosulfatophilum. Biochim Biophys Acta 292:206-217 Prince RC, Gest H and Blankenship RE (1985) Thermodynamic properties of the photochemical reaction center of Heliobacterium chlorum. Biochim Biophys Acta 810:377-384 Swarthoff T, Gast P, Hoff AJ and Amesz J (1981a) An optical and ESR investigation on the acceptor side of the reaction center of the green photosynthetic bacterium Prosthecochloris aestuarii. FEBS Lett 130:93-98 Swarthoff T, van der Veek-Horsley KM and Amesz J (1981b) The primary charge separation, cytochrome oxidation and triplet formation in preparations from the green photosynthetic bacterium Prosthecochloris aestuarii. Biochim Biophys Acta 635:1-12 Trost JT, Brune DC and Blankenship RE (1992) Protein sequences and redox titrations indicate that the electron acceptors in reaction centers from heliobacteria are similar to Photosystem I. Photosynth Res 32:11-12 van de Meent EJ, Kobayashi M, Erkelens C, van Veelen PA, Otte SCM, Inoue K, Watanabe T and Amesz J (1992) The nature of the primary electron acceptor in green sulfur bacteria. Biochim Biophys Acta 1102:371-378 Wahlund TM, Woese CR, Castenholz RW and Madigan MT (1991) A thermophilic green sulfur bacterium from New Zealand hot springs, Chlorobiurn tepidum sp. nov. Arch Microbiol 156: 8190
Regular paper
Spectroscopic studies of bound cytochrome c and an iron-sulfur center in a purified reaction center complex from the green sulfur bacterium
Chlorobium tepidum N o r i a k i K u s u m o t o 1, K a z u h i t o I n o u e 2 & H i d e h i r o S a k u r a i 1
1Department of Biology, School of Education, Waseda University, Tokyo 169-50, Japan; 2Department of Biological Sciences, Faculty of Science, Kanagawa University, 2946 Tsuchiya, Hiratsuka, Kanagawa 259-12, Japan Received 17 October 1994; accepted in revised form 7 December 1994
Key words: cytochrome c, iron-sulfur center, electron transfer, reaction center, flash photolysis spectroscopy, Chlorobium tepidum
Abstract Flash-induced optical kinetics at room temperature ofcytochrome (Cyt) CSsl and an Fe-S center (CFA/CFB) 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 tl/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 (CFA/CFB) which decayed with a h/2 of ,-~400-650 ms. Decay of the latter was effectively accelerated by benzyl viologen (Em - 3 6 0 mV) and methyl viologen ( - 4 4 0 mV), and less effectively by triquat ( - 5 4 0 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 CFA/CFB resembled P430 of PSI, and had a broad negative peak at 430,,~435 nm.
Abbreviations: (B)Chl - (bacterio)chlorophyll; Cyt - cytochrome; FA, FB and Fx - iron-sulfur center A, B and X of Photosystem I; CFA, CFB and CFx - FA-,FB- and Fx-like Fe-S center of Chlorobium; mPMS - l-methoxy-5methylphenazinium methyl sulfate, PSI - Photosystem I; RC - reaction center Introduction There is growing evidence which indicates that the RC of green sulfur bacteria and heliobacteria have similarities, on the acceptor side, to P S I of plants and cyanobacteria (for review of green sulfur bacteria, see Olson (1980), Mathis (1990), Blankenship (1992), Golbeck (1993); for heliobacteria, see Prince et al. (1985), Nitschke et al. (1990b), Trost et al. (1992), Kleinherenbrink et al. (1994) and the references cited therein). The RC of these bacteria can reduce ferredoxin without the requirement of a proton-motive force, and binds Chl-like pigments as the primary electron acceptor (van de Meent et al. 1992) and several Fe-S
centers as early acceptors (Jennings and Evans 1977; Swarthoff et al. 1981a; Prince et al. 1985; Nitschke et al. 1990a,b; Kusumoto et al. 1992; Oh-oka et al. 1993; Kusumoto et al. 1994; Kj~er et al. 1994). Moreover, the deduced amino acid sequences of the RC protein of C. limicola (Btittner et al. 1992) and Heliobacillus mobilis (Liebl et al. 1993) and of Fe-S protein (Illinger et al. 1993) of C. limicola show distinct similarity to those of PS I. In contrast with PS I, the RC of green sulfur bacteria binds Cyt c on the donor side (Hurt and Hauska 1984; Okkels et al. 1992; Oh-oka et al. 1993; Kusumoto et al. 1994). Although functioning of Fe-S centers in RCs of green sulfur bacteria was repeatedly observed by EPR spectroscopy, only
108 a continuous-light induced optical difference spectrum of one of them was reported (Swarthoffet al. 198 la). In Heliobacteria, Kleinherenbrink et al. (1994) recently reported the difference spectrum of Fx from the analysis of a 20-ms decay component in membrane preparations treated with urea which destroyed functional FA and FB. The flash-induced optical difference spectra of Fe-S centers, a counterpart known in P S I as P430 (Hiyama and Ke 197 la), and their reoxidation kinetics have not been determined yet. Until recently, only few preliminary reports had been presented regarding the kinetics of electron transfer by bound Cyt in a purified PC complex. Kinetic studies of these components were hampered by the lack of photochemically efficient RC preparations. In the RC preparation of Swarthoff et al. (1981b) from P aestuarii, the efficiency of photooxidation of Cyt c553 was low. In the RC preparation of Oh-oka et al. from C. limicola (Oh-oka et al. 1993), only 5% of the bound Cyt was oxidizable by flash. With such limited RC preparations, it is difficult to judge if the kinetic properties obtained are related to main pathway or side pathway of electron transfer. By taking great care to maintain anaerobic conditions throughout the course of preparation, we recently succeeded in preparing an RC complex from the thermophilic green sulfur bacterium Chlorobium tepidum which could photooxidize ,,~60% of the bound Cyt c551 with a single flash excitation, as well as with continuous illumination at room temperature. The RC complex contained about 50-70 BChl molecules and 1.21.7 photooxidizable Cyt per P840, three bound Fe-S centers (CFA, CFB, CFx; C for Chlorobium), and was composed of 5--6 peptides (Kusumoto et al. 1994). Since a preliminary investigation indicated the feasibility of this RC preparation for flash kinetic optical studies, in this study we measured reduction kinetics of photooxidized Cyt c551 and oxidation kinetics of a photoreduced Fe-S center, and obtained their optical oxidation-reduction difference spectra.
Materials and methods
Chlorobium tepidum cells were grown (Wahlund et al. 1991) and the RC complex was purified (Kusumoto et al. 1994) as described. Briefly, with N2 gas, cells were disrupted using a cell disintegrator (BioNebulizer, prototype developed by Togasaki, Surzycki and Kitayama at Indiana University). Chromatophore membranes were obtained by centrifugation, and the RC complexes were solubilized from the membranes with Triton
X-100. The RC complexes were purified by sucrose density gradient centrifugation, DEAE chromatography and hydroxyapatite chromatography. All the operations were conducted under anaerobic conditions. The purified RC preparation could be stored frozen at - 8 0 °C for at least 3 months without appreciable loss of activity. Xenon flash-induced absorbance changes in a purified RC complex were measured by a single beam spectrophotometer (Inoue et al. 1986) at ,-,25 °C as previously described (Kusumoto et al. 1994). The RC complex, when kept under anaerobic conditions, was stable and good for repeated measurement lasting for more than 10 h without appreciable loss of activity. In order to estimate the maximal absorbance change in the blue region, data were computer analyzed using MS-Excel (Microsoft, version 5.0). Triquat and mPMS (Hisada and Yagi 1977) were generous gifts from Dr T. Erabi of Tottori University and from Prof. T. Yagi of Shizuoka University, respectively.
Results and discussion
In a previous study, Kusumoto et al. (1994) reported flash kinetic optical measurements of a purified RC complex from C. tepidum, with 2,6-dichlorophenolindophenol as an electron donor, showing decreased absorbance in the blue region which recovered biphasically. One phase of the change appeared to be contributed by bound Cyt c551 and the other by another component decaying with different kinetics at room temperature. In order to determine difference spectra of the two kinetically different components, the effects of concentrations of mPMS (Hisada and Yagi 1977), a photostable analog of 5-methylphenazinium methyl sulfate (phenazine methosulfate), on the decay kinetics after flash excitation of a purified RC complex at room temperature, were studied at 430, 435, and 551 nm (Fig. 1). At 551 nm in the absence of mPMS, the light-induced absorbance change decayed with kinetics approximating a single exponentially decaying component with a tl/2 of-,,200 ms. The tl/2 usually ranged from -,-150 to 400 ms, and might be ascribed to reductants (ascorbate, dithiothreitol etc.) present in the reaction mixture. The decay was accelerated by mPMS with kinetics which are approximately described by the following equation:
109
[mPMS] (/zM) 0
430nm 435nm , , 100ms :.,,~,~>, 500 ms) of charge recombination between Fe-Sand Cyt c551+ in C. tepidum RC. Although the reacting oxidized component differs between the two systems, we may conclude that the spectrum (b) in Fig. 2 represents the difference spectrum of (CFA/CFB)- because the q/2 of charge recombination was very long and CFB was very efficiently reduced at cryogenic temperature (Kusumoto et al. 1994).
Acknowledgement This work was supported in part by a Grant-in-aid for Scientific Research from the Ministry of Education, Science and Culture, Japan (06640852) to H.S.
References Blankenship RE (1992) Origin and early evolution of photosynthesis. Photosynth Res 33:91-111 Biittner M, Xie D-L, Nelson H, Pinther W, Hanska G and Nelson N (1992) Photosynthetic reaction center genes in green sulfur bacteria and in Photosystem 1 are related. Proc Natl Acad Sci USA 89:8135-8139 Golbeck JH (1993) Shared thematic elements in photochemical reaction centers. Proc Natl Acad Sci USA 90:1642-1646 Hisada R and Yagi T (1977) l-methoxy-5-methylphenazinium methyl sulfate. J Biochem 82:1469-1473 Hiyama T and Ke B (197 la) A new photosynthetic pigment,'P430': Its possible role as the primary electron acceptor of Photosystem I. Proc Natl Acad Sci USA 68:1010-1013 Hiyama T and Ke B (197 lb) A further study of P430: a possible primary electron acceptor of Photosystem I. Arch Biochem Biophys 147:99-108 Hurt EC and Hauska G (1984) Purification of membrane-bound cytochromes and a photoactive P840 protein complex of the green sulfur bacterim Chlorobium limicola f. thiosulfatophilum. FEBS Lett 168:149-154 Illinger N, Xie D-L, Hauska G and Nelson N (1993) Identification of the subunit carrying FeS-centers A and B in the P840-reaction center preparation of Chlorobium limicola. Photosynth Res 38: lll-ll4 Inoue K, Sakurai H and Hiyama T (1986) Photoinhibition sites of Photosystem I in isolated chloroplasts. Plant Cell Physiol 27: 961-968 Jennings JV and Evans MCW (1977) The irreversible photoreduction of a low potential component at low temperatures in a preparation of the green photosynthetic bacterium Chlorobium thiosulphatophilum. FEBS Lett 75:33-36 Kjaer B, Jung Y-S, Yu L. Golbeck JH and Scheller HV (1994) Ironsulfur centers in the photosynthetic reaction center complex from Chlorobium vibrioforme. Differences from and similarities to the iron-sulfur centers in Photosystem I. Photosynth Res 41: 105-114 Kleinherenbrink FAM, Chiou H-C, LoBrutto R and Blankenship RE (1994) Spectroscopic evidence for the presence of an iron-sulfur center similar to F x of Photosystem I in Heliobacillus mobilis. Photosynth Res 41:115-123 Kusumoto N, Inoue K, Nasu H, Takano H and Sakurai H (1992) Preparation of reaction center particles containing photoreducible and dithionite-reducible Fe-S centers from a green sulfur bacterium, Chlorobium tepidum. In: Murata N (ed) Research in Photosynthesis, Vol I, pp 389-392. Kluwer Academic Publishers, Dordrecht
112 Kusumoto N, Inoue K, Nasu H and Sakurai H (1994) Preparation of a photoactive reaction center complex containing photoreducible Fe-S centers and photooxidizable cytochrome c from the green sulfur bacterium Chlorobium tepidum. Plant Cell Physio135: 1725 Liebl U, Mockensturm-Wilson M, Trost JT, Brune DC, Blankenship RE and Vermaas W (1993) Single core polypeptide in the reaction center of the photosynthetic bacterium Heliobacillus mobilis: Structual implications and relations to other photosystems. Proc Natl Acad Sci USA 90:7124-7128 Mathis P (1990) Compared structure of plant and bacterial photosynthetic reaction centers. Evolutionary implications. Biochim Biophys Acta 1018:163-167 Nitschke W, Feiler U and Rutherford AW (1990a) Photosynthetic reaction center of green sulfur bacteria studied by EPR. Biochemistry 29:3834-3842 Nitschke W, S6tif P, Liebl U, Feiler U and Rutherford AW (1990b) Reaction center photochemistry of Heliobacterium chlorum. Biochemistry 29:11079-11088 Oh-oka H, Kakutani S, Matsubara H, Malkin R and Itoh S (1993) Isolation of the photoactive reaction center complex that contains three types of Fe-S centers and a cytochrome c subunit from the green sulfur bacterium Chlorobium limicola f. thiosulfatophilum, strain Larsen. Plant Cell Physiol 34:93-101 Okkels JS, Kj~erB, Hansson O, Svendsen I, MOilerBL and Scheller HV (1992) A membrane-bound monoheme cytochrome c551 of a novel type is the immediate electron donor to P840 of the Chlorobium vibrioforme photosynthetic reaction center complex. J Biol Chem 267:21139-21145
Okumura N, Shimada K and Matsuura K (1994) Photo-oxidation of membrane-bound and soluble cytochrome c in the green sulfur bacterium Chlorobium tepidum. Photosynth Res 41:125-134 Olson JM (1980) Chlorophyll organization in green photosynthetic bacteria. Biochim Biophys Acta 594:33-51 Olson JM, Philipson KD and Saner K (1973) Circular dichroism and absorption spectra of bacteriochlorophyll-protein and reaction center complexes from Chlorobium thiosulfatophilum. Biochim Biophys Acta 292:206-217 Prince RC, Gest H and Blankenship RE (1985) Thermodynamic properties of the photochemical reaction center of Heliobacterium chlorum. Biochim Biophys Acta 810:377-384 Swarthoff T, Gast P, Hoff AJ and Amesz J (1981a) An optical and ESR investigation on the acceptor side of the reaction center of the green photosynthetic bacterium Prosthecochloris aestuarii. FEBS Lett 130:93-98 Swarthoff T, van der Veek-Horsley KM and Amesz J (1981b) The primary charge separation, cytochrome oxidation and triplet formation in preparations from the green photosynthetic bacterium Prosthecochloris aestuarii. Biochim Biophys Acta 635:1-12 Trost JT, Brune DC and Blankenship RE (1992) Protein sequences and redox titrations indicate that the electron acceptors in reaction centers from heliobacteria are similar to Photosystem I. Photosynth Res 32:11-12 van de Meent EJ, Kobayashi M, Erkelens C, van Veelen PA, Otte SCM, Inoue K, Watanabe T and Amesz J (1992) The nature of the primary electron acceptor in green sulfur bacteria. Biochim Biophys Acta 1102:371-378 Wahlund TM, Woese CR, Castenholz RW and Madigan MT (1991) A thermophilic green sulfur bacterium from New Zealand hot springs, Chlorobiurn tepidum sp. nov. Arch Microbiol 156: 8190
