Different Sensitivities to Oxygen Between Two Strains of the Photosynthetic Green Sulfur Bacterium Chlorobium vibrioforme NCIB 8327 with Bacteriochlorophyll c and d

Two sub-strains of the anoxygenic photosynthetic green sulfur bacterium Chlorobium vibrioforme NCIB 8327 were derived from the same clone and could be discriminated only by their possession of either bacteriochlorophyll (BChl) c or d as the major pigment in the peripheral light-harvesting antenna system, chlorosome (Saga Y et al. (2003) Anal Sci 19: 1575–1579). In the presence of a proper amount of oxygen in the initial culture medium, the BChl d strain showed longer retardation on its growth initiation than the BChl c strain, indicating that the latter was advantageous for survival under aerobic light conditions which produced reactive oxygen species in vivo. The result would be ascribable to the difference of the midpoint potentials between two kinds of chlorosomes formed by self-aggregates of BChl c and d as measured by their fluorescence quenching.     

Pigment interactions in chlorosomes of various green bacteria

Resonance Raman experiments were performed on different green bacteria. With blue excitation, i.e. under Soret resonance or preresonance conditions, resonance Raman contributions were essentially arising from the chlorosome pigments. By comparing these spectra and those of isolated chlorosomes, it is possible to evaluate how the latter retain their native structure during the isolation procedures. The structure of bacteriochlorophyll oligomers in chlorosomes was interspecifically compared, in bacteriochlorophyllc- and bacteriochlorophylle- synthesising bacteria. It appears that interactions assumed by the 9-keto carbonyl group are identical inChlorobium limicola, Chlorobium tepidum, andChlorobium phaeobacteroides. In the latter strain, the 3-formyl carbonyl group of bacteriochlorophylle is kept free from intermolecular interactions. By contrast, resonance Raman spectra unambiguously indicate that the structure of bacteriochlorophyll oligomers is slightly different in chlorosomes fromChloroflexus auranticus, either isolated or in the whole bacteria.     

Insertional inactivation studies of the csmA and csmC genes of the green sulfur bacterium Chlorobium vibrioforme 8327: the chlorosome protein CsmA is required for viability but CsmC is dispensable

Targeted mutagenesis was used to investigate the roles of the CsmA and CsmC proteins of the chlorosomes of the green bacteria Chlorobium tepidum and Chlorobium vibrioforme 8327. Under the photoautotrophic growth conditions employed, CsmA is required for the viability of the cells but CsmC is dispensable. The absence of CsmC caused a small red shift in the near-infrared absorption maximum of bacteriochlorophyll d in whole cells and chlorosomes, but chlorosomes were assembled in and could be isolated from the csmC mutant. The doubling time of the csmC mutant was approximately twice that of the wild-type strain. Fluorescence emission measurements suggested that energy transfer from the bulk bacteriochlorophyll d to another pigment, perhaps bacteriochlorophyll a, emitting at 800–804 nm, was less efficient in the csmC mutant cells than in wild-type cells. These studies establish that transformation and homologous recombination can be employed in targeted mutagenesis of Chlorobium sp. and further demonstrate that chlorosome proteins play important roles in the structure and function of these light-harvesting organelles.     

Photo-oxidation of membrane-bound and soluble cytochromec in the green sulfur bacteriumChlorobium tepidum

We studied the photosynthetic electron transfer system of membrane-bound and soluble cytochromec inChlorobium tepidum, a thermophilic green sulfur bacterium, using whole cells and membrane preparations. Sulfide and thiosulfate, physiological electron donors, enhanced flash-induced photo-oxidation ofc-type cytochromes in whole cells. In membranes,c-553 cytochromes with two (or three) heme groups served as immediate electron donors for photo-oxidized bacteriochlorophyll (P840) in the reaction center, and appeared to be closely associated with the reaction center complex. The membrane-bound cytochromec-553 had anE _m-value of 180 mV. When isolated soluble cytochromec-553, which has an apparent molecular weight of 10 kDa and seems to correspond to the cytochromec-555 inChlorobium limicola andChlorobium vibrioforme, was added to a membrane suspension, rapid photo-oxidation of both soluble and membrane-bound cytochromesc-553 was observed. The oxidation of soluble cytochromec-553 was inhibited by high salt concentrations. In whole cells, photo-oxidation was observed in the absence of exogenous electron donors and re-reduction was inhibited by stigmatellin, an inhibitor of the cytochromebc complex. These results suggest that the role of membrane-bound and soluble cytochromec inC. tepidum is similar to the role of cytochromec in the photosynthetic electron transfer system of purple bacteria.     

Chlorosome Proteins Studied by MALDI-TOF-MS: Topology of CsmA in Chlorobium tepidum

Chlorosomes, the light-harvesting apparatus of green bacteria, are a unique antenna system, in which pigments are organized in aggregates rather than associated with proteins. Isolated chlorosomes from the green sulphur bacterium Chlorobium tepidum contain 10 surface-exposed proteins. Treatment of chlorosomes from Chlorobium tepidum with protease caused changes in the spectral properties of bacteriochlorophyll c and digestion of chlorosome proteins. Using SDS-PAGE analysis, immunoblotting and matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF-MS) we have investigated the topology of the 59-residue CsmA protein. Our results show that at the N-terminus, the only amino acid available for protease degradation is the methionine. At the C-terminus, amino acids can be removed by protease treatment to produce a residual protein containing at least the sequence between residues 2 and 38. These results indicate that the N-terminal portion of the CsmA protein, which is predicted to be mainly hydrophobic, is buried in the chlorosome envelope.     

Seeing green bacteria in a new light: genomics-enabled studies of the photosynthetic apparatus in green sulfur bacteria and filamentous anoxygenic phototrophic bacteria

Based upon their photosynthetic nature and the presence of a unique light-harvesting antenna structure, the chlorosome, the photosynthetic green bacteria are defined as a distinctive group in the Bacteria. However, members of the two taxa that comprise this group, the green sulfur bacteria (Chlorobi) and the filamentous anoxygenic phototrophic bacteria (Chloroflexales), are otherwise quite different, both physiologically and phylogenetically. This review summarizes how genome sequence information facilitated studies of the biosynthesis and function of the photosynthetic apparatus and the oxidation of inorganic sulfur compounds in two model organisms that represent these taxa, Chlorobium tepidum and Chloroflexus aurantiacus. The genes involved in bacteriochlorophyll (BChl) c and carotenoid biosynthesis in these two organisms were identified by sequence homology with known BChl a and carotenoid biosynthesis enzymes, gene cluster analysis in Cfx. aurantiacus, and gene inactivation studies in Chl. tepidum. Based on these results, BChl a and BChl c biosynthesis is similar in the two organisms, whereas carotenoid biosynthesis differs significantly. In agreement with its facultative anaerobic nature, Cfx. aurantiacus in some cases apparently produces structurally different enzymes for heme and BChl biosynthesis, in which one enzyme functions under anoxic conditions and the other performs the same reaction under oxic conditions. The Chl. tepidum mutants produced with modified BChl c and carotenoid species also allow the functions of these pigments to be studied in vivo.     

Characterization of the csmD and csmE genes from Chlorobium tepidum. The CsmA, CsmC, CsmD, and CsmE proteins are components of the chlorosome envelope

The csmD and csmE genes, encoding two proteins of the chlorosome envelope, have been cloned and sequenced from the green sulfur bacterium Chlorobium tepidum. The csmD gene predicts a hydrophobic protein of 113 amino acids with a molecular mass of 11.1 kDa. The csmE gene was identified immediately upstream from csmD; the csmE gene predicts a protein of 82 amino acids (9.0 kDa) which is 49% identical to CsmA (Chung et al. (1994) Photosynthesis Res 41: 261–275). The CsmE protein is post-translationally processed, most likely in a manner similar to CsmA. The csmE and csmD genes are cotranscribed as a dicistronic mRNA but can also be cotranscribed with an open reading frame upstream from csmE that predicts a protein with sequence similarity to the CheY and SpoOF subclass of regulatory proteins. The CsmA, CsmC, CsmD, and CsmE proteins were overproduced in Escherichia coli, purified, and used to raise polyclonal antibodies in rabbits. Protease susceptibility mapping and agglutination experiments using these antibodies indicate that all four proteins are exposed at the surface of isolated chlorosomes and hence are probably components of the chlorosome envelope. Additionally, antigalactose antibodies were used to confirm that the galactosyl moiety of monogalactosyl diglycerol is exposed at the chlorosome surface; this is consistent with the notion that these lipids are components of the chlorosome envelope.Abbreviations BChl- bacteriochlorophyll - bp- basepair(s) - C.- Chloroflexus - Cb.- Chlorobium - Csm- chlorosome protein - csm gene encoding a chlorosome protein - IPTG isopropyl--d-thiogalatoside - nt nucleotide - ORF open reading frame - PAGE polyacrylamide gel electrophoresis - SDS sodium dodecylsulfate     

Identification of the major chlorosomal bacteriochlorophylls of the green sulfur bacteria Chlorobium vibrioforme and Chlorobium phaeovibrioides; their function in lateral energy transfer

The chlorosomal bacteriochlorophyll (BChl) composition of the green sulfur bacteria Chlorobium vibrioforme and Chlorobium phaeovibrioides was investigated by means of normal-phase high-performance liquid chromatography. From both species a number of homologues was isolated, which were identified by absorption and ²⁵²Cf-plasma desorption mass spectroscopy. Besides BChl d, C. vibrioforme contained a significant amount of BChl c, which may provide an explanation for the previous observation of at least two spectrally different pools of BChl in the chlorosomes of green sulfur bacteria (Otte et al. 1991). C. phaeovibrioides contained various homologues of BChl e only. Absorption spectra in acetone of BChl c, d and e, as well as bacteriopheophytin e are presented. No systematic differences were found for the various homologues of each pigment. In addition to farnesol, the mass spectra revealed the presence of various minor esterifying alcohols in both species, including phytol, oleol, cetol and 4-undecyl-2-furanmethanol, as well as an alcohol of low molecular mass, which is tentatively assumed to be decenol.Abbreviations BChl bacteriochlorophyll - BPh bacteriopheophytin (used as a general name for the Mg-free compound, irrespective of the esterifying alcohol) - HPLC high-performance liquid chromatography     

<Emphasis Type="Italic">Chlorobaculum tepidum</Emphasis> regulates chlorosome structure and function in response to temperature and electron donor availability

Green sulfur bacteria (GSB) rely on the chlorosome, a light-harvesting apparatus comprised almost entirely of self-organizing arrays of bacteriochlorophyll (BChl) molecules, to harvest light energy and pass it to the reaction center. In Chlorobaculum tepidum, over 97% of the total BChl is made up of a mixture of four BChl c homologs in the chlorosome that differ in the number and identity of alkyl side chains attached to the chlorin ring. C. tepidum has been reported to vary the distribution of BChl c homologs with growth light intensity, with the highest degree of BChl c alkylation observed under low-light conditions. Here, we provide evidence that this functional response at the level of the chlorosome can be induced not only by light intensity, but also by temperature and a mutation that prevents phototrophic thiosulfate oxidation. Furthermore, we show that in conjunction with these functional adjustments, the fraction of cellular volume occupied by chlorosomes was altered in response to environmental conditions that perturb the balance between energy absorbed by the light-harvesting apparatus and energy utilized by downstream metabolic reactions.     

Bacteriochlorophyll-c Homolog Composition in Green Sulfur Photosynthetic Bacterium Chlorobium vibrioformeDependent on the Concentration of Sodium Sulfide in Liquid Cultures

Green sulfur photosynthetic bacteria Chlorobium (Chl.) vibrioforme (DSM 263 strain and NCIB 8327 substrain possessing BChl-c) and Chl. tepidum (ATCC 49652) were photoautotrophically grown in liquid cultures containing different concentrations of sodium sulfide (Na₂S). BChl-c homologs possessing a methyl group at the 12-position tended to increase in cells of the two strains of Chl. vibrioforme cultured under high Na₂S concentrations. In contrast, the Na₂S concentration in liquid cultures did not affect the relative composition of BChl-c homologs in Chl. tepidum. 8-Propyl-12-methyl([P,M])-BChl-c homolog, which has been little observed in usual cultivations, could be isolated by reverse-phase high-performance liquid chromatography from the cells of Chl. vibrioforme grown under high Na₂S contents. The [P,M]-BChl-c homolog has the R-configuration at the 3¹-position, which was determined by ¹H-NMR analyses.     

A comparative study of the optical characteristics of intact cells of photosynthetic green sulfur bacteria containing bacteriochlorophyll c,d or e

Energy transfer and pigment arrangement in intact cells of the green sulfur bacteria Prosthecochloris aestuarii, Chlorobium vibrioforme and chlorobium phaeovibrioides, containing bacteriochlorophyll (BChl) c, d or e as main light harvesting pigment, respectively, were studied by means of absorption, fluorescence, circular dichroism and linear dichroism spectroscopy at low temperature. The results indicate a very similar composition of the antenna in the three species and a very similar structure of main light harvesting components, the chlorosome and the membrane-bound BChl a protein. In all three species the Q_y transition dipoles of BChl c, d or e are oriented approximately parallel to the long axis of the chlorosome. Absorption and fluorescence excitation spectra demonstrate the presence of at least two BChl c-e pools in the chlorosomes of all three species, long-wavelength absorbing BChls being closest to the membrane. In C. phaeovibrioides, energy from BChl e is transferred with an efficiency of 25% to the chlorosomal BChl a at 6 K, whereas the efficiency of transfer from BChl e to the BChl a protein is 10%. These numbers are compatible with the hypothesis that the chlorosomal BChl a is an intermediary in the energy transfer from the chlorosome to the membrane.Abbreviations BChl bacteriochlorophyll - Chl chlorophyll - CD circular dichroism - LD linear dichroism     

Characterization of csmB genes,encoding a 7.5-kDa protein of the chlorosome envelope,from the green sulfur bacteria Chlorobium vibrioforme 8327D and Chlorobium tepidum

The csmB gene, encoding the 7.5-kDa “Gerola-Olson” protein of chlorosomes, has been cloned and sequenced from the green sulfur bacteria Chlorobium vibrioforme strain 8327D and Chlorobium tepidum. Two potential start codons were identified, and the csmB gene may be translated into a preprotein with an amino-terminal extension. Two forms of the mature CsmB protein (74 or 75 amino acids in length) were identified that differ by the presence or absence of a methionine residue at the amino terminus. The csmB gene of Chl. tepidum is transcribed as an abundant monocistronic mRNA of approximately 350 nucleotides; primer extension mapping of the 5′ endpoint of the csmB mRNA suggests there is strong similarity between the csmB promoter and the σ⁷⁰ promoters of Escherichia coli. The CsmB protein of Chl. tepidum was overproduced as a histidine-tagged fusion protein in E. coli, purified to homogeneity by Ni²⁺ chelation affinity chromatography, and used to raise polyclonal antibodies in rabbits. Protease susceptibility mapping and agglutination experiments with isolated chlorosomes using anti-CsmB antibodies indicate that the CsmB protein is a component of the chlorosome envelope. Received: 28 May 1996 / Accepted: 17 July 1996     

Quinones in chlorosomes of green sulfur bacteria and their role in the redox-dependent fluorescence studied in chlorosome-like bacteriochlorophyll c aggregates

The light-harvesting chlorosome antennae of anaerobic, photosynthetic green sulfur bacteria exhibit a highly redox-dependent fluorescence such that the fluorescence intensity decreases under oxidizing conditions. We found that chlorosomes from Chlorobium tepidum contain three isoprenoid quinone species (chlorobiumquinone, menaquinone-7, and an unidentified quinone that probably is a chlorobiumquinone derivative) at a total concentration of approximately 0.1 mol per mol bacteriochlorophyll c. Most of the cellular chlorobiumquinone was found in the chlorosomes and constituted about 70% of the total chlorosome quinone pool. When the quinones were added to artificial, chlorosome-like bacteriochlorophyll c aggregates in an aqueous solution, a high redox dependency of the fluorescence was observed. Chlorobiumquinones were most effective in this respect. A lesser redox dependency of the fluorescence was still observed in the absence of quinones, probably due to another unidentified redox-active component. These results suggest that quinones play a significant, but not exclusive role in controlling the fluorescence and in inhibiting energy transfer in chlorosomes under oxic conditions. Chlorosomes from Chloroflexus aurantiacus contained menaquinone in an amount similar to that of total quinone in Chlorobium tepdium chlorosomes, but did not contain chlorobiumquinones. This may explain the much lower redox-dependent fluorescence observed in Chloroflexus chlorosomes. Received: 4 November 1996 / Accepted: 18 February 1997     

X-ray scattering and electron cryomicroscopy study on the effect of carotenoid biosynthesis to the structure of Chlorobium tepidum chlorosomes

Chlorosomes, the main antenna complexes of green photosynthetic bacteria, were isolated from null mutants of Chlorobium tepidum, each of which lacked one enzyme involved in the biosynthesis of carotenoids. The effects of the altered carotenoid composition on the structure of the chlorosomes were studied by means of x-ray scattering and electron cryomicroscopy. The chlorosomes from each mutant strain exhibited a lamellar arrangement of the bacteriochlorophyll c aggregates, which are the major constituents of the chlorosome interior. However, the carotenoid content and composition had a pronounced effect on chlorosome biogenesis and structure. The results indicate that carotenoids with a sufficiently long conjugated system are important for the biogenesis of the chlorosome baseplate. Defects in the baseplate structure affected the shape of the chlorosomes and were correlated with differences in the arrangement of lamellae and spacing between the lamellar planes of bacteriochlorophyll aggregates. In addition, comparisons among the various mutants enabled refinement of the assignments of the x-ray scattering peaks. While the main scattering peaks come from the lamellar structure of bacteriochlorophyll c aggregates, some minor peaks may originate from the paracrystalline arrangement of CsmA in the baseplate.     

Circular dichroism of green bacterial chlorosomes

Positive and negative bands in previously measured circular dichroism (CD) spectra of Chlorobium limicola chlorosomes appeared to be sign-reversed relative to those of Chloroflexus aurantiacus chlorosomes in the 740–750 nm spectral region where bacteriochlorophyll (BChl) c absorbs maximally. It was not clear, however, whether this difference was intrinsic to the chlorosomes or was due to differences in the procedures used to prepare them. We therefore repeated the CD measurements using chlorosomes isolated from both Cb. limicola f. thiosulfatophilum and Cf. aurantiacus using the method of Gerola and Olson (1986, Biochim. Biophys. Acta 848: 69–76). Contrary to the earlier results, both types of chlorosomes had very similar CD spectra, suggesting that both have similar arrangements of BChl c molecules. The previously reported difference between the CD spectra of Chlorobium and Chloroflexus chlorosomes is due to the instability of Chlorobium chlorosomes, which can undergo a hypsochromic shift in their near infrared absorption maximum accompanied by an apparent inversion in their near infrared CD spectrum during isolation. Treating isolated chlorosomes with the strong ionic detergent sodium dodecylsulfate, which removes BChl a, does not alter the arrangement of BChl c molecules in either Chloroflexus or Chlorobium chlorosomes, as indicated by the lack of an effect on their CD spectra.Abbreviations BChl bacteriochlorophyll - Cb. Chlorobium - CD circular dichroism - Cf. Chloroflexus - NIR near infrared     

Anisotropic distribution of emitting transition dipoles in chlorosome from <Emphasis Type="Italic">Chlorobium tepidum</Emphasis>: fluorescence polarization anisotropy study of single chlorosomes

The polarization anisotropy of fluorescence spectra from single chlorosomes isolated from a green sulfur bacterium, Chlorobium (Cb.) tepidum, was observed at 13 K. As the polarizer was rotated, the intensities of the fluorescence bands of both bacteriochlorophyll (BChl)-c self-aggregates and BChl-a in baseplate proteins showed clear oscillations. From the oscillation, the values of the degree of polarization (DP) and the phase shift (PS) between the BChl-c and BChl-a bands were determined for each single chlorosome. The DP versus PS plot for Cb. tepidum chlorosomes showed linear correlations between the PS and the DP values for both BChl-c and BChl-a fluorescence bands. This tendency could be explained from a simulation assuming a random orientation of chlorosomes and a triaxial orientation distribution of emitting transition dipoles within a single chlorosome. The intensity ratios among the X-/Y-/Z-principal transition dipoles were estimated to be 0.3/0.5/1 and 1/0.6/0.1 for the BChl-c and BChl-a fluorescence bands, respectively. Here, the X-, Y-, and Z-axes are perpendicular, parallel to the cytoplasmic membrane, and parallel to the chlorosome long axis, respectively. A theoretical calculation based on the exciton theory was conducted to reproduce the observed triaxial orientation distribution of emitting transition dipoles. The simulation revealed that a deformation introduced to the circular cross section of the rod-shaped BChl-c self-aggregates could qualitatively reproduce results of this study.     

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     

Characterization of the chlorosome antenna of the filamentous anoxygenic phototrophic bacterium<Emphasis Type="Italic"> Chloronema</Emphasis> sp. strain UdG9001

The absorption and fluorescence properties of chlorosomes of the filamentous anoxygenic phototrophic bacterium Chloronema sp. strain UdG9001 were analyzed. The chlorosome antenna of Chloronema consists of bacteriochlorophyll (BChl) d and BChl c together with -carotene as the main carotenoid. HPLC analysis combined with APCI LC-MS/MS showed that the chlorosomal BChls comprise a highly diverse array of homologues that differ in both the degree of alkylation of the macrocycle at C-8 and/or C-12 and the alcohol moiety esterified to the propionic acid group at C-17. BChl c and BChl d from Chloronema were mainly esterified with geranylgeraniol (33% of the total), heptadecanol (24%), octadecenol (19%), octadecanol (14%), and hexadecenol (9%). Despite this pigment heterogeneity, fluorescence emission of the chlorosomes showed a single peak centered at 765 nm upon excitation at wavelengths ranging from 710 to 740 nm. This single emission, assigned to BChl c, indicates an energy transfer from BChl d to BChl c within the same chlorosome. Likewise, incubation of chlorosomes under reducing conditions caused a weak increase in fluorescence emission, which indicates a small redox-dependent fluorescence. Finally, protein analysis of Chloronema chlorosomes using SDS-PAGE and MALDI-TOF-MS revealed the presence of a chlorosomal polypeptide with a molecular mass of 5.7 kDa, resembling the CsmA protein found in Chloroflexus aurantiacus and Chlorobium tepidum chlorosomes. Several minor polypeptides were also detected but not identified. These results indicate that, compared with other members of filamentous anoxygenic phototrophic bacteria and green sulfur bacteria, Chloronema possesses an antenna system with novel features that may be of interest for further investigations.Abbreviations APCI LC-MS/MS Atmospheric pressure chemical ionization liquid chromatography mass spectrometry - BChl Bacteriochlorophyll - Chl. Chlorobium - Cfl. Chloroflexus - MALDI-TOF-MS Matrix assisted laser desorption/ionization time-of-flight mass spectrometry - [Et] Ethyl - [i-Bu] Isobutyl - [Me] Methyl - [neo-Pent] Neopentyl - [n-Pr] Propyl - t _R Retention time     

The light-harvesting antenna of Chlorobium tepidum: Interactions between the FMO protein and the major chlorosome protein CsmA studied by surface plasmon resonance

Green sulfur bacteria possess two external light-harvesting antenna systems, the chlorosome and the FMO protein, which participate in a sequential energy transfer to the reaction centers embedded in the cytoplasmic membrane. However, little is known about the physical interaction between these two antenna systems. We have studied the interaction between the major chlorosome protein, CsmA, and the FMO protein in Chlorobium tepidum using surface plasmon resonance (SPR). Our results show an interaction between the FMO protein and an immobilized synthetic peptide corresponding to 17 amino acids at the C terminal of CsmA. This interaction is dependent on the presence of a motif comprising six amino acids that are highly conserved in all the currently available CsmA protein sequences.