Heliobacillus Mobilis Research Articles

Structure and evolution of ribonuclease P RNA in Gram-positive bacteria.

The sequences and structures of RNase P RNAs of some Gram-positive bacteria, e.g. Bacillus subtilis, are very different than those of other bacteria. In order to expand our understanding of the structure and evolution of RNase P RNA in Gram-positive bacteria, gene sequences encoding RNase P RNAs from 10 additional species from this evolutionary group have been determined, doubling the number of sequences available for comparative analysis. The enlarged data set allows refinement of the secondary structure model of these unusual RNase P RNAs and the identification of potential tertiary interactions between P10.1 and L12, and between L5.1 and L15.1. The newly-obtained sequences suggest that RNase P RNA underwent an abrupt, dramatic restructuring in the ancestry of the low-G+C Gram-positive bacteria after the divergence of the branches leading to the 'Clostridia and relatives' and the remaining low-G+C Gram-positive species. The unusual structures of the RNase P RNAs of Mycoplasma hyopneumoniae and M.floccularre are apparently derived from RNAs with Bacillus-like structure rather than from intermediate, partially restructured ancestral RNAs. The structure of the RNase P RNA from the photosynthetic Heliobacillus mobilis supports the relationship of this specie with Bacillus and Staphylococcus rather than the 'Clostridia and relatives' as suggested by the sequences of their small-subunit ribosomal RNAs.

FTIR spectroscopy of primary donor photooxidation in Photosystem I, Heliobacillus mobilis, and Chlorobium limicola. Comparison with purple bacteria

The photooxidation of the primary electron donor in several Photosystem I-related organisms (Synechocystis sp. PCC 6803, Heliobacillus mobilis, and Chlorobium limicola f. sp. thiosulphatophilum) has been studied by light-induced FTIR difference spectroscopy at 100 K in the 4000 to 1200 cm(-1) spectral range. The data are compared to the well-characterized FTIR difference spectra of the photooxidation of the primary donor P in Rhodobacter sphaeroides (both wild type and the heterodimer mutant HL M202) in order to get information on the charge localization and the extent of coupling within the (bacterio)chlorophylls constituting the oxidized primary donors. In Rb. sphaeroides RC, four marker bands mostly related to the dimeric nature of the oxidized primary donor have been previously observed at ≈2600, 1550, 1480, and 1295 cm(-1). The high-frequency band has been shown to correspond to an electronic transition (Breton et al. (1992) Biochemistry 31: 7503-7510) while the three other marker bands have been described as phase-phonon bands (Reimers and Hush (1995) Chem Phys 197: 323-332). The absence of these bands in PS I as well as in the heterodimer HL M202 demonstrates that in P700(+) the charge is essentially localized on a single chlorophyll molecule. For both H. mobilis and C. limicola, the presence of a high-frequency band at ≈ 2050 and 2450 cm(-1), respectively, and of phase-phonon bands (at ≈ 1535 and 1300 cm(-1) in H. mobilis, at ≈ 1465 and 1280 cm(-1) in C. limicola) indicate that the positive charge in the photooxidized primary donor is shared between two coupled BChls. The structure of P840(+) in C. limicola, in terms of the resonance interactions between the two BChl a molecules constituting the oxidized primary donor, is close to that of P(+) in purple bacteria reaction centers while for H. mobilis the FTIR data are interpreted in terms of a weaker coupling of the two bacteriochlorophylls.

Energy and electron transfer upon selective femtosecond excitation of pigments in membranes of Heliobacillus mobilis.

Excitation energy transfer steps in membranes of Heliobacillus mobilis were directly monitored by transient absorption spectroscopy with a time resolution of 30 fs under selective excitation within the inhomogeneously broadened bacteriochlorophyll g QY band. The initial anisotropy was found to be > 0.4, indicating that the pigments are excitonically coupled. After initial decay of this anisotropy in < 50 fs, major sub-picosecond components associated with spectral equilibration were identified, corresponding to uphill energy transfer with a 300 fs time constant (812 nm excitation) and downhill energy transfer with 100 and 500 fs components (770 nm excitation). These equilibrations are ascribed predominantly to single excitation transfer steps, as anisotropy measurements showed that equilibration within spectrally similar pigments occurs on the same time scale as spectral equilibration, a situation which contrasts with that in photosystem I. Downhill energy transfer occurs to a significant extent directly to an energetically heterogeneous population of excited states as well as in a sequential way via gradually lower-lying pools of bacteriochlorophyll g. This finding supports a description in which all pigments, including the bluemost absorbing, are spatially organized in a random way rather than in clusters of spectrally similar species. Spectral equilibration is not entirely completed prior to formation of the primary radical pair P798 + A0-, which was found to proceed in a multiexponential way (time constants of 5 and 30 ps). No indication for the formation of radical species other than P798 + A0- on the time scale up to 100 ps was found.

Secondary electron transfer processes in membranes of Heliobacillus mobilis.

Picosecond transient absorption difference spectroscopic experiments were performed on membranes of the antenna/reaction center complex of Heliobacillus mobilis to study the electron transfer processes. Particular emphasis was placed on the blue spectral region, where the difference spectra of iron-sulfur centers and quinones are significantly different. Spectra were measured at room temperature in the wavelength region from 400 to 470 nm and from 630 to 730 nm. Laser excitation was into the 788 nm Q gamma band of the bacteriochlorophyll g of the reaction center complex. Global analysis in both wavelength regions reveals three kinetic components. A 25 ps phase originates from the decay of the excited state of antenna to form the primary charge-separated state P798+A0-; a 600 ps component is assigned to the electron transfer from the primary electron acceptor A0 to a secondary electron acceptor; a nondecaying component on the time scale measured represents the formation of the secondary charge-separated state. When the secondary electron acceptors were reduced by adding dithionite at pH 11, the 600 ps component disappeared. Only a 25 ps component and a constant were observed in the 630-730 nm region. The 25 ps component is assigned to the excitation decay in the antenna and the formation of P798+A0-, just as in the nonreduced sample. In the reduced sample, the P798+A0- state does not decay on the time scale measured. In the 400-470 nm region, the same kinetic behavior was observed. The absorption difference spectra of the primary and the secondary electron acceptor were constructed from different charge-separated states.(ABSTRACT TRUNCATED AT 250 WORDS)

Membrane-Bound c-Type Cytochromes in Heliobacillus mobilis. In Vivo Study of the Hemes Involved in Electron Donation to the Photosynthetic Reaction Center

The amount of heme per photosynthetic reaction center (RC) was examined in whole cells of Heliobacillus mobilis, and a stoichiometry of 5-6 hemes c and 1-3 hemes b per RC was found. Virtually the full complement of heme was seen to be functionally connected to the pool of electron donors to the photosynthetic RC. The kinetic parameters of electron transfer between reduced c-type hemes and the photooxidized primary donor P798+ were studied in whole cells and membrane fragments. The in vivo half-times of electron donation (50% with t 1/2 = 110 microseconds, 50% with t 1/2 = 600 microseconds) were seen to slow down to half-times in the range of several and several tens of milliseconds following disruption of cells. A severe conformational alteration or a change in the identity of the donating heme is discussed. Redox titrations of the flash-induced absorption changes performed on whole cells in the presence of mediators yielded the following redox midpoint potentials: P798, Em = +240 mV; heme c553, Em = +190, +170, and +90 mV for the heme components oxidized after the first, second, and third flash, respectively. The results demonstrate that the pool of c553 hemes donating electrons to the RC is heterogeneous and that it consists of either several distinguishable cytochromes or multiheme cytochromes or both. The number of hemes reduced and the kinetics of heme rereduction after flash-induced oxidation were found to depend strongly on the degree of anaerobicity in the interior compartment of the cell. A model rationalizing the obtained results in terms of a set of differing redox components is proposed.

Isolation, Characterization, and Primary Structure of Rubredoxin from the Photosynthetic Bacterium, Heliobacillus mobilis

Rubredoxin is a small nonheme iron protein that serves as an electron carrier in bacterial systems. Rubredoxin has now been isolated and characterized from the strictly anaerobic phototroph, Heliobacillus mobilis. The molecular mass (5671.3 Da from the amino acid sequence) was confirmed and partial formylation of the N-terminal methionyl residue was established by matrix-assisted laser desorption mass spectroscopy. The complete 52-amino-acid sequence was determined by a combination of N-terminal sequencing by Edman degradation and C-terminal sequencing by a novel method using carboxypeptidase treatment in conjunction with amino acid analysis and laser desorption time of flight mass spectrometry. The molar absorption coefficient of Hc. mobilis rubredoxin at 490 nm is 6.9 mM−1 cm−1 and the midpoint redox potential at pH 8.0 is −46 mV. The EPR spectrum of the oxidized form shows resonances at g = 9.66 and 4.30 due to a high-spin ferric iron. The amino acid sequence is homologous to those of rubredoxins from other species, in particular, the gram-positive bacteria, and the phototrophic green sulfur bacteria, and the evolutionary implications of this are discussed.

Spectral heterogeneity and time-resolved spectroscopy of excitation energy transfer in membranes of Heliobacillus mobilis at low temperatures

Transient absorption difference spectra in the Qy absorption band from membranes of Heliobacillus mobilis were recorded at 140 and 20 K upon 200 fs laser pulse excitation at 590 nm. Excitation transfer from short wavelength absorbing forms of bacteriochlorophyll g to long wavelength bacteriochlorophyll g occurred within 1-2 ps at both long wavelength bacteriochlorophyll g occurred within 1-2 ps at both temperatures. In addition, a slower energy transfer process with a time constant of 15 ps was observed at 20 K within the pool of long wavelength-absorbing bacteriochlorophyll g. Energy transfer from long wavelength antenna pigments to the primary electron donor P798 was observed, yielding the primary charge-separated state P798+A0-. The time constant for this process was 30 ps at 140 K and about 70 ps at 20 K. A decay component with smaller amplitude and a lifetime of up to hundreds of picoseconds was observed that was centered around 814 nm at 20 K. Kinetic simulations using simple lattice models reproduce the observed decay kinetics at 295 and 140 K, but not at 20 K. The kinetics of energy redistribution within the spectrally heterogeneous antenna system at low temperature argue against a simple "funnel" model for the organization of the antenna of Heliobacillus mobilis and favor a more random spatial distribution of spectral forms. However, the relatively high rate of energy transfer from long wavelength antenna bacteriochlorophyll g to the primary electron donor P798 at low temperature is difficult to explain with either of these models.

Gram-positive cell wall structure of the A3 gamma type in heliobacteria.

The amino acid composition and structure of the peptidoglycan from Heliobacillus mobilis was determined by one- and two-dimensional thin-layer chromatography of completely and partially hydrolysed cell wall preparations. The structure was found to be of the A3 gamma type, with L,L-diaminopimelate in position 3, D-alanine in position 4 and a glycine interpeptide bridge, as found in certain groups of Gram-positive bacteria including Clostridium perfringens and Nocardioides simplex. The presence of a Gram-positive type of cell wall in heliobacteria is consistent with their phylogenetic relationship to the 'low G + C' Gram-positive bacteria, as previously demonstrated by 16S rRNA sequencing.

Spectroscopic evidence for the presence of an iron-sulfur center similar to Fx of Photosystem I in Heliobacillus mobilis.

Treatment of membranes of Heliobacillus mobilis with high concentrations of the chaotropic agent urea resulted in the removal of the iron-sulfur centers FA and FB from the reaction center, as indicated by EPR spectra under strongly reducing conditions. In urea-treated membranes, transient absorption measurements upon a laser flash indicated a recombination between the photo-oxidized primary donor P798+ and a reduced acceptor with a time constant of 20 ms at room temperature. Benzylviologen, vitamin K-3 and methylene blue were found to accept electrons from the reduced acceptor efficiently. A differential extinction coefficient of 225-240 mM-1 cm-1 at 798 nm was determined from experiments in the presence of methylene blue. Transient absorption difference spectra between 400 and 500 nm in the presence and absence of artificial acceptors indicated that the electron acceptor involved in the 20 ms recombination has an absorption spectrum similar to that of an iron-sulfur center. This iron-sulfur center was assigned to be analogous to Fx of Photosystem I. Our results provide evidence in support of the presence of Fx in heliobacteria, which was proposed on the basis of the reaction center polypeptide sequence (Liebl et al. (1993) Proc. Natl. Acad. Sci. USA 90: 7124-7128). Implications for the electron transfer pathway in the reaction center of heliobacteria are discussed.

Spurious circular dichroism signals with intact cells of heliobacteria

Strong anomalous circular dichroism (CD) signals were observed with suspensions of intact cells of the heliobacteria Heliobacillus mobilis and Heliobacterium chlorum. They were an order of magnitude larger than with isolated membranes at 6 K and changed dramatically with temperature. From the fact that suspensions of intact cells of heliobacteria showed linear dichroism (LD), it was concluded that the cells were oriented. The signals were proven to be artifacts, caused by coupling between LD and strain linear birefringence together with a smaller direct contribution of LD. The same effects were observed with oriented membrane fragments. The orientation of the cells was caused by gravity and could be eliminated by suspending the cells in an isopycnic solution. This way the real CD of intact cells could be measured. As expected, this spectrum was very similar to that of membrane fragments.

Delayed Fluorescence from Fe-S Type Photosynthetic Reaction Centers at Low Redox Potential

Fluorescence kinetics were measured in membranes of a photosystem II-deletion mutant of the cyanobacterium Synechocystis sp. PCC 6803 containing photosystem I as the only reaction center and of the anoxygenic photosynthetic bacterium Heliobacillus mobilis. The measurements were performed under conditions where forward electron transfer to secondary acceptors was inhibited by the strong reductant sodium dithionite. Delayed fluorescence due to recombination of the primary radical pair P+A0- in both species was found to consist of several kinetic components. The longest-lived component had a lifetime of 35 ns in photosystem I and 18 ns in H. mobilis, respectively, which corresponds with the lifetime of the primary radical pair. Delayed fluorescence components with lifetimes of about 4 ns, 1 ns, and 200 ps were also observed in both species and were attributed to relaxations within the radical pair. A standard free energy difference of 0.18 eV was calculated for both species between the relaxed primary radical pair and the excited antenna at room temperature. A value of 0.25 eV was estimated for the free energy difference between the relaxed primary radical pair and the excited primary donor. The temperature dependence of the delayed fluorescence between 25 and 2 degrees C indicated that more than half of the free energy difference is due to enthalpy. Our measurements indicate an overall similarity between the primary electron transfer process in the Fe-S type (or low potential) reaction centers and the (bacterio)pheophytin-quinone type (or high potential) reaction centers found in purple photosynthetic bacteria and photosystem II.

Time-resolved spectroscopy of energy and electron transfer processes in the photosynthetic bacterium Heliobacillus mobilis

The kinetics of excitation energy transfer and electron transfer processes within the membrane of Heliobacillus mobilis were investigated using femtosecond transient absorption difference spectroscopy at room temperature. The kinetics in the 725- to 865-nm region, upon excitation at 590 and 670 nm, were fit using global analysis. The fits returned three kinetic components with lifetimes of 1-2 ps and 27-30 ps, and a component that does not decay within several nanoseconds. The 1- to 2-ps component is attributed to excitation equilibration to form a thermally relaxed excited state. The 27- to 30-ps phase corresponds to the decay of the relaxed excited state to form a charge-separated state. The intrinsic energy and electron transfer rates were estimated using the experimental results and theoretical models for excitation migration and trapping dynamics. Taking into account the number of antenna pigments and their spectral distribution, an upper limit of 1.2 ps for the intrinsic time constant for charge separation in the reaction center is calculated. This upper limit corresponds with the trapping-limited case for excitation migration and trapping. Reduction of the primary electron acceptor A0 was observed in the 640 to 700 nm region using excitation at 780 nm. An instantaneous absorbance increase followed by a decay of about 30 ps was observed over a broad wavelength region due to the excited state absorption and decay of BChl g molecules in the antenna. In addition, a narrow bleaching band centered at 670 nm grows in with an apparent time constant of about 1.0 ps, superimposed on the 30-ps absorbance increase due to excited state absorption. Measurements on a longer time scale showed that besides the 670 nm pigment a BChl g molecule absorbing near 785 nm may be involved in the primary charge separation, and that this pigment may be in equilibrium with the 670 nm pigment. The bleaching bands at 670 nm and 785nm recovered with a time constant of about 600 ps, due to forward electron transport to a secondary electron acceptor. Energy and electron transfer properties of H. mobilis membranes are compared with Photosystem 1, to which the heliobacteria bear an evolutionary relationship.

Single core polypeptide in the reaction center of the photosynthetic bacterium Heliobacillus mobilis: structural implications and relations to other photosystems.

The gene for a reaction center core polypeptide from the anoxygenic photosynthetic bacterium Heliobacillus mobilis was cloned and sequenced. The deduced amino acid sequence consists of 609 residues with a molecular mass of 68 kDa. An adjacent open reading frame is not transcribed under our experimental conditions. No evidence for a second related reaction center core gene was found. The primary sequence of the reaction center protein (P800 protein) shows a high percentage of sequence identity to photosystem I in a cysteine-containing loop, which is the putative binding site of the iron-sulfur center FX and in the preceding hydrophobic region. Our data imply a homodimeric organization of the reaction center. This is fundamentally different from photosystem I and most other photosynthetic reaction centers, where the reaction center core is composed of two similar but nonidentical subunits.

Evidence for an alternative nitrogenase inHeliobacterium gestii

The anoxygenic phototrophic bacterium Heliobacterium gestii grew and fixed dinitrogen in molybdenum-deficient media when either iron or iron plus vanadium were present. Cultures grown on N2 with iron or iron plus vanadium reduced acetylene to both ethylene and ethane; molybdenum-grown cultures did not produce ethane. Cultures of Heliobacterium chlorum and Heliobacillus mobilis were unable to grow and fix dinitrogen in media devoid of molybdenum. It is concluded that H. gestii can synthesize an iron-only alternative nitrogenase under appropriate culture conditions.

Nitrogen fixation and nitrogen metabolism in heliobacteria

Three species of anoxygenic phototrophic heliobacteria, Heliobacterium chlorum, Heliobacterium gestii, and Heliobacillus mobilis, were studied for comparative nitrogen-fixing abilities and regulation of nitrogenase. Significant nitrogenase activity (acetylene reduction) was detected in all species grown photoheterotrophically on N2, although cells of H. mobilis consistently had higher nitrogenase activity than did cells of either H. chlorum or H. gestii. Nitrogen-fixing cultures of all three species of heliobacteria were subject to “switch-off” of nitrogenase activity by ammonia; glutamine also served to “switch-off” nitrogenase activity but only in cells of H. mobilis and H. gestii. Placing photosynthetically grown heliobacterial cultures in darkness also served to “switch-off” nitrogenase activity. Dark-mediated “switch-off” was complete in lactate-grown heliobacteria but in pyruvate-grown cells substantial rates of nitrogenase activity continued in darkness. In all heliobacteria examined ammonia was assimilated primarily through the glutamine synthetase/glutamate synthase (GS/GOGAT) pathway although significant levels of alanine dehydrogenase were present in extracts of cells of H. gestii, but not in the other species. The results suggest that heliobacteria, like phototrophic purple bacteria, are active N2-fixing bacteria and that despite their gram-positive phylogenetic roots, heliobacteria retain the capacity to control nitrogenase activity by a “switch-off” type of mechanism. Because of their ability to fix N2 both photosynthetically and in darkness, it is possible that heliobacteria are significant contributors of fixed nitrogen in their paddy soil habitat.

Protein sequences and redox titrations indicate that the electron acceptors in reaction centers from heliobacteria are similar to Photosystem I

Photosynthetic reaction centers isolated from Heliobacillus mobilis exhibit a single major protein on SDS-PAGE of 47 000 Mr. Attempts to sequence the reaction center polypeptide indicated that the N-terminus is blocked. After enzymatic and chemical cleavage, four peptide fragments were sequenced from the Heliobacillus mobilis apoprotein. Only one of these sequences showed significant specific similarity to any of the protein and deduced protein sequences in the GenBank data base. This fragment is identical with 56% of the residues, including both cysteines, found in the highly conserved region that is proposed to bind iron-sulfur center FX in the Photosystem I reaction center peptide that is the psaB gene product. The similarity to the psaA gene product in this region is 48%.Redox titrations of laser-flash-induced photobleaching with millisecond decay kinetics on isolated reaction centers from Heliobacterium gestii indicate a midpoint potential of -414 mV with n=2 titration behavior. In membranes, the behavior is intermediate between n=1 and n=2, and the apparent midpoint potential is -444 mV. This is compared to the behavior in Photosystem I, where the intermediate electron acceptor A1, thought to be a phylloquinone molecule, has been proposed to undergo a double reduction at low redox potentials in the presence of viologen redox mediators.These results strongly suggest that the acceptor side electron transfer system in reaction centers from heliobacteria is indeed analogous to that found in Photosystem I. The sequence similarities indicate that the divergence of the heliobacteria from the Photosystem I line occurred before the gene duplication and subsequent divergence that lead to the heterodimeric protein core of the Photosystem I reaction center.

Identification of 8 1-hydroxychlorophyll a as a functional reaction center pigment in heliobacteria

Chromatographic analysis of membranes and antenna-reaction center complexes of Heliobacterium chlorum and membranes of Heliobacillus mobilis revealed the presence of a small amount of a polar pigment with an absorption spectrum which was essentially identical to that of chlrophyll (Chl) a. Its structure was elucidated by means of 1H-NMR Correlated Spectroscopy, 13C-NMR and 252Cf-Plasma Desorption Mass Spectrometry as 81-hydroxychlorophyll a (81-OH-Chl a), esterified with farnesol. It is concluded that 81-OH-Chl a is responsible for the band near 670 nm in the absorption spectrum of heliobacteria. The molar ratio bacteriochlorophyll (BChl) g: 81-OH-Chl a was approx. 17 for rapidly growing cells of H. chlorum and Hb. mobilis. On the assumption that the antenna consists of 35–40 BChls g, this means that two molecules of 81-OH-Chl a are present per reaction center. At least one of them functions as electron acceptor in the primary charge separation.

Bacteriochlorophyll g epimer as a possible reaction center component of heliobacteria

Normal-phase HPLC analysis of acetone extracts of cells, membranes and antenna-reaction center complexes of Heliobacterium chlorum and Heliobacillus mobilis showed the presence of bacteriochlorophyll (BChl) g′, the 132-epimer of BChl g. The molar ratio of BChl g:BChl g′ in these preparations was approx. 18 in cells and membranes of both species. This value, when combined with a molar ratio of BChl g to the primary electron donor, P-798, of 35–40, yields a BChl g′:P-798 ratio of 2, suggesting that P-798 may be a dimer of BChl g′. The amount of BChl g′ in the antenna-reaction center complex of H. chlorum was slightly higher, suggesting that some epimerization may have occurred during the isolation of the complex. In contrast, bacteriopheophytins (BPhe) g and g′ were present in too small amouts to be essential components of the photosynthetic apparatus. This confirms the idea that the reaction center of heliobacteria is basically different from those of purple bacteria and Photosystem II. A detailed interpretation, based on Correlated Spectroscopy and Double Resonance experiments is given of the 1H-NMR spectra of BChl g and BChl g′, confirming the structure and identity of both pigments. Absorption and circular dichroism spectra of BChl g and BChl g′ are also presented.

Fatty acid and lipopolysaccharide analyses of three Heliobacterium spp

The cellular fatty acids from Heliobacterium chlorum, Heliobacterium gestii, and Heliobacillus mobilis were analyzed. The fatty acid contents of the three organisms were essentially the same, consisting of large amounts of branched chain and some mono-unsaturated fatty acids. Neither a phenol-water nor a phenol-petroleum ether-chloroform extraction of whole cells yielded lipopolysaccharide.

Purification and properties of an antenna-reaction center complex from heliobacteria

Antenna-reaction center complexes were prepared by incubation of membranes of Heliobacterium chlorum and Heliobacillus mobilis with the detergent sulfobetaine-12, followed by sucrose gradient centrifugation in the presence of sulfobetaine-12 and sodium cholate. Similar results were obtained with n-octyl β- d-glucopyranoside. Further purification was obtained by size-exclusion HPLC. From the elution volume, the molecular masses for the purified complexes solubilized with sulfobetaine-12 were estimated to be 335 kDa for both species, not corrected for the weight of the associated detergent molecules. SDS-polyacrylamide gel electrophoresis showed the presence of a dominant peptide located at 94 kDa, together with minor and variable bands near 50 kDa and above 100 kDa. The spectral properties of the isolated complexes were very similar for the two species and also almost identical to those of the membranes used as starting material. Fluoresence emission and excitation spectra showed efficient energy transfer within the complexes. The reaction center activity, measured by the yield and extent of photo-oxidation of the primary electron donor P-798, was completely conserved. These observations indicate that the structure of the antenna and of the reaction center was fully retained during solubilization and isolation. It is concluded that the complexes probably contain the complete pigment complement of a single photosynthetic unit, together with the associated reaction center. Although the estimated molecular weights of the antenna-reaction center complexes are similar to that of the Photosystem I core, the peptide composition appears to suggest a basically different structure.