Biosynthesis of the photosynthetic membranes of Rhodopseudomonas sphaeroides

Selective expression of light-harvesting complexes alters phospholipid composition in the intracytoplasmic membrane and core complex of purple phototrophic bacteria

Phospholipid-protein interactions play important roles in regulating the function and morphology of photosynthetic membranes in purple phototrophic bacteria. Here, we characterize the phospholipid composition of intracytoplasmic membrane (ICM) from Rhodobacter (Rba.) sphaeroides that has been genetically altered to selectively express light-harvesting (LH) complexes. In the mutant strain (DP2) that lacks a peripheral light-harvesting (LH2) complex, the phospholipid composition was significantly different from that of the wild-type strain; strain DP2 showed a marked decrease in phosphatidylglycerol (PG) and large increases in cardiolipin (CL) and phosphatidylcholine (PC) indicating preferential interactions between the complexes and specific phospholipids. Substitution of the core light-harvesting (LH1) complex of Rba. sphaeroides strain DP2 with that from the purple sulfur bacterium Thermochromatium tepidum further altered the phospholipid composition, with substantial increases in PG and PE and decreases in CL and PC, indicating that the phospholipids incorporated into the ICM depend on the nature of the LH1 complex expressed. Purified LH1-reaction center core complexes (LH1-RC) from the selectively expressing strains also contained different phospholipid compositions than did core complexes from their corresponding wild-type strains, suggesting different patterns of phospholipid association between the selectively expressed LH1-RC complexes and those purified from native strains. Effects of carotenoids on the phospholipid composition were also investigated using carotenoid-suppressed cells and carotenoid-deficient species. The findings are discussed in relation to ICM morphology and specific LH complex-phospholipid interactions.

Restricted Localization of Photosynthetic Intracytoplasmic Membranes (ICMs) in Multiple Genera of Purple Nonsulfur Bacteria

In bacteria and eukaryotes alike, proper cellular physiology relies on robust subcellular organization. For the phototrophic purple nonsulfur bacteria (PNSB), this organization entails the use of a light-harvesting, membrane-bound compartment known as the intracytoplasmic membrane (ICM). Here we show that ICMs are spatially and temporally localized in diverse patterns among PNSB. We visualized ICMs in live cells of 14 PNSB species across nine genera by exploiting the natural autofluorescence of the photosynthetic pigment bacteriochlorophyll (BChl). We then quantitatively characterized ICM localization using automated computational analysis of BChl fluorescence patterns within single cells across the population. We revealed that while many PNSB elaborate ICMs along the entirety of the cell, species across as least two genera restrict ICMs to discrete, nonrandom sites near cell poles in a manner coordinated with cell growth and division. Phylogenetic and phenotypic comparisons established that ICM localization and ICM architecture are not strictly interdependent and that neither trait fully correlates with the evolutionary relatedness of the species. The natural diversity of ICM localization revealed herein has implications for both the evolution of phototrophic organisms and their light-harvesting compartments and the mechanisms underpinning spatial organization of bacterial compartments.IMPORTANCE Many bacteria organize their cellular space by constructing subcellular compartments that are arranged in specific, physiologically relevant patterns. The purple nonsulfur bacteria (PNSB) utilize a membrane-bound compartment known as the intracytoplasmic membrane (ICM) to harvest light for photosynthesis. It was previously unknown whether ICM localization within cells is systematic or irregular and if ICM localization is conserved among PNSB. Here we surveyed ICM localization in diverse PNSB and show that ICMs are spatially organized in species-specific patterns. Most strikingly, several PNSB resolutely restrict ICMs to regions near the cell poles, leaving much of the cell devoid of light-harvesting machinery. Our results demonstrate that bacteria of a common lifestyle utilize unequal portions of their intracellular space to harvest light, despite light harvesting being a process that is intuitively influenced by surface area. Our findings therefore raise fundamental questions about ICM biology and evolution.

Intrinsic curvature properties of photosynthetic proteins in chromatophores

In purple bacteria, photosynthesis is carried out on large indentations of the bacterial plasma membrane termed chromatophores. Acting as primitive organelles, chromatophores are densely packed with the membrane proteins necessary for photosynthesis, including light harvesting complexes LH1 and LH2, reaction center (RC), and cytochrome bc(1). The shape of chromatophores is primarily dependent on species, and is typically spherical or flat. How these shapes arise from the protein-protein and protein-membrane interactions is still unknown. Now, using molecular dynamics simulations, we have observed the dynamic curvature of membranes caused by proteins in the chromatophore. A membrane-embedded array of LH2s was found to relax to a curved state, both for LH2 from Rps. acidophila and a homology-modeled LH2 from Rb. sphaeroides. A modeled LH1-RC-PufX dimer was found to develop a bend at the dimerizing interface resulting in a curved shape as well. In contrast, the bc(1) complex, which has not been imaged yet in native chromatophores, did not induce a preferred membrane curvature in simulation. Based on these results, a model for how the different photosynthetic proteins influence chromatophore shape is presented.

Employing Rhodobacter sphaeroides to functionally express and purify human G protein-coupled receptors

G protein-coupled receptors (GPCRs) represent the largest class of cell surface receptors and play crucial roles in many cellular and physiological processes. Functional production of recombinant GPCRs is one of the main bottlenecks to obtaining structural information. Here, we report the use of a novel bacterial expression system based on the photosynthetic bacteriumRhodobacter sphaeroidesfor the production of human recombinant GPCRs. The advantage of employingR. sphaeroidesas a host lies in the fact that it provides much more membrane surface per cell compared to other typical expression hosts. The system was tailored to overexpress recombinant receptors under the control of the moderately strong and highly regulated superoperonic photosynthetic promoterpufQ. We tested this system for the expression of some class A GPCRs, namely, the human adenosine A2a receptor (A2aR), the human angiotensin AT1a receptor (AT1aR) and the human bradykinin B2 receptor (B2R). Several different constructs were examined and functional production of the recombinant receptors was achieved. The best-expressed receptor, AT1aR, was solubilized and affinity-purified. To the best of our knowledge, this is the first report of successful use of a bacterial host –R. sphaeroides– to produce functional recombinant GPCRs under the control of a photosynthetic gene promoter.

Transcriptome dynamics during the transition from anaerobic photosynthesis to aerobic respiration in Rhodobacter sphaeroides 2.4.1

Rhodobacter sphaeroides 2.4.1 is a facultative photosynthetic anaerobe that grows by anoxygenic photosynthesis under anaerobic-light conditions. Changes in energy generation pathways under photosynthetic and aerobic respiratory conditions are primarily controlled by oxygen tensions. In this study, we performed time series microarray analyses to investigate transcriptome dynamics during the transition from anaerobic photosynthesis to aerobic respiration. Major changes in gene expression profiles occurred in the initial 15 min after the shift from anaerobic-light to aerobic-dark conditions, with changes continuing to occur up to 4 hours postshift. Those genes whose expression levels changed significantly during the time series were grouped into three major classes by clustering analysis. Class I contained genes, such as that for the aa3 cytochrome oxidase, whose expression levels increased after the shift. Class II contained genes, such as those for the photosynthetic apparatus and Calvin cycle enzymes, whose expression levels decreased after the shift. Class III contained genes whose expression levels temporarily increased during the time series. Many genes for metabolism and transport of carbohydrates or lipids were significantly induced early during the transition, suggesting that those endogenous compounds were initially utilized as carbon sources. Oxidation of those compounds might also be required for maintenance of redox homeostasis after exposure to oxygen. Genes for the repair of protein and sulfur groups and uptake of ferric iron were temporarily upregulated soon after the shift, suggesting they were involved in a response to oxidative stress. The flagellar-biosynthesis genes were expressed in a hierarchical manner at 15 to 60 min after the shift. Numerous transporters were induced at various time points, suggesting that the cellular composition went through significant changes during the transition from anaerobic photosynthesis to aerobic respiration. Analyses of these data make it clear that numerous regulatory activities come into play during the transition from one homeostatic state to another.

Development of the bacterial photosynthetic apparatus

Anoxygenic photosynthetic bacteria have provided us with crucial insights into the process of solar energy capture, pathways of metabolic and societal importance, specialized differentiation of membrane domains, function or assembly of bioenergetic enzymes, and into the genetic control of these and other activities. Recent insights into the organization of this bioenergetic membrane system, the genetic control of this specialized domain of the inner membrane and the process by which potentially photosynthetic and non-photosynthetic cells protect themselves from an important class of reactive oxygen species will provide an unparalleled understanding of solar energy capture and facilitate the design of solar-powered microbial biorefineries.

A second and unusual pucBA operon of Rhodobacter sphaeroides 2.4.1: genetics and function of the encoded polypeptides

A new operon (designated the puc2BA operon) displaying a high degree of similarity to the original pucBA genes of Rhodobacter sphaeroides 2.4.1 (designated puc1) was identified and studied genetically and biochemically. The puc2B-encoded polypeptide is predicted to exhibit 94% identity with the original beta-apoprotein. The puc2A-encoded polypeptide is predicted to be much larger (263 amino acids) than the 54-amino-acid puc1A-encoded polypeptide. In the first 48 amino acids of the puc2A-encoded polypeptide there is 58% amino acid sequence identity to the original puc1A-encoded polypeptide. We found that puc2BA is expressed, and DNA sequence data suggested that puc2BA is regulated by the PpsR/AppA repressor-antirepressor and FnrL. Employing genetic and biochemical approaches, we obtained evidence that the puc2B-encoded polypeptide is able to enter into LH2 complex formation, but neither the full-length puc2A-encoded polypeptide nor its N-terminal 48-amino-acid derivative is able to enter into LH2 complex formation. Thus, the sole source of alpha-polypeptides for the LH2 complex is puc1A. The role of the puc1C-encoded polypeptide was also determined. We found that the presence of this polypeptide is essential for normal levels of transcription and translation of the puc1 operon but not for transcription and translation of the puc2 operon. Thus, the puc1C gene product appears to have both transcriptional and posttranscriptional roles in LH2 formation. Finally, the absence of any LH2 complex when puc1B was deleted in frame was surprising since we know that in the presence of functional puc2BA, approximately 30% of the LH2 complexes normally observed contain a puc2B-encoded beta-polypeptide.

Supramolecular organization of the photosynthetic chain in chromatophores and cells of Rhodobacter sphaeroides

Flash-induced kinetics of the membrane potential increase related to electron transfer within the cytochrome (cyt) b/c1 complex (Phase III) and that of cyt c1+c2 reduction have been measured as a function of myxothiazol concentration in isolated chromatophores and whole cells of Rhodobacter sphaeroides. Upon addition of nonsaturating concentrations of myxothiazol, kinetics of Phase III display two phases, Phase IIIa and Phase IIIb. The amplitude of Phase IIIa, completed in about 10 ms, is proportional to the fraction of non-inhibited cyt b/c1 complexes, while its half-time is independent of the myxothiazol concentration. A fast cyt c1+c2 reduction phase is correlated to Phase IIIa. These experiments demonstrate that, in a range of time of several ms, diffusion of cyt c2 is restricted to domains formed by a supercomplex including two reaction centers (RCs) and a single cyt b/c1 complex, as proposed by Joliot et al. (Biochim Biophys Acta 975: 336-345, 1989). Phase IIIb, completed in about 100 ms, shows that positive charges or inhibitor molecules are exchanged between supercomplexes in this range of time. These exchanges occur within domains including 2 to 3 supercomplexes, i.e. in membrane domains smaller than a single chromatophore. These conclusions apply to both isolated chromatophores and whole cells.

Isolation and expression of the Rhodobacter sphaeroides gene (pgsA) encoding phosphatidylglycerophosphate synthase

The Rhodobacter sphaeroides pgsA gene (pgsARs), encoding phosphatidylglycerophosphate synthase (PgsARs), was cloned, sequenced, and expressed in both R. sphaeroides and Escherichia coli. As in E. coli, pgsARs is located immediately downstream of the uvrC gene. Comparison of the deduced amino acid sequences revealed 41% identity and 69% similarity to the pgsA gene of E. coli, with similar homology to the products of the putative pgsA genes of several other bacteria. Comparison of the amino acid sequences of a number of enzymes involved in CDP-diacylglycerol-dependent phosphatidyltransfer identified a highly conserved region also found in PgsARs. The pgsARs gene carried on multicopy plasmids was expressed in R. sphaeroides under the direction of its own promoter, the R. sphaeroides rrnB promoter, and the E. coli lac promoter, and this resulted in significant overproduction of PgsARs activity. Expression of PgsARs activity in E. coli occurred only with the E. coli lac promoter. PgsARs could functionally replace the E. coli enzyme in both a point mutant and a null mutant of E. coli pgsA. Overexpression of PgsARs in either E. coli or R. sphaeroides did not have dramatic effects on the phospholipid composition of the cells, suggesting regulation of the activity of this enzyme in both organisms.

Import and assembly of the α and β-polypeptides of the light-harvesting complex I (B870) in the membrane system of Rhodobacter capsulatus investigated in an in vitro translation system

Transcripts of the genes pufBA, pufB or pufA from Rhodobacter capsulatus were translated in a cell-free system of R. capsulatus. The incorporation of the nascent polypeptides LHIα and β in various types of membranes and the assembly of the light-harvesting (LH) complex I (B870) were investigated. The highest rate of stable incorporation of LHIα and β into the membrane was observed with membranes from the wild type strain grown under chemotrophic conditions. Addition of membranes from cells defective in biosynthesis of pigment-binding proteins resulted in a less efficient or less stable incorporation of LHIαβ. The single polypeptides LHIα or β were synthesized and inserted into the membrane but were extractable to a higher percentage by 6 M urea than the pairwise inserted LHI polypeptides.If the ribosomes and the S135 extract were depleted of DnaK the rate of synthesis of both polypeptides, LHIα and β, was strongly reduced. Removal of GroEL from the cell-free system did not impair the synthesis and membrane association of both proteins, but affected the stable insertion. A high percentage of the LHIαβ polypeptides became extractable by 6 M urea if the cell-free system was depleted of GroEL. Addition of GroEL to the cell-free system restored the capacity of stable insertion of both proteins into the membrane. GroEL interacted with LHIα and β before membrane targeting as shown by immunological means.A protein fraction, which can be removed from the membrane with a low-salt buffer, supported the effective and stable incorporation of LHIαβ into the membrane. It is concluded that the assembly of the LHI complex in the membrane system of R. capsulatus is a multistep process guided and supported by polypeptides located in the cytoplasm and in the membrane. In the cell-free in vitro system not only the correct insertion of the LHI polypeptides but also an assembly with bacteriochlorophyll was observed. BChl was synthesized from δ-amino levulinate in the cell free system.

cis-acting regulatory elements involved in oxygen and light control of puc operon transcription in Rhodobacter sphaeroides

Transcriptional expression of the puc operon in Rhodobacter sphaeroides is highly regulated by both oxygen and light. The approximately 600 bp of DNA upstream of the 5' ends of the two puc-specific transcripts encompasses two functionally separable cis-acting domains. The upstream regulatory region (URS) (-629 to -150) is responsible for enhanced transcriptional regulation of puc operon expression by oxygen and light. The more proximal upstream region (downstream regulatory region [DRS]), containing putative promoter(s), operator(s), and factor binding sites (-150 to -1), is involved in unenhanced transcriptional expression of the puc operon under aerobic and anaerobic conditions. Thus, the DRS shows normal derepression of puc operon expression when cells are shifted from aerobic to photosynthetic growth conditions in terms of percent change but does not show the potential range of expression that is only observed when elements of the URS are present. Because of these observations, we have made a distinction between anaerobic control (describing the shift) and oxygen control (describing the magnitude of derepression). Promoter(s) and/or activator function(s) of the puc operon is associated with a 35-bp DNA region between -92 and -57. Homologous sequences at -10 to -27 and -35 to -52 appear to involve additional regulatory elements: mutations at -12 (A to C) and -26 (G to A) result in partial derepression of puc operon expression under conditions of high aeration. Both point mutations require the upstream regulatory region (-629 to -150) to be present in cis for partial derepression of puc operon transcription under aerobic conditions. Immediately upstream of the promoter and/or activator region are overlapping consensus sequences for IHF (integratin host factor) and FNR (fumarate nitrate reductase) (-105 to -129). This region appears to be essential for enhanced expression of the puc operon. Thus, these two regulatory domains (URS and DRS) appear to involve approximately seven unique regulatory elements. In addition, the data reveal a direct interaction between the URS (-629 to -150) and the DRS (-150 to -1).

Construction, expression, and localization of a CycA::PhoA fusion protein in Rhodobacter sphaeroides and Escherichia coli

We demonstrated the utility of Escherichia coli alkaline phosphatase, encoded by phoA, as a reporter molecule for genetic fusions in Rhodobacter sphaeroides. A portion of the R. sphaeroides cycA gene was fused to phoA, yielding a fusion protein comprising the putative signal sequence and first 10 amino acids of the cytochrome c2 apoprotein joined to the sixth amino acid of alkaline phosphatase. The fusion protein was efficiently transported to the periplasm of R. sphaeroides as determined by enzyme activity, Western immunoblot analysis, and immunogold electron microscopy. We also documented the ability of an R. sphaeroides mutant, RS104, with gross defects in photosynthetic membrane morphology to efficiently recognize and translocate the fusion protein to the periplasmic compartment. The inclusion of 500 base pairs of R. sphaeroides DNA in cis to the cycA structural gene resulted in a 2.5-fold increase in alkaline phosphatase activity in photosynthetically grown cells compared with the activity in aerobically grown cells, demonstrating that the fusion protein is regulated in a manner similar to that of cytochrome c2 regulation. We also constructed two pUC19-based plasmids suitable for the construction of translational fusions to phoA. In these plasmids, translational fusions of phoA to the gene under consideration can be made in all three reading frames, thus facilitating construction and expression of fusion protein systems utilizing phoA.

Characterization and localization of phosphatidylglycerophosphate and phosphatidylserine synthases in Rhodobacter sphaeroides

Catalytic properties and membrane associations of the phosphatidylglycerophosphate (PGP) and phosphatidylserine (PS) synthases of Rhodobacter sphaeroides were examined to further characterize sites of phospholipid biosynthesis. In preparations of cytoplasmic membrane (CM) enriched in these activities, apparent Km values of PGP synthase were 90 microM for sn-glycerol-3-phosphate and 60 microM for CDP-diacylglycerol; the apparent Km of PS synthase for L-serine was near 165 microM. Both enzymes required Triton X-100 with optimal PS synthase activity at a detergent/CDP-diacylglycerol (mol/mol) ratio of 7.5:1.0, while for optimal PGP synthase, a range of 10-50:1.0 was observed. Unlike the enzyme in Escherichia coli and several other Gram-negative bacteria, the PS synthase activity had a specific requirement for magnesium and was tightly associated with membranes rather than ribosomes in crude cell extracts. Sedimentation studies suggested that the PGP synthase was distributed uniformly over the CM in both chemoheterotrophically and photoheterotrophically grown cells, while the PS synthase was confined mainly to a vesicular CM fraction. Solubilized PGP synthase activity migrated as a single band with a pI value near 5.5 in a chromato-focusing column and 5.8 on isoelectric focusing; in the latter procedure, the pI was shifted to 5.3 in the presence of CDP-diacylglycerol. The PGP synthase activity gave rise to a single polypeptide band in lithium dodecyl sulfate-polyacrylamide gel electrophoresis at 4 degrees C.

Posttranscriptional control of puc operon expression of B800-850 light-harvesting complex formation in Rhodobacter sphaeroides

The puc operon of Rhodobacter sphaeroides comprises the pucBA structural genes which encode B800-850 light-harvesting beta and alpha polypeptides, respectively. Northern (RNA) blot hybridization analysis of puc operon expression has identified two pucBA-specific transcripts. The small (0.5-kilobase [kb]) transcript encodes the beta and alpha polypeptides and, under photoheterotrophic growth conditions, was approximately 200-fold more abundant than the large (2.3-kb) transcript. The 5' end of the 0.5-kb transcript was mapped at 117 nucleotides upstream from the start of pucB. The 3' ends of the 0.5-kb transcript were mapped to two adjacent nucleotides, which follow a stem-loop structure immediately 3' to the pucA stop codon. Two mutant strains, PUC705-BA and PUC-Pv, were constructed by replacement of the pucBA genes and adjacent DNA in the former case or by insertional interruption of the DNA downstream of the pucBA genes in the latter case. The two mutant strains were devoid of B800-850 complexes during photosynthetic growth but were otherwise apparently normal. The B800-850 phenotype of both PUC705-BA and PUC-Pv was not complemented in trans with a 2.5-kb PstI restriction endonuclease fragment extending from 0.75 kb upstream of pucBA to 1.3 kb downstream of pucBA, despite the presence of the 0.5-kb pucBA-specific transcript. Both of the mutant strains, however, showed restoration of B800-850 expression with a 10.5-kb EcoRI restriction endonuclease fragment in trans encompassing the 2.5-kb PstI fragment. Western immunoblot analysis revealed no B800-850-beta polypeptide as well as no polypeptide designated 15A in either mutant. Nonetheless, under photoheterotrophic growth conditions, the 0.5-kb pucBA-specific transcript was present in PUC-Pv, although no 2.3-kb transcript was detectable. We suggest that the DNA region immediately downstream of pucBA encodes a gene product(s) essential for translational or posttranslational expression of the B800-850 beta and alpha polypeptides.

Molecular genetics of photosynthetic membrane biosynthesis in Rhodobacter sphaeroides

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Cloning, DNA sequence, and expression of the Rhodobacter sphaeroides light-harvesting B800-850-alpha and B800-850-beta genes

Two deoxyoligonucleotide probes were synthesized in accordance with the available amino acid sequence of the B800-850-beta polypeptide from Rhodobacter sphaeroides and were used to isolate a 2.6-kilobase PstI fragment from R. sphaeroides 2.4.1 chromosomal DNA. Identification of the B800-850-beta and B800-850-alpha structural genes, pucB and pucA, was confirmed by DNA sequencing. Northern (RNA) blot analysis, using restriction endonuclease fragments from the cloned genes as probes, revealed a single puc-operon-specific, highly stable transcript of approximately 640 bases present in photosynthetically grown cells. In vitro transcription-translation analysis of the puc operon revealed that the maximum synthesis of the puc operon gene products was achieved when the entire 2.6-kilobase PstI fragment was used as the template, although a 537-base-pair XmaIII fragment was sufficient to direct the synthesis of pucB and pucA fusion product.

DNA sequence and in vitro expression of the B875 light-harvesting polypeptides of Rhodobacter sphaeroides

The genes for the Rhodobacter sphaeroides light-harvesting B875-beta, and B875-alpha polypeptides (pufB and pufA) are closely linked to the genes for the reaction center L and reaction center M polypeptides (pufL and pufM) on what has been termed the puf operon (gene order, pufB, A, L, M). The DNA sequence of the pufB and pufA structural genes from wild-type R. sphaeroides 2.4.1 was determined and aligned with the available amino acid sequence of the wild-type B875-beta and B875-alpha polypeptides. The relative levels of the B875-beta and B875-alpha and the reaction center L and reaction center M polypeptides synthesized in a homologous cell-free transcription-translation system were compared with those found in vivo. Analysis of the gene products produced in vitro with plasmids containing deletions upstream of the pufB structural gene identified a region of DNA required for expression of the B875-beta and B875-alpha polypeptides. These results support the hypothesis that the mapped 5' termini of the large and small puf operon transcripts represent transcription initiation sites.

Phospholipid transfer activity in synchronous populations of Rhodobacter sphaeroides

Studies of intracytoplasmic membrane biogenesis employing steady-state synchronously dividing populations of Rhodobacter sphaeroides reveal that the translocation of pre-existing phospholipid into the growing membrane is concurrent with cell division (Cain, B.D., Deal, C.D., Fraley, R.T. and Kaplan, S. (1981) J. Bacteriol. 145, 1154-1166), yet the mechanism of phospholipid movement is unknown. However, the discovery of phospholipid transfer protein activity in R. sphaeroides (Cohen, L.K., Lueking, D.R. and Kaplan, S. (1979) J. Biol. Chem. 254, 721-728) provides one possible mechanism for phospholipid movement. Therefore the level of phospholipid transfer activity in cell lysates of synchronized cultures was measured and was shown to increase stepwise coinciding precisely with the increase in cell number of the culture. Although the amount of transfer activity per cell remained constant throughout the cell cycle, the specific activity of the phospholipid transfer activity showed a cyclical oscillation with its highest value coincident with the completion of cell division. Purified intracytoplasmic membrane can be used as phospholipid acceptor in the developed phospholipid transfer assay by employing either cytoplasmic membrane or liposomes as the phospholipid donor. Intracytoplasmic membrane isolated from the cells prior to division (high protein to phospholipid ratio) served as a better phospholipid acceptor in the phospholipid transfer system when compared with membranes derived from the cells following cell division (low protein to phospholipid ratio).

Purification and properties of acyl coenzyme A thioesterase II from Rhodopseudomonas sphaeroides

A high molecular weight acyl coenzyme A (acyl-CoA) thioesterase, designated thioesterase II, has been purified 5300-fold from photoheterotrophically grown cells of Rhodopseudomonas sphaeroides. In contrast to R. sphaeroides acyl-CoA thioesterase I [Boyce, S.G., & Lueking, D.R. (1984) Biochemistry 23, 141-147], thioesterase II has a native molecular mass (Mr) of 120,000, is capable of hydrolyzing saturated and unsaturated acyl-CoA substrates with acyl chain lengths ranging from C4 to C18, and is completely insensitive to the serine esterase inhibitor diisopropyl fluorophosphate. Palmitoyl-CoA and stearoyl-CoA are the preferred (lowest Km) saturated acyl-CoA substrates and vaccenoyl-CoA is the preferred unsaturated substrate. However, comparable Vmax values were obtained with a variety of acyl-CoA substrates. Unlike a similar thioesterase present in cells of Escherichia coli [Bonner, W.M., & Bloch, K. (1972) J. Biol. Chem. 247, 3123-3133], R. sphaeroides thioesterase II displays a high ratio of decanoyl-CoA to palmitoyl-CoA activities and exhibits little ability to hydrolyze 3-hydroxyacyl-CoA substrates. Only 3-hydroxydodecanoyl-CoA supported a measurable rate of enzyme activity. With the purification of thioesterase II, the enzymes responsible for greater than 90% of the acyl-CoA thioesterase activity present in cell-free extracts of R. sphaeroides have now been identified.

Intracellular localization of phospholipid transfer activity in Rhodopseudomonas sphaeroides and a possible role in membrane biogenesis

The cellular content of phospholipid transfer activity in Rhodopseudomonas sphaeroides was examined as a function of both oxygen partial pressure and light intensity used for growth. Cells grown under high light conditions (100 W/m2) had over two times the cellular level of phospholipid transfer activity when compared with cells grown under other conditions. Although cells grown under low light conditions (3 W/m2) had the lowest amount of total phospholipid transfer activity, they had the highest level (49%) of membrane-associated transfer activity. The soluble phospholipid transfer activity was further localized into periplasmic and cytoplasmic fractions. The distribution of phospholipid transfer activity in cells grown under medium light intensity (10 W/m2) was calculated as 15.1% membrane-associated, 32.4% in the periplasm, and 52.5% in the cytoplasm. The phospholipid transfer activities in the periplasmic and cytoplasmic fractions had distinctly different properties with respect to their molecular weights (56,000 versus 27,000) and specificities of transfer (phosphatidylethanolamine greater than phosphatidylglycerol versus phosphatidylglycerol greater than phosphatidylethanolamine).

Excitation energy transfer in Rhodopseudomonas sphaeroides chromatophore membranes fused with liposomes

The role of phospholipid in the structural organization of the light-harvesting complexes of Rhodopseudomonas sphaeroides was examined in photosynthetic (chromatophore) membrane vesicles fused with liposomes. Photochemically active preparations with progressive phospholipid enrichment up to greater than 15-fold were obtained by both polyethylene glycol- and acidic-pH-induced fusion. Their fluorescence emission at approximately 300 and 77 K was increased by 2-3.5-fold from the peripheral B800-850 antenna relative to that from the core B875 antenna. Up to 30-40% reduction in the efficiency of excitation energy transfer between B850 and B875 was also observed at 77 K suggesting a selective, phospholipid-induced dissociation of a portion of the B800-850 from the rest of the light-harvesting system.

Electron transport in chromatophores from Rhodopseudomonas sphaeroides GA fused with liposomes

Chromatophores from Rhodopseudomonas sphaeroides GA were fused with liposomes in order to dilute the components of the cyclic photosynthetic electron-transport chain within the membrane. This dilution led to a decrease in the rate of cytochrome b-561 reduction. The original rates could be restored at potentials around 100 mV (where a large part of the quinone pool is chemically reduced), if ubiquinone was incorporated into the liposomes prior to fusion. Similar dilution effects could be observed in synchronized cultures. The membrane obtained after division contained about twice the amount of phospholipids per reaction center when compared to chromatophores prepared from cells harvested just before division. Chromatophores from synchronized cultures are more uniform with respect to the concentration of the different electron-transport components in the membrane than the membranes from normally grown cells. The kinetic behaviour both of fused chromatophores and of membranes from synchronized cultures are in agreement with a modified Q-cycle model for photosynthetic electron transport in Rps. sphaeroides. The results presented in this paper cannot be explained by postulating the presence of a firmly bound quinone, Qz, in the ubiquinol: cytochrome c2 oxidoreductase, as previously proposed.