Protein components of bacterial photosynthetic membranes

Stabilization of Cereibacter sphaeroides Photosynthetic Reaction Center by the Introduction of Disulfide Bonds

The photosynthetic reaction center of the purple nonsulfur bacterium Cereibacter sphaeroides is a useful model for the study of mechanisms of photoinduced electron transfer and a promising component for photo-bio-electrocatalytic systems. The basic research and technological applications of this membrane pigment-protein complex require effective approaches to increase its structural stability. In this work, a rational design approach to genetically modify the reaction centers by introducing disulfide bonds is used. This resulted in significantly increasing the thermal stability of some of the mutant pigment-protein complexes. The formation of the S-S bonds was confirmed by X-ray crystallography as well as SDS-PAGE, and the optical properties of the reaction centers were studied. The genetically modified reaction centers presented here preserved their ability for photochemical charge separation and could be of interest for basic science and biotechnology.

Evolution of photosystem I and the control of global enthalpy in an oxidizing world

Life on earth is governed by light, chemical reactions, and the second law of thermodynamics, which defines the tendency for increasing entropy as an expression of disorder or randomness. Life is an expression of increasing order, and a constant influx of energy and loss of entropic wastes are required to maintain or increase order in living organisms. Most of the energy for life comes from sunlight and, thus, photosynthesis underlies the survival of all life forms. Oxygenic photosynthesis determines not only the global amount of enthalpy in living systems, but also the composition of the Earth's atmosphere and surface. Photosynthesis was established on the Earth more than 3.5 billion years ago. The primordial reaction center has been suggested to comprise a homodimeric unit resembling the core complex of the current reaction centers in Chlorobi, Heliobacteria, and Acidobacteria. Here, an evolutionary scenario based on the known structures of the current reaction centers is proposed.

Photosystem I reaction center from the thermophilic cyanobacterium Mastigocladus laminosus

Photosystem I reaction center was isolated from the cyanobacterium Mastigocladus laminosus. It contained four different subunits with molecular masses (as determined by sodium dodecyl sulfate gels) of about 70,000 (subunit I), 16,000 (subunit II), 11,000 (subunit III), and 10,000 (subunit IV) daltons. The purified reaction center contained about 100 chlorophyll a molecules per P(700); however, they could be readily depleted down to about 50 chlorophyll a per P(700) without loss in the photochemical activities. The reaction center was active in cytochrome c photooxidation, but the photooxidation of an acidic cytochrome, like the Euglena cytochrome 552, required the presence of cations. The purified reaction center was found to be similar in several respects to the photosystem I reaction centers from higher plants and, especially, to the one isolated from green algae. Subunit I appeared on sodium dodecyl sulfate gels in the same position and possessed the same shape of an apparent double band as the corresponding subunits I of green plants and of algae. Subunits I and II of photosystem I reaction centers from Mastigocladus, higher plants, and green algae showed immunological crossreactivity. This observation might serve as biochemical evidence for the common evolution of the photosystem I reaction centers. In higher plants and green algae subunit II is a product of cytoplasmic ribosomes and therefore, a high degree of homology should have been preserved upon transfer of its gene from the prokaryote to the nucleus of the eukaryotes.

Structure and function of photosystems I and II

Oxygenic photosynthesis, the principal converter of sunlight into chemical energy on earth, is catalyzed by four multi-subunit membrane-protein complexes: photosystem I (PSI), photosystem II (PSII), the cytochrome b(6)f complex, and F-ATPase. PSI generates the most negative redox potential in nature and largely determines the global amount of enthalpy in living systems. PSII generates an oxidant whose redox potential is high enough to enable it to oxidize H(2)O, a substrate so abundant that it assures a practically unlimited electron source for life on earth. During the last century, the sophisticated techniques of spectroscopy, molecular genetics, and biochemistry were used to reveal the structure and function of the two photosystems. The new structures of PSI and PSII from cyanobacteria, algae, and plants has shed light not only on the architecture and mechanism of action of these intricate membrane complexes, but also on the evolutionary forces that shaped oxygenic photosynthesis.

Forty-five years of developmental biology of photosynthetic bacteria

Developmental biology and cell differentiation of photosynthetic prokaryotes are less noticed fields than the showpieces of eukaryotes, e.g. Drosophila melanogaster. The large metabolic versatility of the facultative purple bacteria and their great capability to adapt to different ecological conditions, however, aroused the inquisitiveness to investigate the process of cell differentiation and to use these bacteria as model system to study structure, function and biosynthesis of the photosynthetic apparatus. The great progress in research in this field paved the way to study principal mechanisms of cellular organization and differentiation in these bacteria. In this article, the history of the research on membrane structure and development of anoxygenic photosynthetic prokaryotes during the last 45 years is described. A personal account of how I entered the field through research on the phototaxis of cyanobacteria is given. Intracytoplasmic membranes (ICM) were detected by electron microscopy in cyanobacteria and in purple non-sulfur bacteria. The formation of ICM by invagination of the cytoplasmic membrane in purple bacteria was observed for the first time. Investigations on the effect of changes in oxygen tension and light intensity on the formation of pigments and intracytoplasmic membranes followed. The isolation, purification, and analysis of light-harvesting complexes and of pigment-binding proteins was the next step of our research. Lipopolysaccharides and peptidoglycans were detected and analyzed in the outer membrane of photosynthetic bacteria. Functional membrane differentiation includes variations in the rates of photophosphorylation and electron transport. Molecular genetic approaches have initiated the investigation of transcriptional regulation and the analysis of correlation between pigment and protein synthesis. Molecular analysis of assembly of light-harvesting complexes and membrane differentiation are the present aspects of our research. Cell differentiation has been considered under evolutionary view.

The antenna system of Rhodospirillum rubrum. Detection of macromolecular constituents not stainable by Coomassie brilliant blue in solubilized preparations of the B880 complex

A solubilized preparation of the major Rhodospirillum rubrum antenna complex (B880) was obtained by a described procedure and its polypeptide composition was analyzed by polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulfate. Only two polypeptides of molecular weights close to 7000 were detected after staining the gels with Coomassie brilliant blue. However, several other constituents could be visualized by silver staining or by an immunochemical method. When the preparation was chromatographed on Sephacryl, some of the resulting fractions exhibited the characteristic B880 absorption spectrum and contained only the two proteins that were detectable with Coomassie brilliant blue. In those fractions the A 280/A 880ratio was 0.4, which indicated a significant improvement of the bacteriochlorophyll to protein ratio over the unchromatographed preparation (A 280/A 880=0.7). Other chromatography fractions lacked bacteriochlorophyll and contained a carotenoid which seemed to be bound to protein. The macromolecular constituents present in these latter fractions differed from those associated to the purified B880 complex in their electrophoretic moblities and/or in their staining properties. That suggested the possible existence of a carotenoprotein that did not result from the B880 complex upon loss of bacteriochlorophyll.

Differential protein insertion into developing photosynthetic membrane regions of Rhodopseudomonas sphaeroides

Previous studies have suggested that much of the B800-850 light-harvesting bacteriochlorophyll a-protein complex is inserted directly into the intracytoplasmic photosynthetic membrane of Rhodopseudomonas sphaeroides. In contrast, the B875 light-harvesting and reaction center complexes are assembled preferentially at peripheral sites of photosynthetic membrane growth initiation. The basis for this apparent site-specific polypeptide insertion was examined during the inhibition of RNA and protein syntheses. The pulse labeling of polypeptides at the membrane growth initiation sites was significantly less sensitive to inhibition by rifampicin, chloramphenicol, or kasugamycin than in the intracytoplasmic or outer membranes. This suggests increased stability for the translation machinery at these membrane invagination sites. Similar differential effects in polypeptide insertion were observed during inhibition of bacteriochlorophyll synthesis through deprival of delta-aminolevulinate to R sphaeroides mutant H-5, which requires this porphyrin precursor. The pulse-labeling patterns observed during the inhibition of both RNA and pigment syntheses were consistent with the uncoupling of polypeptide insertion into the membrane invagination sites from their growth and maturation into intracytoplasmic membranes.

Alignment of genetic and restriction maps of the photosynthesis region of the Rhodopseudomonas capsulata chromosome by a conjugation-mediated marker rescue technique

The restriction map of a 46-kilobase fragment of the Rhodopseudomonas capsulata chromosome was aligned with the genetic map of the photosynthesis region of that chromosome by a marker rescue technique. Marker rescue was effected by mobilization of vectors bearing fragments of R. capsulata DNA from Escherichia coli to a set of R. capsulata mutants. Plasmids pDPT51 and pDPT55 were constructed to mediate the intergeneric mobilization of pBR322 derivatives, and a mutant of R. capsulata with improved intergeneric recipient activity was isolated. Four previously unmapped genes affecting bacteriochlorophyll synthesis and two genes affecting photochemical reaction center synthesis have been located by marker rescue. Some of the fragments of R. capsulata DNA are capable of vector-independent complementation, implying that promoters are located on these fragments. Other fragments complement only in one orientation of insertion in the vector, implying transcription from promotors on the vectors and thereby fixing the direction of transcription for those fragments. Still other fragments of DNA show rescue only via recombination between homologous plasmid-borne DNA fragments and chromosomal mutations. The physical dimensions of the genetic map are 3.0 megadaltons per map unit, which agrees with previous estimates based on the size of the R. capsulata gene transfer agent.

Rhodopseudomonas sphaeroides membranes: alterations in phospholipid composition in aerobically and phototrophically grown cells

The effects of growth conditions on phospholipid composition in Rhodopseudomonas sphaeroides have been reexamined. The levels of phosphatidylethanolamine (27 to 28%), phosphatidylglycerol (23 to 24%), and phosphatidylcholine (11 to 18%) were very similar in cells grown aerobically or phototrophically at a high light intensity, consistent with findings for another member of Rhodospirillaceae. In addition, an unknown phospholipid species was detected which comprised 20 to 30% of the total phospholipid in these cells. In cells growing phototrophically at low-intensity illumination, the level of phosphatidylethanolamine increased by about 1.6-fold and that of the unknown phospholipid markedly decreased. Although the synthesis of photosynthetic pigments, light-harvesting protein, and intracytoplasmic photosynthetic membranes also increased markedly, the ratios of individual phospholipid species were essentially identical in photosynthetic membrane and cell wall fractions purified from these cells. Since a significant exchange of lipids apparently did not occur during the isolation of these fractions, it was suggested that the changes in cellular phospholipid accumulation were not due to a unique composition within the photosynthetic membrane. Instead, these phosphoglyceride changes were found to be related to overall phospholipid metabolism and could be accounted for principally by differences in biosynthetic rates. These results, together with studies in nutrient-restricted aerobic cells, suggested that the mechanism by which phospholipid levels are regulated may be related to radiant energy flux rather than cellular energy limitation.

The photoreaction center of Rhodospirillum rubrum mutant strain F24.1

Rhodospirillum rubrum strain F24.1 is a spontaneous revertant of nonphototrophic mutant F24 derived from wild-type strain S1. Strain F24 shows no detectable photochemical activity and contains, at most, traces of the photoreaction center polypeptides. Strain F24.1 has a phototrophic growth rate close to that of the wild-type strain (Picorel, R., del Valle-Tascón, S. and Ramírez, J.M. (1977) Arch. Biophys. Biochem. 181, 665-670) but shows little photochemical activity. Light-induced absorbance changes in the near-infrared, photoinduced EPR signals and ferricyanide-elicited absorbance changes indicate that strain F24.1 has a photoreaction center content of 7-8% as compared to strain S1. Polyacrylamide gel electrophoresis of isolated F24.1 chromatophores shows the photoreaction center polypeptides to be present in amounts compatible with this value. Photoreaction center was prepared from strain F24.1 and showed no detectable difference with that of strain S1. It is concluded that strain F24.1 photosynthesis is due entirely to its residual 7-8% of typical photoreaction center.

Membranes of Rhodopseudomonas sphaeroides. VII. Photochemical properties of a fraction enriched in newly synthesized bacteriochlorophyll a-protein complexes

Previous pulse-chase studies have shown that bacteriochlorophyll a-protein complexes destined eventually for the photosynthetic (chromatophore) membrane of Rhodopseudomonas sphaeroides appear first in a distinct pigmented fraction. This rapidly labeled material forms an upper band when extracts of phototrophically grown cells are subjected directly to rate-zone sedimentation. In the present investigation, flash-induced absorbance changes at 605 nm have demonstrated that the upper fraction is enriched two-fold in photochemical reaction center activity when compared to chromotophores; a similar enrichment in the reaction center-associated B-875 antenna bacteriochlorophyll complex was also observed. Although b- and c-type cytochromes were present in the upper pigmented band, no photoreduction of the b-type components could be demonstrated. The endogenous c-type cytochrome (Em = +345 mV) was photooxidized slowly upon flash illumination. The extent of the reaction was increased markedly with excess exogenous ferrocytochrome c but only slightly in chromatophores. Only a small light-induced carotenoid band shift was observed. These results indicate that the rapidly labeled fraction contains photochemically competent reaction centers associated loosely with c-type and unconnected to b-type cytochrome. It is suggested that this fraction arises from new sites of cytoplasmic membrane invagination which fragment to form leaky vesicles upon cell disruption.

Structural studies on reconstituted reaction center-phosphatidylcholine membranes

Reaction center protein, isolated from the photosynthetic bacterium Rhodopseudomonas sphaeroides R26 mutant, was incorporated into phosphatidylcholine bilayers forming a homogeneous population of unilamellar vesicles. Cytochrome c, added to preformed reaction center-phosphatidylcholine vesicles, rapidly reduced up to 90% of the laser-generated (BChl)2+ of the reaction center (with kinetics of electron transfer similar to those in the chromatophore membrane) which suggests that the portion of the reaction center which accommodates functional cytochrome c binding sites is exposed predominantly on the exterior of the vesicles. Unit cell electron density profiles were derived from lamellar X-ray diffraction from oriented reaction center-phosphatidylcholine membrane multilayers at varying lipid/protein ratios. The analysis of these profiles showed that the reaction center protein incorporates into the phosphatidylcholine membrane with unique sidedness and that the profile of the reaction center protein itself is asymmetric and spans the membrane.

Membranes of Rhodopseudomonas sphaeroides. VI. Isolation of a fraction enriched in newly synthesized bacteriochlorophyll alpha-protein complexes

Radioactivity eventually destined for the chromatophore membrane of Rhodopseudomonas sphaeroides was shown in pulse-chase studies to appear first in a distinct pigmented fraction. The material formed an upper pigmented band which sedimented more slowly than chromatophores when cell-free extracts were subjected directly to rate-zone sedimentation on sucrose density gradients. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis indicated that the purified fraction contained polypeptide bands of the same mobility as light-harvesting bacteriochlorophyll alpha and reaction center-associated protein components of chromatophores; these were superimposed upon cytoplasmic membrane polypeptides. The pulse-chase relation was confined mainly to the polypeptide components of these pigment-protein complexes. It is suggested that the isolated fraction may be derived from sites at which new membrane invagination is initiated.

Characterisation of reaction centers and their phospholipids from Rhodospirillum rubrum

1. Reaction centers from Rhodospirillum rubrum have been extracted with the zwitterionic detergent lauryl dimethyl amine oxide. Subsequent purification has been achieved by gel filtration and ion-exchange chromatography. The pure reaction centers are composed of three protein subunits (L, M, H), bacteriocholorophyll and bacteriopheophytin in the ratio 2 : 1 and phospholipids. 2. The phospholipid composition has been found to be similar to that of whole chromatophore membrane, except that diphosphatidyl glycerol is present in higher amount in the isolated complex. When the detergent treatment of the chromatophore membrane is done in the presence of NaCl, a lower phospholipid content in isolated reaction centers has been found together with a lower stability in the association among the protein subunits. In this complex, the largest subunit H is easily split off and a LM complex is obtained. It is concluded that the phospholipids play an important role in the stability of reaction center complexes.

Modifiable chromatophore proteins in photosynthetic bacteria

The chromatophores of Chromatium vinosum, as well as six other photosynthetic bacteria, contained two or more proteins which were insoluble when heated in the presence of sodium dodecyl sulfate (SDS) and 2-mercaptoethanol (beta-ME). When the chromatophores were dissolved at room temperature in SDS-beta-ME, these proteins were present in the SDS-polyacrylamide gel electrophoresis profiles, but when the samples were dissolved at 100 degrees C, they were absent or considerably diminished. When one-dimensional gels of chromatophores solubilized at room temperature were soaked in the SDS-beta-ME solution and heated to 100 degrees C and the gels were run in a second dimension, the proteins became immobilized in the original first-dimension gel, where they could be detected by staining. The two major proteins so affected in C. vinosum had apparent molecular weights of 28,000 and 21,000. The chromatophores of several other photosynthetic bacteria also contained predominant proteins between 30,000 and 19,000 molecular weight, which became insoluble when heated in the presence of SDS and beta-ME. In at least two of the species examined, these appeared to be reaction center proteins. The conditions causing the proteins to become insoluble were complex and involved temperature, SDS concentration, and the presence of sulfhydryl reagents. The chromatophores of four of the Chromatiaceae species and two strains of one of the Rhodospirillaceae species examined had a protein-pigment complex that was visible in SDS-polyacrylamide gel profiles of samples dissolved at room temperature but was absent in samples dissolved at 100 degrees C.

The incorporation of reaction centres into membranes from a bacteriochlorophyll-less mutant of Rhodopseudomonas sphaeroides

Reaction centres purified from a blue-green mutant R-26 of Rhodopseudomonas sphaeroides can be incorporated into bacteriochlorophyll-less membranes purified from an aerobically-grown bacteriochlorophyll-less mutant 01 of R. sphaeroides. This can be accomplished by raising the temperature of the mixture or by addition of the detergent sodium cholate and its subsequent removal by dilution or dialysis. Optimum conditions for the reconstitution are at 4 degrees C in the presence of 1% cholate and soybean phospholipid (2 : 1, w/w, with membrane protein). Isopycnic sucrose density gradient centrifugation of such preparations shows that reaction centres and light-harvesting pigment-protein complex bind to the membranes. Reconstituted membranes exhibit light-induced steady-state cytochrome absorbance changes resembling those observed in chromatophores prepared from the photosynthetically-grown mutant R-26. The effect on these absorbance changes of varying reaction centre content in the membrane has been studied, and the time course of the interaction between 01 membrane cytochrome c2 and added reaction centre examined. Cytochrome b photoreduction and cytochrome c2 photo-oxidation were observed in the reconstituted preparation; each increased following the addition of antimycin A, suggesting that a cyclic light-driven system had been reconstituted.

Effect of heat and 2-mercaptoethanol on intracytoplasmic membrane polypeptides of Rhodopseudomonas sphaeroides

Solubilization at 75 degrees C of Rhodopseudomonas sphaeroides chromatophores in the presence of sodium dodecyl sulfate (SDS) and 2-mercaptoethanol (beta-ME) resulted in the selective absence of reaction center B and C polypeptides from SDS-polyacrylamide gel electrophoresis profiles. A newly identified, chromatophore-specific polypeptide, with a mass of 35.2 kdaltons, was also missing under these conditions of chromatophore solubilization. Solubilization at 27 degrees C in the presence of SDS and beta-ME also resulted in the disappearance of these three polypeptides, but at much slower rates. Disappearance of either endogenous or exogenously supplied reaction center polypeptides B and C during SDS solubilization of whole chromatophores at either 27 or 75 degrees C was shown to be entirely dependent upon the presence of beta-ME. After chromatophore solubilization in the presence of beta-ME and subsequent SDS-polyacrylamide gel electrophoresis, exogenously added reaction centers B and C could be localized in a complex of no less than 100 to 200 kdaltons. However, the precise size of the complex was influenced by the stoichiometry of the reacting components. The disappearance of the 35.2-kdalton polypeptide was neither dependent upon the presence of beta-ME nor dependent upon the presence of any additional chromatophore polypeptides. The 35.2-kdalton polypeptide underwent a heat-induced oligomerization to yield several high-molecular-weight species.

Membranes of Rhodopseudomonas sphaeroides. V. Identification of bacteriochlorophyll alpha-depleted cytoplasmic membrane in phototrophically grown cells

The separation of membrane fragments was investigated in extracts of phototropically grown Rhodopseudomonas sphaeroides to determine if the plasma membrane contains discrete regions. A highly purified fraction of bacteriochlorophyll alpha-deficient membrane fragments was isolated by differential centrifugation, chromatography on Sepharose 2B, reaggregation, and isopycnic sedimentation on sucrose gradients. Significant levels of b- and c-type cytochromes and succinate dehydrogenase were demonstrated in the isolated membrane fragments and their appearance in electron micrographs, their polypeptide profile in dodecyl sulfate-polyacrylamide gel electrophoresis, and overall chemical composition were essentially identical to a similar fraction isolated from aerobically grown cells. Their polypeptide profiles were distinct from those of the intracytoplasmic chromatophore and outer membranes, and on the basis of bacteriochlorophyll content the phototrophic fraction was contaminated with chromatophores by less than 9%. The membrane fragments contained no diaminopimelic acid or glucosamine. It is condluded that the membrane fragments isolated from phototrophically growing Rp. sphaeroides have arisen from photosynthetic pigment-depleted regions of the plasma membrane structurally and functionally differentiated from the intracytoplasmic chromatophore membrane. These regions represent conserved chemotrophic cytoplasmic membrane whose synthesis continues under photoheterotrophic conditions.

Properties of photochemical reaction centers purified from Rhodopseudomonas gelatinosa

Reaction centers were isolated from a carotenoidless mutant of Rhodopseudomonas gelatinosa by hydroxyapatite chromatography of purified chromatophores treated with lauryl dimethyl amine oxide. Absorption spectra and spectra of light-induced absorbance changes are similar to those of reaction centers from Rhodopseudomonas sphaeroides. The ratio of absorbance at 280 nm to that at 799 nm was 1.8 in the purest preparations. The extinction coefficient at the 799 nm absorption maximum was estimated to be 305 +/- 20 mM--1 . CM--1. The molecular weight based on protein and chromophore assays was found to be 1.5 . 10(5); the reaction center protein accounted for 6% of the total membrane protein. These reaction centers contained no cytochrome and showed just two components of apparent molecular weights 33 000 and 25 000 in polyacrylamide gel electrophoresis. The chromatophores contained 42 molecules of antenna bacteriochlorophyll for each reaction center.

Membrane-bound, pyridine nucleotide-independent L-lactate dehydrogenase of Rhodopseudomonas sphaeroides

Rhodopseudomonas sphaeroides has a pyridine nucleotide-independent L-lactate dehydrogenase associated with the membrane fraction of cells grown either aerobically or phototrophically. The dehydrogenase is present in cells grown on a variety of carbon sources, but at levels less than 20% of that found in cells grown with DL-lactate. The dehydrogenase has been purified 45-fold from membranes of strain L-57, a non-photosynthetic mutant, by steps involving solubilization with lauryl dimethylamine oxide and three anion-exchange chromatography steps. The purified enzyme was specific for the L-isomer of lactate. The Km of the purified enzyme for L-lactate is 1.4 mM, whereas that of the membrane-associated enzyme is 0.5 mM. The enzyme activity was inhibited competitively by D-lactate and non-competitively by oxalate and oxamate. Quinacrine, a flavin analog, also inhibited the activity. The inducible enzyme may serve as a marker of membrane protein in studies of membrane development.

Localisation of the subunits of the photosynthetic reaction centers in the chromatophore membrane of Rhodospirillum rubrum

Reaction centers were isolated with the detergent lauryl dimethyl amine oxide from chromatophore membranes of Rhodospirillum rubrum. The subunit composition of these reaction centers is similar to the one obtained from Rhodopseudomonas spheroides: three subunits with the molecular weights of 21 000, 24 000 and 29 000. Reaction centers prepared from chromatophores labeled with 131I were heavely labeled in their large subunit (H). The smaller subunits (L and M) contained only little label. Sonication during labeling yielded a slightly higher incorporation of 131I in subunit H compared to the smaller ones. It is concluded that the H protein is largely exposed at the cytoplasmic side of the membrane but might also be accessible for iodination on the inside of the membrane while the L and M proteins are almost completely embedded in the membrane. Iodination of spheroplasts results in only a slight binding of 131I to chromatophores and reaction centers.

Effects of incident light levels on photosynthetic membrane polypeptide composition and assembly in Rhodopseudomonas sphaeroides

Cells of Rhodopseudomonas sphaeroides were grown anaerobically with incident light levels ranging between 4,500 and 400 footcandles (ca. 48,420 and 4,304 lux). Cells grown with the higher light levels had lower contents of total bacteriochlorophyll and incorporated L-[U-14C]leucine into membrane protein at higher rates than cells grown with lower light levels. The former cells also contained relatively lower amounts of light-harvesting membrane polypeptides as compared with the latter cells. In contrast, the relative amounts of reaction center membrane polypeptides were approximately the same with varying incident light levels. The relative amounts of these membrane polypeptides were correlated with differences in rates of synthesis and assembly of the polypeptides into membrane by measuring the rates of incorporation of L-[U-14C]leucine into the membrane-bound polypeptides. No significant differences in rates of turnover of these polypeptides were detected under the varying incident light levels as measured in pulse-chase radioactive labeling experiments.

Membranes of Rhodopseudomonas sphaeroides. IV. Assembly of chromatophores in low-aeration cell suspensions

Chromatophore membrane formation was induced in low-aeration suspensions of Rhodopseudomonas sphaeroides and highly purified chromatophore preparations were isolated at various intervals between 4 and 18 h. The levels of several functional components associated with the isolated strucures were investigated. B-875, the light-harvesting bacteriochlorophyll complex associated with the reaction center, was preferentially inserted into the chromatophore membrane during the early stages of induction, and thereafter its levels reached a steady state; b- and c-type cytochromes were also maintained at essentially constant levels. In contrast, the levels of B-850, the accessory light-harvesting bacteriochlorophyll, together with its associated protein, continued to increase throughout the induction process. Increases in the levels of the major carotenoid component followed a similar course. These findings are consistent with a stepwise assembly mechanism for associated bacteriochlorophyll and protein components and suggest that separate regulatory mechanisms control the levels of functionally essential and accessory components within the membrane.

Temperature dependency of the rate of electron transport as a monitor of protein motion

The temperature dependency of the rate of biological electron transport is interpreted as evolving from a contraction of the electron transport components. A theoretical expression for this temperature dependency is derived in terms of the coefficient of linear expansion (a) of the protein components. Using this expression alpha is calculated for several electron transport systems and shown to be similar to alpha-values of synthetic polymers. A discontinuity in alpha is shown to be present in all biological electron transport reactions at ca. 150 K. This discontinuity is interpreted as a change in the intramolecular bonding of the electron transport protein units.

Control of composition and activity of the photosynthetic apparatus of Rhodopseudomonas capsulata grown in ammonium-limited continuous culture

Rhodopseudomonas capsulata was grown either phototropically in the light or chemotrophically in the dark at oxygen tensions of 5 mm and 3 mm Hg in ammonium-limited continuous culture. During growth limitation bacteriochlorophyll content of cells and membranes varied dependent on growth rate drastically in chemotrophic cultures. Concomittantly, the ratio of membrane protein to total protein varied in the range of 30-41%. This dependence of membrane differentiation on growth rate was less evident in phototrophically grown cells. The incorporation of the bulk of bacteriochlorophyll was shown to be quantitatively correlated to the incorporation of 1-3 low molecular weight proteins with molecular weights in the range of 14 to less than 10 k daltons. Supported by similar findings of other authors it is proposed, that these proteins are to be attributed to the species of antenna bacteriochlorophyll and represent components of the photosynthetic apparatus. With decreasing growth rates the size of the photosynthetic unit with respect to the population of bacteriochlorophyll- and protein molecules was reduced subsequent to a reduction in the rate of incorporation of antenna-bacteriochlorophyll and the low molecular weight proteins, the reaction-center bacteriochlorophyll content of the membranes remaining constant. A parallel decrease in potential phosphorylating capacity was observed. It is concluded, that under these conditions, primary photochemical reactions in the reaction center were not the rate-limiting step in photophosphorylation. The interaction of growth limitation by an anabolic precursor (NH+4) and control of membrane differentiation by light intensity or oxygen tension is discussed.

The nature of a pgiment-protein complex excreted from mutants of Rhodopseudomonas sphaeroides

Pigment-protein complexes excreted from three bacteriochlorophyll-less mutants (mutants 8, 8-29, and 8-47) of Rhodopseudomonas sphaeroides have been isolated and purified. In the absence of detergents the complexes remained in an aggregated state, but were disaggregated by 0.2% Triron X-100. Sepharose 6B gel filtration indicated that the disaggregated complex from each of the mutants had a particle weight of about 165000, and contained 30 +/- 3% protein. This complex was further dissociated by 1% sodium dodecyl sulfate. Sephadex G-100 gel filtration now indicated that the majority of the protein was present as a small polypeptide with a molecular weight of about 9000. The pigment-protein complex from one of the mutants was treated with a bacteriochlorophyll extract. The bacteriochlorophyll was converted to bacteriopheophytin and became bound to the protein, replacing the endogenous tetrapyrrole (a bacteriocholorophyll precursor). The red absorption maximum of the bacteriopheophytin was shifted during this process to 840-865 nm. These properties are consistent with the possibility that the pigment-protein complexes contain a protein normally associated with light-harvesting bacteriochlorophyll in the wide-type strain.

Cytoplasmic and outer membranes separation in Rhodopseudomonas sphaeroides

A cell envelope fraction had been prepared after mechanical disruption of lysozyme-EDTA spheroplasts from depigmented Rhodopseudomonas sphaeroides (aerobically grown in the light). On linear sucrose gradients this fraction can be separated in a cytoplasmic membrane fraction and an outer membrane fraction. The cytoplasmic fraction (buoyant density: 1.18 g/cm3) has been characterized by its succinic dehydrogenase activity and by its composition. The outer membrane fraction (buoyant density: 1.21 g/cm3) does not contain any respiratory activity nor hemoproteins. The same fractionation has been done on cells repigmented in the dark by lowering the O2 pressure. In that case the same two fractions have been detected in addition to the chromatophore fraction (buoyant density: 1.14 g/cm3). However both, and specially the outer membrane fraction, were contaminated by chromatophore material.

Photochemical reaction centers from Rhodopseudomonas capsulata

A photochemically active bacteriochlorophyll-protein complex (reaction center) has been isolated from the carotenoidless mutant A1a+ of Rhodopseudomonas capsulata by treatment of membranes with lauryl dimethyl amine oxide. Three proteins with molecular weights of 20,500, 24,000 and 28,000 (molar ratio 1:1:1) were detected in the reaction center preparations. After mild treatment of intracytoplasmic membranes with Na-dodecyl sulfate (0.5%, 30 degrees C, 1 min) succeeded by polyacrylamide gel electrophoresis two pigmented bands were obtained. Material of one fraction could be bleached reversibly by actinic light and contained two proteins with molecular weights of 20,500 and 24000. The second band is photochemically inactive.

Formation of reaction centers and light-harvesting bacteriochlorophyll-protein complexes in Rhodopseudomonas capsulata

Formation of the photosynthetic apparatus was induced in aerobically grown dark cultures of Rhodopseudomonas capsulata by lowering of the oxygen tension. Besides the wild type strain the carotenoid-less mutant strain A1a+ was investigated. Both strains exhibited initially a decrease of the molar ratio of total bacteriochlorophyll (Bchl) to reaction center (RC) Bchl, followed by an increase. Synthesis of RC-Bchl preceded the synthesis of light-harvesting (LH) Bchl. Activities of photophosphorylation in membrane preparations, isolated from cultures after different periods of incubation at low aeration, decreased on the basis of total Bchl from about 9 to 2 mumole ATP/mumole total Bchl-min, whereas the rate on the basis of RC-Bchl remained constant (about 500 mumole ATP/mumole RC-Bchl-min). Under the same conditions the membrane proteins were labelled with U-14C-protein hydrolysate. Corresponding to RC-Bchl the synthesis of RC-proteins dominated during the first 30 min of incubation at PO2 below 3 mm Hg. After 45-60 min of membrane formation at low aeration the synthesis of LH-complex proteins exceeded the synthesis of RC proteins. The correlations between protein and Bchl synthesis in the sequential formation of RC- and LH-complexes are discussed.

Isolation and characterization of a temperate bacteriophage specific for Rhodopseudomonas spheroides

The isolation of a temperature phage specific for the photosynthetic bacterium Rhodopseudomonas spheroides is reported. This phage, Rphi-1, establishes a state of lysogeny and can be induced from the prophage state by exposure to mitomycin C or UV irradiation. Mutants of Rphi-1 which grow on a standard laboratory strain (2.4.1) of Rhodopseudomonas spheroides were isolated. Although the original Rphi-1 isolated was chloroform sensitive, the mutant which plates on strain 2.4.1 is chloroform resistant. Rphi-1 does not grow on closely related bacteria, such as Rhodopseudomonas palustris or Rhodopseudomonas capsulata. Rphi-1 mutants forms plaques with the same efficiency whether the plates are incubated under aerobic conditions in the dark or under anaerobic conditions in the light (phototropic conditions).