Intracytoplasmic membrane synthesis in synchronous cell populations of Rhodopseudomonas sphaeroides. Polypeptide insertion into growing membrane

Complete DNA sequence, specific Tn5 insertion map, and gene assignment of the carotenoid biosynthesis pathway of Rhodobacter sphaeroides

The carotenoid biosynthesis genes form a cluster within the genome of Rhodobacter sphaeroides, lying in the middle of a larger cluster and 45 kb in length, which contains genes for bacteriochlorophyll biosynthesis and for the reaction center and light-harvesting apoproteins. The positions and approximate limits of the carotenoid genes were determined previously by localized transposon Tn5 mutagenesis and by comparison with the closely related Rhodobacter capsulatus carotenoid gene cluster. In this report, analysis of the DNA and deduced amino acid sequences of the carotenoid genes in R. sphaeroides are presented. Twenty-five Tn5 insertion mutants were used to produce a base-specific Tn5 insertion map of this region, and carotenoid gene assignment was supported by spectroscopic, ultrastructural, and high-pressure liquid chromatography analyses of these mutants. A region in the 3' end of crtD which affects bacteriochlorophyll biosynthesis was discovered, and CrtA was found to possess a proline-rich C-terminal region containing a repeated (Ala-Pro)n motif. CrtF also showed a high degree of sequence conservation with eukaryotic O-methyltransferases. This study provides gene sequences and assignments based upon a comprehensive structural, spectroscopic, and biochemical analysis of a range of carotenoid biosynthetic mutants; in each mutation, the point of Tn5 insertion is determined accurate to 1 bp on the gene cluster.

Early steps in carotenoid biosynthesis: sequences and transcriptional analysis of the crtI and crtB genes of Rhodobacter sphaeroides and overexpression and reactivation of crtI in Escherichia coli and R. sphaeroides

In the purple photosynthetic bacterium Rhodobacter sphaeroides, the desaturation of phytoene has already been implicated in the assembly of the light-harvesting 2 complex (H.P. Lang and C.N. Hunter, Biochem. J. 298:197-205, 1994). The phytoene synthase and desaturase enzymes mediate the first steps specific for carotenoid biosynthesis up to and including the synthesis of the colored carotenoid neurosporene. In this report, we present the DNA and deduced amino acid sequences of the genes encoding these proteins, namely, crtB and crtI, from R. sphaeroides and present evidence for the existence of a crtIB operon. Both genes have been shown to possess putative puc and puf operon-like promoter sequences, and oxygen regulation and the point of initiation of the crtI transcript have been demonstrated. The complete crtI gene has been overexpressed in Escherichia coli and R. sphaeroides and shown to catalyze three desaturations of phytoene to give neurosporene. This activity was shown to be ATP dependent, and the cofactor requirement was investigated by using a spectroscopic assay for in vitro carotenogenic activity. Although the crtI and crtB genes have been sequenced from a number of different organisms, the transcriptional organization and regulation of these genes have not been analyzed in detail. In this report, we have located the transcription initiation point and have shown that R. sphaeroides possesses an oxygen-regulated CrtI-type phytoene desaturase gene that forms a transcriptional operon with crtB.

Cell-cycle-specific fluctuation in cytoplasmic membrane composition in aerobically grown Rhodospirillum rubrum

Aerobic growth with synchronous cell division was induced in Rhodospirillum rubrum by starvation methods. Cells were harvested at different points in the cell cycle. Analysis of the composition of the cell envelope prepared by differential centrifugation or density gradient-purified cytoplasmic membrane obtained from cells at different times indicated that the protein/phospholipid ratio fluctuated with the cell cycle. The protein/phospholipid ratio of cell envelope from selection-synchronized cells also fluctuated with the cell cycle. These studies indicate that the phenomenon of cell-cycle-dependent fluctuation in membrane composition is not restricted to the intracytoplasmic chromatophore membrane of phototrophic cells.

Membrane adenosine triphosphatase in synchronous cultures of Rhodobacter sphaeroides

Studies of intracytoplasmic membrane biogenesis utilizing synchronized cultures of Rhodobacter sphaeroides have revealed that most intracytoplasmic membrane proteins accumulate continuously throughout the cell cycle while new phospholipid appears discontinuously within the intracytoplasmic membrane. The resulting changes in the structure of the membrane lipids was proposed to influence the activities of enzymes associated with the intracytoplasmic membranes (Wraight, C.A., Leuking, D.R., Fraley, R.T. and Kaplan, S. (1978) J. Biol. Chem. 253, 465-471). We have extended the study of intracytoplasmic membrane biogenesis in R. sphaeroides to include the membrane adenosine triphosphatase. The membrane bound Mg2+-dependent, oligomycin-sensitive adenosine triphosphatase activity was measured throughout the cell cycle for steady-state synchronized cells of R. sphaeroides and found to accumulate discontinuously. Following treatment with an uncoupling reagent (2,4-dinitrophenol) the intracytoplasmic membrane associated adenosine triphosphatase activity was stimulated uniformly in membranes isolated at different stages of the cell cycle. The adenosine triphosphatase was also measured by quantitative immunoblots utilizing specific antibody to compare the enzyme activity and enzyme protein mass. Immunologic measurement of the adenosine triphosphatase in isolated membranes indicated a constant ratio of enzyme to chromatophore protein exists during the cell cycle in contrast to the discontinuous accumulation of adenosine triphosphatase activity. These results are discussed in light of the cell-cycle specific synthesis of the intracytoplasmic membrane.

Control of metF gene expression in maxicell preparations of Escherichia coli K-12: reversible action of the metJ protein and effect of vitamin B12

Expression of methionine regulon elements was controlled by the metJ protein gpMetJ. A maxicell system with cloned copies of the metF transcription unit allowed reversible action of gpMetJ. Expression of the metF transcription unit in maxicells was reduced by exogenous vitamin B12 at concentrations of 0.5 nM or greater.

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).

Role of apparent membrane growth initiation sites during photosynthetic membrane development in synchronously dividing Rhodopseudomonas sphaeroides

Sites of intracytoplasmic membrane growth and temporal relations in the assembly of photosynthetic units were examined in synchronously dividing Rhodopseudomonas sphaeroides cells. After rate-zone sedimentation of cell-free extracts, apparent sites of initiation of intracytoplasmic membrane growth formed an upper pigmented band that sedimented more slowly than the intracytoplasmic membrane-derived chromatophore fraction. Throughout the cell cycle, the levels of the peripheral B800-850 light-harvesting pigment-protein complex relative to those of the core B875 complex in the upper pigmented fraction were only about half those of chromatophores. Pulse-labeling studies with L-[35S]methionine indicated that the rates of assembly of proteins in the upper pigmented fraction were much higher than those of chromatophores throughout the cell cycle; rates for the reaction center polypeptides were estimated to be approximately 3.5-fold higher than in chromatophores when the two membrane fractions were equalized on a protein basis. In pulse-chase studies, radioactivity of the reaction center and B875 polypeptides increased significantly in chromatophores and decreased in the upper pigmented band during cell division. These data suggest that the B875 reaction center cores of the photosynthetic units are inserted preferentially into sites of membrane growth initiation isolated in the upper pigmented band and that the incomplete photosynthetic units are transferred from their sites of assembly into the intracytoplasmic membrane during cell division. These results suggested further that B800-850 is added directly to the intracytoplasmic membrane throughout the cell cycle.

Cell-cycle-specific oscillation in the composition of chromatophore membrane in Rhodospirillum rubrum

Synchrony in phototrophic cultures of Rhodospirillum rubrum was induced by stationary-phase cycling or by alterations in light intensity. Intracytoplasmic chromatophore membranes were prepared by differential centrifugation. Analysis of the composition of chromatophores obtained from cells at different times indicated that the protein/bacteriochlorophyll a ratio was constant throughout the cell cycle but that the protein/phospholipid ratio oscillated. This cell-cycle-dependent fluctuation in chromatophore membrane composition was reflected in the buoyant densities of the isolated chromatophores.

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).

Cell-cycle-specific biosynthesis of the photosynthetic membrane of Rhodopseudomonas sphaeroides. Structural implications

Structural changes association with the intracytoplasmic membrane during the cell cycle of the photosynthetic bacterium Rhodopseudomonas sphaeroides have been studied by freeze-fracture electron microscopy. The isolated intracytoplasmic membrane vesicles, chromatophores, were fused in order to obtain large fracture faces, allowing more precise measurements and statistical analysis of both intramembrane particle density and size determinations. The intramembrane particle density of the protoplasmic face (PF) of the intracytoplasmic membrane, (from 4970 to 8290/micrometers 2), was shown to be a linear function of the protein/phospholipid ratio (from 2.5 to 5.1, w/w) of the intracytoplasmic membrane. Under constant light intensity, both the average particle size and particle size distribution remained unchanged during the cell cycle. These results provide the structural basis for the earlier reported cell-cycle-specific variations in both protein/phospholipid ratio and alternation in phospholipid structure of the intracytoplasmic membrane of R. sphaeroides during photosynthetic growth. The average particle diameter in the PF face of the intracytoplasmic membrane was 8.25, 9.08 and 9.75 nm at incident light intensities of 4000, 500 and 30 ft X cd, respectively. When chromatophores were fused with small, unilamellar liposomes, the intramembrane particle density decreased as input liposome phospholipid increased, whereas the particle size remained constant and particle distribution became random.

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.

Induction of the photosynthetic membranes of Rhodopseudomonas sphaeroides: biochemical and morphological studies

Cells of Rhodopseudomonas sphaeroides grown in a 25% O2 atmosphere were rapidly subjected to total anaerobiosis in the presence of light to study the progression of events associated with the de novo synthesis of the inducible intracytoplasmic membrane (ICM). This abrupt change in physiological conditions resulted in the immediate cessation of cell growth and whole cell protein, DNA, and phospholipid accumulation. Detectable cell growth and whole cell protein accumulation resumed ca. 12 h later. Bulk phospholipid accumulation paralleled cell growth, but the synthesis of individual phospholipid species during the adaptation period suggested the existence of a specific regulatory site in phospholipid synthesis at the level of the phosphatidylethanolamine methyltransferase system. Freeze-fracture electron microscopy showed that aerobic cells contain small indentations within the cell membrane that appear to be converted into discrete ICM invaginations within 1 h after the imposition of anaerobiosis. Microscopic examination also revealed a series of morphological changes in ICM structure and organization during the lag period before the initiation of photosynthetic growth. Bacteriochlorophyll synthesis and the formation of the two light-harvesting bacteriochlorophyll-protein complexes of R. sphaeroides (B800-850 and B875) occurred coordinately within 2 h after the shift to anaerobic conditions. Using antibodies prepared against various ICM-specific polypeptides, the synthesis of reaction center proteins and the polypeptides associated with the B800-850 complex was monitored. The reaction center H polypeptide was immunochemically detected at low levels in the cell membrane of aerobic cells, which contained no detectable ICM or bacteriochlorophyll. The results are discussed in terms of the oxygen-dependent regulation of gene expression in R. sphaeroides and the possible role of the reaction center H polypeptide and the cell membrane indentations in the site-specific assembly of ICM pigment-protein complexes during the de novo synthesis of the ICM.

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.

Effect of cerulenin on macromolecule synthesis in chemoheterotrophically and photoheterotrophically grown Rhodopseudomonas sphaeroides

The antibiotic cerulenin causes the immediate cessation of phospholipid biosynthesis in both chemoheterotrophic and photoheterotrophic cultures of Rhodopseudomonas sphaeroides. Macromolecule biosynthesis in photoheterotrophic cells was unaffected by cerulenin for the first 2 h after antibiotic addition and then continued at a reduced rate for an additional 8 h. In contrast, macromolecule biosynthesis in chemoheterotrophic cells was severely affected by cerulenin within the first 2 h of treatment. Pulse-labeling of protein after cerulenin addition revealed that all subcellular fractions were equally affected by the action of cerulenin with chemoheterotrophic cell fractions more profoundly affected than those derived from photoheterotrophic cells. Protein insertion into the intracytoplasmic membrane of photoheterotrophic cells continued for up to 6 h after the onset of cerulenin treatment. Residual macromolecule synthesis was correlated with the presence of the photosynthetic membrane system under all conditions of growth.

In vivo metabolic intermediates of phospholipid biosynthesis in Rhodopseudomonas sphaeroides

The in vivo metabolic pathways of phospholipid biosynthesis in Rhodopseudomonas sphaeroides have been investigated. Rapid pulse-chase-labeling studies indicated that phosphatidylethanolamine and phosphatidylglycerol were synthesized as in other eubacteria. The labeling pattern observed for N-acylphosphatidylserine (NAPS) was inconsistent with the synthesis of this phospholipid occurring by direct acylation of phosphatidylserine (PS). Rather, NAPS appeared to be kinetically derived from an earlier intermediate such as phosphatidic acid or more likely CDP-diglyceride. Tris-induced NAPS accumulation specifically reduced the synthesis of PS. Treatment of cells with a bacteriostatic concentration of hydroxylamine (10 mM) greatly reduced total cellular phospholipid synthesis, resulted in accumulation of PS, and stimulated the phosphatidylglycerol branch of phospholipid metabolism relative to the PS branch of the pathway. When the cells were treated with a lower hydroxylamine dosage (50 microM), total phospholipid synthesis lagged as PS accumulated, however, phospholipid synthesis resumed coincident with a reversal of PS accumulation. Hydroxylamine alone was not sufficient to promote NAPS accumulation but this compound allowed continued NAPS accumulation when cells were grown in medium containing Tris. The significance of these observations is discussed in terms of NAPS biosynthesis being representative of a previously undescribed branch of the phospholipid biosynthetic sequence.

Light-dependent regulation of the synthesis of soluble and intracytoplasmic membrane proteins of Rhodopseudomonas sphaeroides

Cells of Rhodopseudomonas sphaeroides grown under saturating light conditions (30 W/m2) and then shifted to low light intensity (3 W/m2) required 2.5 h to adapt to the new lower light conditions. After the shift, cell growth, whole cell protein accumulation, and bacteriochlorophyll accumulation ceased immediately. Approximately midway into the adaptation period, bacteriochlorophyll synthesis commenced at a new, higher rate, which continued through the beginning of the low-light growth period until new steady-state levels were reached. Immediately after the downshift, the rate of cellular protein synthesis declined to 22% of its preshift rate. Pulse-labeling of protein throughout the adaptation period and comparison with a steady-state prelabel culture revealed that synthesis of two of the three light-harvesting proteins, as well as two additional high-molecular-weight photosynthetic membrane proteins, was derepressed three- to fivefold compared with bulk cellular protein. Finally, the synthesis of at least three soluble proteins showed light-dependent regulation after the light downshift. These results are discussed in terms of the light-dependent regulation of synthesis of the photosynthetic membrane macromolecular components and the division of protein synthesis between the photosynthetic membranes and the soluble cell phase.

Biosynthesis of the photosynthetic membranes of Rhodopseudomonas sphaeroides

The steady-state biosynthesis of the photosynthetic membrane (ICM) of Rhodopseudomonas sphaeroides has been reviewed. At moderate light intensities, 500 ft-c, preexisting ICM serves as the insertion matrix for newly synthesized membrane components. Whereas the bulk of the membrane protein, protein-pigment complexes, and pigments are inserted into preexisting ICM throughout the cell cycle, phospholipid is transferred from outside the ICM to the ICM only at the time of cell division. Because the site of cellular phospholipid synthesis is the cytoplasmic membrane, these results infer that despite the physical continuity of cytoplasmic membrane and ICM, there must exist between these membranous domains a "barrier" to the free diffusion of cellular phospholipid. The cyclical alternation in protein to phospholipid ratio of the ICM infers major structural and functional alternations, such as changes in the protein to lipid ratio of the membrane, specific density of the membrane, lipid structure within the membrane, and the rate of cyclic electron flow. When biochemical studies are correlated with detailed electron microscopic investigations we can further conclude that the number of photosynthetic units within the plane of the membrane can vary by nearly a factor of two over the course of the cell cycle. The average physical size of the photosynthetic units is constant for a given light intensity but inversely proportional to light intensity. The distribution of photosynthetic unit size classes within the membrane can be interpreted as suggesting that the "core" of the photosynthetic unit (reaction center plus fixed antenna complex) is inserted into the membrane coordinately as a structural entity. The variable antenna complex is, on the other hand, inserted independent of the "core" and randomly associates with both old and new core complexes. Finally, we conclude that there is substantial substructure to te distribution of photosynthetic units within the ICM, ie, they are highly ordered and exist in a defined spatial orientation to one another.

Expression of the transposable lac operon Tn951 in Rhodopseudomonas sphaeroides

The transposon Tn951 (lac) was introduced into the photosynthetic bacterium Rhodopseudomonas sphaeroides 2.4.1, which is normally Lac-, via the P-group plasmid RP1. beta-Galactosidase was produced constitutively in both chemotrophically and phototrophically grown cells, and the levels were found to be the same but low. Mutants were isolated, however, that were able to grow on lactose minimal medium and which expressed different levels of beta-galactosidase when grown chemotrophically or phototrophically. The beta-galactosidase levels found in all R. sphaeroides strains were much less than those found in Escherichia coli.

Kinetic analysis of N-acylphosphatidylserine accumulation and implications for membrane assembly in Rhodopseudomonas sphaeroides

The accumulation of N-acylphosphatidylserine (NAPS) in response to the inclusion of Tris in the growth medium of Rhodopseudomonas sphaeroides strain M29-5 has been examined. In the accompanying paper (Donohue et al., J. Bacteriol. 152:000--000, 1982), we show that in response to Tris, NAPS accumulated to as much as 40% of the total cellular phospholipid content. NAPS accumulation began immediately upon addition of Tris and was reflected as an abrupt 12-fold increase in the apparent rate of NAPS accumulation. We suggest that Tris altered the flow of metabolites through a preexisting and previously unknown metabolic pathway. NAPS accumulation ceased immediately upon the removal of Tris; however, accumulated NAPS remained largely metabolically stable. Importantly, under conditions in which NAPS was not accumulated, the intracytoplasmic membrane was shown to be virtually devoid of newly synthesized NAPS. The significance of this observation is discussed in terms of its physiological implications on phospholipid transfer and membrane biogenesis in R. sphaeroides.

Purification and characterization of an N-acylphosphatidylserine from Rhodopseudomonas sphaeroides

A new phospholipid that can account for up to 40% of the total cellular phospholipid of Rhodopseudomonas sphaeroides has been identified. Purification of the phospholipid was accomplished by column chromatography on silicic acid and diethylaminoethylcellulose followed by preparative thin-layer chromatography. A combination of spectroscopic and chemical techniques were used to identify the unknown phospholipid as an N-acylphosphatidylserine. Infrared spectroscopy revealed the presence of both ester and amide bonds in the phospholipid. Interpretation of the proton nuclear magnetic resonance spectrum of the new phospholipid indicated the presence of three acyl chains per phospholipid and in all other respects was compatible with the proposed structure of the molecule. Chemical studies confirmed the presence of a glycerylphosphorylserine moiety in the molecule and yielded three fatty acyl chains per hydrolyzed phospholipid. The fatty acid composition of the phospholipid was approximately 85% vaccenic acid, 9% stearic acid, 5% palmitic acid, and 1% palmitoleic acid, which is essentially identical with the fatty acid composition of whole cell phospholipid preparations from R. sphaeroides. Chemical synthesis of an N-acylphosphatidylserine from beef brain phosphatidylserine and palmitic anhydride gave a product with characteristics similar to those of the naturally occurring material isolated from R. sphaeroides.

Light-induced division and genomic synchrony in phototrophically growing cultures of Rhodopseudomonas sphaeroides

An experimental procedure for rapidly obtaining cell populations of phototrophically growing Rhodopseudomonas sphaeroides which display division and genomic synchrony has been developed. The basis of the procedure resides with the normal physiological response displayed by cells of R. sphaeroides that have been subjected to an immediate decrease in incident light intensity. After an abrupt high- to low-light transition of an asynchronously dividing cell population, an immediate cessation of increases in culture turbidity, total cell number, and net accumulations of culture deoxyribonucleic acid and phospholipid occurs. Total cell number remains constant for 2.5 h after the transition to low light, after which time, it undergoes a sharp increase. Reinitiation of high-light conditions of growth 1 h subsequent to this increase in total cell number results in a cell population possessing a high degree of division and genomic synchrony. A characterization of this procedure, together with a demonstration of its utility for studies on intracytoplasmic membrane assembly, is presented.

In vivo intermembrane transfer of phospholipids in the photosynthetic bacterium Rhodopseudomonas sphaeroides

The kinetics of accumulation of phospholipids into the intracytoplasmic membrane of Rhodopseudomonas sphaeroides have been examined. We have previously demonstrated that accumulation of phospholipids in the intracytoplasmic membrane is discontinuous with respect to the cell cycle. In this study we demonstrated a sevenfold increase in the rate of phospholipid incorporation into the intracytoplasmic membrane concurrent with the onset of cell division. Pulse-chase labeling studies revealed that the increase in the rate of phospholipid accumulation into the intracytoplasmic membrane results from the transfer of phospholipid from a site other than the intracytoplasmic membrane, and that the transfer of phospholipid, rather than synthesis of phospholipid, is most likely subject to cell cycle-specific regulation. The rates of synthesis of the individual phospholipid species (phosphatidylethanolamine, phosphatidyglycerol, and an unknown phospholipid) remained constant with respect to one another throughout the cell cycle. Similarly, each of these phospholipid species appeared to be transferred simultaneously to the intracytoplasmic membrane. We also present preliminary kinetic evidence which suggested that phosphatidylethanolamine may be converted to phosphatidycholine within the intracytoplasmic membrane.

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.

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.

A rapid method for the isolation of intracytoplasmic membranes from Rhodopseudomonas sphaeroides using an air-driven ultracentrifuge

A method has been developed for the isolation intracytoplasmic (ICM) vesicles (chromatophores) from Rhodopseudomonas sphaeroides using an air-driven ultracentrifuge. Application of conventional techniques used for preparative scale equipment to the air-driven ultracentrifuge allows the rapid isolation of ICM vesicles from reduced quantities of starting material. Sodium dodecyl sulfatepolyacrylamide gel electrophoresis profiles of ICM vesicles isolated in this fashion are essentially indistinguishable from those isolated by conventional means.

The use of the fluorescent probe alpha-parinaric acid to determine the physical state of the intracytoplasmic membranes of the photosynthetic bacterium, Rhodopseudomonas sphaeroides

alpha-Parinaric acid has been used to determine the degree of ordering of the hydrocarbon region of purified intracytoplasmic membranes of Rhodopseudomonas sphaeroides. The usefulness of alpha-parinaric acid as a probe of membrane fluidity was established by comparison of its fluorescent properties in phosphatidylcholine vesicles with those of the more commonly used fluorescent probe, 1,6-diphenyl-1,3,5-hexatriene. Both fluorescent probes were shown to monitor similar environments in the phosphatidylcholine vesicles when the phospholipids were maintained at temperatures above their phase transition temperature. The rotational mobility of alpha-parinaric acid in the intracytoplasmic membranes was determined from 0 to 50 degrees C, a region where no phase transitions were detectable. The rotational mobility of alpha-parinaric acid dissolved in vesicles formed from total extracted intracytoplasmic membrane phospholipids, was 2--3-fold greater than that measured in the intact intracytoplasmic membranes; demonstrating that the presence of protein greatly reduces the mobility of the phospholipid acyl chains of the intracytoplasmic membranes. Due to the high protein content of these membranes, the perturbing effect of protein on acyl chain mobility may extend to virtually all the intracytoplasmic membrane phospholipid.