Formation and ultrastructure of extra membranes in Escherichia coli

Revisiting the mesosome as a novel site of hydrogen peroxide accumulation in Escherichia coli

The major source of endogenous hydrogen peroxide is generally thought to be the respiratory chain of bacteria and mitochondria. In our previous works, mesosome structure was induced in cells during rifampicin effect, and the mesosome formation is always accompanied by excess hydrogen peroxide accumulation in bacterial cells. However, the underlying mechanisms of hydrogen peroxide production and the rationale behind it remain still unknown. Here we report that hydrogen peroxide can specifically accumulate in the mesosome in vitro. Mesosomes were interpreted earlier as artifacts of specific cells under stress through TEM preparation, while, in the current study, mesosomes were shown as intracellular compartments with specific roles and features by using quickly freezing preparation of TEM. Formation of hydrogen peroxide was observed in suspension of mesosomal vesicles by using either a fluorescence-based reporter assay or a histochemical method, respectively. Our investigation provides experimental evidence that mesosomes can be a novel site of hydrogen peroxide accumulation.

Massive formation of intracellular membrane vesicles in Escherichia coli by a monotopic membrane-bound lipid glycosyltransferase

The morphology and curvature of biological bilayers are determined by the packing shapes and interactions of their participant molecules. Bacteria, except photosynthetic groups, usually lack intracellular membrane organelles. Strong overexpression in Escherichia coli of a foreign monotopic glycosyltransferase (named monoglycosyldiacylglycerol synthase), synthesizing a nonbilayer-prone glucolipid, induced massive formation of membrane vesicles in the cytoplasm. Vesicle assemblies were visualized in cytoplasmic zones by fluorescence microscopy. These have a very low buoyant density, substantially different from inner membranes, with a lipid content of > or = 60% (w/w). Cryo-transmission electron microscopy revealed cells to be filled with membrane vesicles of various sizes and shapes, which when released were mostly spherical (diameter approximately 100 nm). The protein repertoire was similar in vesicle and inner membranes and dominated by the glycosyltransferase. Membrane polar lipid composition was similar too, including the foreign glucolipid. A related glycosyltransferase and an inactive monoglycosyldiacylglycerol synthase mutant also yielded membrane vesicles, but without glucolipid synthesis, strongly indicating that vesiculation is induced by the protein itself. The high capacity for membrane vesicle formation seems inherent in the glycosyltransferase structure, and it depends on the following: (i) lateral expansion of the inner monolayer by interface binding of many molecules; (ii) membrane expansion through stimulation of phospholipid synthesis, by electrostatic binding and sequestration of anionic lipids; (iii) bilayer bending by the packing shape of excess nonbilayer-prone phospholipid or glucolipid; and (iv) potentially also the shape or penetration profile of the glycosyltransferase binding surface. These features seem to apply to several other proteins able to achieve an analogous membrane expansion.

In vivo assembly of a biological membrane of defined size, shape, and lipid composition

At restrictive temperature, mutant ts1 of bacteriophage PM2 makes membrane vesicles inside infected Alteromonas espejiana. A shift from restrictive to permissive temperature resulted in rapid maturation to infectious virions. The membrane vesicles were isolated from cellular membranes by sucrose density gradient centrifugation. Analysis of the unique peak at rho = 1.190 g/cm3 showed spheres of two diameters, 50 nm and 54 nm. The wild-type virus is icosahedral with an average diameter of 60 nm. Gel electrophoresis indicated the absence in the vesicles of the coat and spike proteins. sp27 and sp43, respectively, and the presence of only one viral structural protein, sp6.6. DNA was also present. The lipid in the vesicles was composed of phosphatidylglycerol and phosphatidylethanolamine in a proportion similar to that of the wild-type virus, whose ratio is nearly the inverse of that found in the host membrane. Thus, membrane vesicles made by mutant ts1 resembled the membrane of the wild-type virus in size, shape, and lipid composition, but contained only one of the four structural proteins of the virus. This hydrophobic protein, sp6.6 may be responsible for stimulating membrane morphogenesis.

Escherichia coli mutants defective in membrane phospholipid synthesis: binding and metabolism of 1-oleoylglycerol 3-phosphate by a plsB deep rough mutant

Mutants of Escherichia coli containing a defective sn-glycerol 3-phosphate acyltransferase are conditionally defective in the synthesis of acylglycerol phosphate (acylglycerol-P). Incubation of a deep rough derivative of one of these plsB strains with 1-[3H]oleoylglycerol-32P resulted in the binding of up to 70 nmol of oleoylglycerol-P per 100 nmol of cellular phospholipid. The binding was dependent on time, oleoylglycerol-P concentration, and the quantity of cells employed. The rate and extent of oleoylglycerol-P binding was affected by the deep rough mutation. The altered phospholipid composition due to oleoylglycerol-P binding was without consequence on cell growth and viability, but caused the appearance of intracellular multilamellar structures. Use of the double-labeled oleoylglycerol P demonstrated that the entire molecule was bound to the cell. Intact [3H]-oleoylglycerol-32P was converted to phosphatidylethanolamine and phosphotidyl-glycerol at a rate about 40% of that of de novo phospholipid synthesis. These data demonstrate the transmembrane movement of oleoylglycerol-P to the inner surface of the cytoplasmic membrane and suggest that it may become possible to supplement plsB strains of E. coli with acylglycerol-P's.

Physiological characterization of an Escherichia coli mutant altered in the structure of murein lipoprotein

Studies using isogenic transductant strains mlpA+ and mlpA as well as reversion analysis suggested that the physiological consequences of a structural gene mutation in murein lipoprotein include (i) increased sensitivity toward chelating agents ethylenediaminetetraacetic acid and ethyleneglycol-bis (beta-aminoethyl ether)-N,N-tetraacetic acid, (ii) leakage of periplasmic enzyme ribonuclease, (iii) weakened association between the outer membrane and the rigid layer accentuated by Mg2+ starvation, resulting in the formation of outer membrane blebs, and (iv) decreased growth rate in media of low ionic strength or low osmolarity. It is suggested that the bound form of lipoprotein plays an important role in the maintenance of the structural integrity of the outer membrane of the Escherichia coli cell envelope. Other outer membrane components may also contribute to the anchorage of outer membrane to the rigid layer, probably through ionic interactions with divalent cations. Using the phenotype of ribonuclease leakage as an unselected marker in a three-factor cross with P1 transduction, we were able to establish the gene order of man mlpA aroD pps on the E. coli chromosome.

Genetic and physiological classification of periplasmic-leaky mutants of Salmonella typhimurium

Mutants of Salmonella typhimurium that leaked periplasmic proteins were isolated. Four classes of mutants were identified by their increased sensitivity to dyes, detergents, or antibiotics. Conjugation studies indicated that representatives of two classes mapped in the proA-galE region of the Salmonella chromosome and two in the cysI-argE region. According to their bacteriophage sensitivity pattern, all four of the mutant classes appear to retain the smooth lipopolysaccharide characteristic. One class of mutants has an abnormal cell envelope in which the outer membrane balloons away from the murein layer.

Mesosomes: membranous bacterial organelles

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Ultrastructural properties of the extra membranes of Escherichia coli O111a as revealed by freeze-fracturing and negative-staining techniques

Escherichia coli O111a is a thermosensitive strain which, when grown at 40 C, accumulates large quantities of intracellular membranes. The ultrastructure of these membranes in cells which have been chemically fixed, embedded, and examined as thin sections has been compared with that of membranes in cells negatively stained or freeze-fractured. Results indicate that the extra membranes are present in the three types of preparations examined and, therefore, clearly are not artifacts of chemical fixation. Negative staining has proved also to be a valuable tool as a rapid means of monitoring cells for the accumulation of large amounts of extra membranes. Also, examination of thin sections has shown that distinct continuities between the plasma membrane and the extra membranes exist. In general, membrane surfaces in freeze-fractured cells containing extra membranes appear smooth and lack the particles associated with the plasma membranes of many cells.

Giant cells of Escherichia coli: a morphological study

Bacterial growth without division was observed in a giant cell-producing strain of Escherichia coli K-12. Giant cell production is controlled by the lon(-) (failure of cell division after irradiation) and mon(-) (formation of irregularly shaped cells) genes. Irradiation of a lon(-)mon(-) strain (P678-A(4)) with low doses of ultraviolet or ionizing radiation results in the production of large, amorphous giant cells with 500 to 1,000 times the volume of the nonirradiated parents. The concentration of NaCl in the growth medium was found to influence irradiated-cell morphology. Low concentrations (0.2% NaCl) resulted in elongated cells, whereas spherical giant cells were produced in the presence of high salt (1% NaCl) concentrations.Thin-section electron microscopy revealed an extensive network of intracellular membranes forming vacuoles, vesicles, and cisternae. These structures bear a striking resemblance to the rough and smooth membranes (endoplasmic reticulum, Golgi complex, vacuoles, etc.) found in eucaryotic cells.

Mutants of Rhodospirrillum rubrum obtained after long-term anaerobic, dark growth

Rhodospirillum rubrum S(1) cells were grown for more than 100 generations under strict anaerobic, dark conditions in liquid medium with sodium pyruvate. During this time, growth became nonpigmented. When cells were streaked onto the surface of solid growth medium in anaerobic bottles and placed in the dark, a few light-red colonies developed, but the majority was nonpigmented. Mutants were obtained from colonies selected on the basis of pigmentation and bacteriochlorophyll a content. The growth, ultrastructure, and light reactivity of two mutants were examined. Mutant C synthesized bacteriochlorophyll a (7.2 mumoles per mg of protein), altered membrane structures, and chromatophores during dark growth. Examination of light-induced changes of the absorption spectrum of this mutant suggested that only an electron transport pathway, which included the low potential cytochrome-like pigment C428, could be detected. Mutant C grew anaerobically in the light, whereas mutant G1 was light sensitive and produced only trace amounts of bacteriochlorophyll a (0.6 mumole per ml of protein). Poorly pigmented G1 cells contained unusual membrane structures. When dark-grown G1 colonies were placed in the light, deep-red colored papillae developed in the nonpigmented colonies. During anaerobic, dark growth with sodium pyruvate, both C and G1 synthesized poly-beta-hydroxybutyrate and produced acetate, carbon dioxide, and hydrogen gas.

Presence of bacteriophage-like inhibitory particles in Escherichia coli

Bacteriophage-like particles were found in the supernatant fluids of Escherichia coli O111a and O111:B(4). Caution is urged in the study of deoxyribonucleic acid synthesis and replication in these strains.

Ultrastructure of deoxyribonucleic acid-membrane associations in Escherichia coli

Areas of contact between deoxyribonucleic acid (DNA) and intracytoplasmic membrane are frequently seen in the "extra" membrane-forming strain Escherichia coli 0111a(1). By examination of serial sections, it has been estimated that these DNA-membrane associations occur in at least 60% of the extra membrane-containing cells. Most of the DNA masses contained only one contact area. Several cells in which the DNA had been stretched revealed individual fibers connecting to the membrane, suggesting a firm attachment of DNA to membrane. The areas of membrane associated with DNA fibers were usually between 100 and 500 nm in diameter, although some smaller areas were seen. Electron microscopic autoradiography of cells in which the replication forks were labeled showed grains over 24% of the profiles containing a contact area, whereas there were grains over only 16% of the profiles without a contact area. Data from autoradiographs of cells in which the label was "chased" away from the replication fork showed the reverse labeling pattern. These data indicate that the areas of contact between DNA and intracytoplasmic membranes seen in electron micrographs contain the DNA replication forks.

Effect of low temperature on the growth and fine structure of Bacillus subtilis

Logarithmically growing cultures of Bacillus subtilis transferred from 37 to 15 C present atypical growth curves, and ultrathin sections of such cells reveal structural modifications involving mesosome deterioration and double cell wall formation. After a time, optical density and viable count increase, and cells regain the appearance typical of control cells, indicating a recovery from thermal stress. Subcultures of such recovered cells continue to grow well at 15 C. Cultures transferred from 37 to 12 C show atypical growth and fine structure, although no recovery from this stress is seen. Cultures previously grown at 15 C continue to grow at 12 C, and, furthermore, do not show the ultrastructural alterations seen in similar cells with a 37 C thermal history. The results of these studies suggest that low temperatures induce structural modifications in B. subtilis, that the response of a population to thermal stress may change during the period of the stress, and that thermal history may influence the response of a population to thermal stress.