Proposed mechanism for generation and localization of new cell division sites during the division cycle of Escherichia coli

Native cellular architecture of Treponema denticola revealed by cryo-electron tomography

Using cryo-electron tomography, we are developing a refined description of native cellular structures in the pathogenic spirochete Treponema denticola. Tightly organized bundles of periplasmic flagella were readily observed in intact plunge-frozen cells. The periplasmic space was measured in both wild-type and aflagellate strains, and found to widen by less than the diameter of flagella when the latter are present. This suggests that a structural change occurs in the peptidoglycan layer to accommodate the presence of the flagella. In dividing cells, the flagellar filaments were found to bridge the cytoplasmic cylinder constriction site. Cytoplasmic filaments, adjacent to the inner membrane, run parallel to the tightly organized flagellar filaments. The cytoplasmic filaments may be anchored by a narrow plate-like structure. The tapering of the cell ends was conserved between cells, with a patella-shaped structure observed in the periplasm at the tip of each cytoplasmic cylinder. Several incompletely characterized structures have been observed in the periplasm between dividing cells, including a cable-like structure linking two cytoplasmic cylinders and complex foil-shaped structures.

Duplication and segregation of the actin (MreB) cytoskeleton during the prokaryotic cell cycle

The bacterial actin homolog MreB exists as a single-copy helical cytoskeletal structure that extends between the two poles of rod-shaped bacteria. In this study, we show that equipartition of the MreB cytoskeleton into daughter cells is accomplished by division and segregation of the helical MreB array into two equivalent structures located in opposite halves of the predivisional cell. This process ensures that each daughter cell inherits one copy of the MreB cytoskeleton. The process is triggered by the membrane association of the FtsZ cell division protein. The cytoskeletal division and segregation events occur before and independently of cytokinesis and involve specialized MreB structures that appear to be intermediates in this process.

Positioning of chemosensory clusters in E. coli and its relation to cell division

Chemotaxis receptors and associated signalling proteins in Escherichia coli form clusters that consist of thousands of molecules and are the largest native protein complexes described to date in bacteria. Clusters are located at the cell poles and laterally along the cell body, and play an important role in signal transduction. Much work has been done to study the structure and function of receptor clusters, but the significance of their positioning and the underlying mechanisms are not understood. Here, we used fluorescence imaging to study cluster distribution and follow cluster dynamics during cell growth. Our data show that lateral clusters localise to specific periodic positions along the cell body, which mark future division sites and are involved in the localisation of the replication machinery. The chemoreceptor cluster positioning is thus intricately related to the overall structure and division of an E. coli cell.

The Escherichia coli baby cell column: a novel cell synchronization method provides new insight into the bacterial cell cycle

We describe a new method for synchronizing bacterial cells. Cells that have transiently expressed an inducible mutant 'sticky' flagellin are adhered to a volume of glass beads suspended in a chromatography column though which growth medium is pumped. Following repression of flagellin synthesis, newborn cells are eluted from the column in large quantities exceeding that of current baby machine techniques by approximately 10-fold. Eluted cultures of 'baby cells' are highly synchronous as determined by analysis of DNA replication, cell division and other events, over time after elution from the column. We also show that use of 'minutes after elution' as a time metric permits much greater temporal resolution among sequential chromosomal events than the commonly used metric of cell size (length). The former approach reveals the existence of transient intermediate stages that are missed by the latter approach. This finding has two important implications. First, at a practical level, the baby cell column is a particularly powerful method for temporal analysis. Second, at a conceptual level, replication-related events are more tightly linked to cell birth (i.e. cell division) than to absolute cell mass.

Bacterial chromosome segregation

Recent studies have made great strides toward our understanding of the mechanisms of microbial chromosome segregation and partitioning. This review first describes the mechanisms that function to segregate newly replicated chromosomes, generating daughter molecules that are viable substrates for partitioning. Then experiments that address the mechanisms of bulk chromosome movement are summarized. Recent evidence indicates that a stationary DNA replication factory may be responsible for supplying the force necessary to move newly duplicated DNA toward the cell poles. Some factors contributing to the directionality of chromosome movement probably include centromere-like-binding proteins, DNA condensation proteins, and DNA translocation proteins.

Nucleoid-independent identification of cell division sites in Escherichia coli

The mechanism used by Escherichia coli to determine the correct site for cell division is unknown. In this report, we have attempted to distinguish between a model in which septal position is determined by the position of the nucleoids and a model in which septal position is predetermined by a mechanism that does not involve nucleoid position. To do this, filaments with extended nucleoid-free regions adjacent to the cell poles were produced by simultaneous inactivation of cell division and DNA replication. The positions of septa that formed within the nucleoid-free zones after division was allowed to resume were then analyzed. The results showed that septa were formed at a uniform distance from cell poles when division was restored, with no relation to the distance from the nearest nucleoid. In some cells, septa were formed directly over nucleoids. These results are inconsistent with models that invoke nucleoid positioning as the mechanism for determining the site of division site formation.

Hemi-methylated oriC DNA binding activity found in non-specific acid phosphatase

The lacZ-hobH fusion clone, containing an Escherichia coli DNA segment located at 92 min on the chromosomal map, was screened as a producer of E. coli oriC hemi-methylated binding activity. We have purified the protein encoded by this locus to near homogeneity. The protein corresponds to the monomeric form of a non-specific acid phosphatase (NAP) whose gene has been designated aphA. oriC DNA footprinting experiments showed protection of hemi-methylated probe by partially purified NAP, but not by purified preparations. Yet, gel retardation experiments with an oriC oligonucleotide demonstrated DNA binding activity of purified NAP in the presence of Mg2+. This experiment also showed an increased affinity of the protein for the hemi-methylated probe compared with the fully or unmethylated form. Indirect immunofluorescene microscopy revealed the existence of discrete NAP foci at mid-cell in cells with two nucleoids, but at cell poles in those with one nucleoid.

The biophysics of the gram-negative periplasmic space

When subject to an osmotic 'up-shock', water flows outward from bacterial cytoplasm of the bacterium. Lipid bilayers can shrink very little in area and therefore must wrinkle to accommodate the smaller volume. The usual consequence is that all the layers of the cell envelope must become wrinkled together because they adhere to each other and must now cover a smaller surface. Plasmolysis spaces are formed if the cytoplasmic membrane (CM) separates from the other components of the wall. However, because the CM bilayer is essentially an incompressible two-dimensional liquid, this constraint restricts the location and shape of plasmolysis spaces. With mild up-shocks they form at the pole and around constricting regions in the cell. Elsewhere their creation requires the formation of endocytotic or exocytotic vesicles. The formation of endocytotic vesicles occurs in animal and plant cells as well as in bacterial cells. With stronger up-shocks tubular structures (Bayer adhesion sites), or other special geometric shapes (e.g., Scheie structures) allow the bilayer to surround an irregular shaped cytoplast. Periosmotic agents, that is, those that extract water from the periplasm as well as the cytoplasm, are molecules such as poly-vinyl-pyrrolidone and alpha-cyclodextrin that are too large to pass through the porins in the outer membrane. They were found to significantly inhibit the formation of plasmolysis spaces. Presumably, they inhibit the plasmolysis process, which requires that extracellular fluid enter between the CM and the outer membrane (OM). In the extreme case, with the dehydrating action of both osmotic agents and periosmotic agents, periplasmic space formation tends to be prevented and a new kind of space develops within the cytoplasm. We have designated these as 'cytoplasmic voids'. These novel structures are not bounded by lipid bilayers, in contrast to the endocytotic vesicles. These new spaces appear to result from the negative turgor pressure generated by the application of the combination of osmotic and periosmotic agents causing bubble formation. Several ideas in the literature about the wall biology (periseptal annuli, leading edge, osmotic pressure in the periplasm) are presented and critiqued. The basic criticism of these is that much of the phenomena can be explained because of the physics of the phospholipid bilayers and osmotic forces and thus does not imply the existence of a special control mechanism to regulate growth and division.

Bacterial cell division

Bacteria usually divide by building a central septum across the middle of the cell. This review focuses on recent results indicating that the tubulin-like FtsZ protein plays a central role in cytokinesis as a major component of a contractile cytoskeleton. Assembly of this cytoskeletal element abutting the membrane is a key point for regulation. The characterization of FtsZ homologues in Mycoplasmas, Archaea, and chloroplasts implies that the constriction mechanism is conserved and that FtsZ can constrict in the absence of peptidoglycan synthesis. In most Eubacteria, the internal cytoskeleton must also regulate synthesis of septal peptidoglycan. The Escherichia coli septum-specific penicillin-binding protein 3 (PBP3) forms a complex with other enzymes involved in murein metabolism, suggesting a centrally located transmembrane complex capable of splicing multiple new strands of peptidoglycan into the cell wall. Important questions remain about the spatial and temporal control of bacterial division.

Maintenance of intestinal epithelium structural integrity and mucosal leukocytes during chemotherapy by oral administration of muramyl tripeptide phosphatidylethanolamine

The systemic administration of doxorubicin (DXR) decreases the number of epithelial cells and leukocytes in the small intestine of mice. Oral administration of muramyl tripeptide phosphatidylethanolamine (MTP-PE) prevented both disruption of intestinal architecture, and a decrease in the number of macrophages, and it induced the expression of IL-6, G-CSF, GM-CSF, and TNF-alpha in the intestinal tissue. The data suggest that the oral administration of MTP-PE can prevent chemotherapy-induced toxicity to the intestinal mucosa and hence infections due to translocation of aerobic bacteria from the intestine to the blood.

Overview of controls in the Escherichia coli cell cycle

The harmonious growth and cell-to-cell uniformity of steady-state bacterial populations indicate the existence of a well-regulated cell cycle, responding to a set of internal signals. In Escherichia coli, the key events of this cycle are the initiation of DNA replication, nucleoid segregation and the initiation of cell division. The replication initiator is the DnaA protein. In nucleoid segregation, the MukB protein, required for proper partitioning, may be a member of the myosin-kinesin superfamily of mechanoenzymes. In cell division, the FtsZ protein has a tubulin motif, is a GTPase and polymerizes in a ring around midcell during septation; the FtsA protein has an actin-like structure. The nature of the internal signals triggering these events is not known but candidates include cell mass, the superhelical density of the chromosome and the concentration of two regulatory nucleotides, cyclic AMP and ppGpp. The involvement of cytoskeletal-like proteins in key cycle events encourages the notion of a fundamental biological unity in cell cycle regulation in all organisms.

Development of the cell-division site in FtsA- filaments

Early changes at cell-division sites were studied in non-septate filaments induced by growth of ftsATs mutant cells under non-permissive conditions. The positions of localized regions of plasmolysis were used as markers for the locations of partial and complete annular structures that are thought to be precursors of the periseptal annuli that flank the septum during cytokinesis. The results confirmed that these structures were localized at potential division sites and suggested a model in which older division sites play a role in the generation of new sites for future use, with each older site being used only once for this purpose. The results also suggest that the details of division-site development can profitably be studied in cells in which early events in the differentiation process are uncoupled from the septation event.

Early stages in development of the Escherichia coli cell-division site

Development of the Escherichia coli cell division site was studied in wild-type cells and in non-septate filaments of ftsZnull and ftsZTs mutant cells. Localized regions of plasmolysis were used as markers for the positions of annular structures that are thought to be related to the periseptal annuli that flank the ingrowing septum during cytokinesis. The results show that these structures are localized at potential division sites in non-septate filaments of FtsZ- cells, contrary to previous reports. The positions of the structures along the long axis of the cells in both wild-type cells and FtsZ- filaments were unaffected by the presence of plasmolysis bays at the cell poles. These results do not agree with a previous suggestion that the apparent association of plasmolysis bays with future division sites was artefactual. They support the view that division sites begin to differentiate before the initiation of septal ingrowth and that plasmolysis bays and the annular attachments that define them, mark the locations of these early events in the division process.

Significance of plasmolysis spaces as markers for periseptal annuli and adhesion sites

During hyperosmotic shock, the protoplast and stretched-out peptidoglycan layer first shrink together until the turgor pressure in the cell is relieved. Being non-compressible, the outer and inner membranes must fold their superfluous surfaces. While the protoplast contracts further, the inner membrane rearranges into plasmolysis spaces visible by phase-contrast microscopy. Two opposing theories predict a similar positioning of spaces in dividing cells and filaments: the 'periseptal annulus model', based on adhesion zones, involved in the predetermination of the division site; and a 'restricted, random model', based on physical properties of the protoplast. Strong osmotic shock causes retraction of the inner membrane over almost the entire surface forming the so-called 'Bayer bridges'. These tubular adhesion sites are preserved by chemical fixation, and can be destroyed by cryofixation and freeze-substitution of unfixed cells. Both the regular positioning of the plasmolysis spaces and the occurrence of tubular adhesion sites can be explained on the basis of physical properties of the membrane which necessitate rearrangements by membrane flow during shrinkage of the protoplast.

Plasmolysis bays in Escherichia coli: are they related to development and positioning of division sites?

Plasmolysis bays, induced in Escherichia coli by hypertonic treatment, are flanked by zones of adhesion between the plasma membrane and the cell wall. To test the proposition of Cook et al. (W. R. Cook, F. Joseleau-Petit, T. J. MacAlister, and L. I. Rothfield, Proc. Natl. Acad. Sci. USA 84:7144-7148, 1987) that these zones, called periseptal annuli, play a role in determining the division site, we analyzed the positions of these zones by phase-contrast and electron microscopy. In situ treatment of cells grown in agar showed that the youngest cell pole was the most susceptible to plasmolysis, whereas the constriction site was resistant. Lateral bays occurred only at some distance from a polar bay or a resistant constriction site. Orienting cells with their most prominently plasmolyzed polar bay in one direction showed that the lateral bays were always displaced away from the polar bay at about half the distance to the other cell pole. If no poles were plasmolyzed, lateral bays occurred either in the centers of nonconstricting cells or at the 1/4 or 3/4 position of cell length in constricting cells. The asymmetric positions of lateral plasmolysis bays, caused by their abrupt displacement in the presence of polar bays or constriction sites, does not confirm the periseptal annulus model (Cook et al.), which predicts a gradual and symmetric change in the position of lateral bays with increasing cell length. Our analysis indicates that plasmolysis bays have no relation to the development and positioning of the future division site.

Polar location of the chemoreceptor complex in the Escherichia coli cell

The eukaryotic cell exhibits compartmentalization of functions to various membrane-bound organelles and to specific domains within each membrane. The spatial distribution of the membrane chemoreceptors and associated cytoplasmic chemotaxis proteins in Escherichia coli were examined as a prototypic functional aggregate in bacterial cells. Bacterial chemotaxis involves a phospho-relay system brought about by ligand association with a membrane receptor, culminating in a switch in the direction of flagellar rotation. The transduction of the chemotaxis signal is initiated by a chemoreceptor-CheW-CheA ternary complex at the inner membrane. These ternary complexes aggregate predominantly at the cell poles. Polar localization of the cytoplasmic CheA and CheW proteins is dependent on membrane-bound chemoreceptor. Chemoreceptors are not confined to the cell poles in strains lacking both CheA and CheW. The chemoreceptor-CheW binary complex is polarly localized in the absence of CheA, whereas the chemoreceptor-CheA binary complex is not confined to the cell poles in strains lacking CheW. The subcellular localization of the chemotaxis proteins may reflect a general mechanism by which the bacterial cell sequesters different regions of the cell for specialized functions.

Identification and characterization of the smbA gene, a suppressor of the mukB null mutant of Escherichia coli

The mukB gene encodes a protein involved in chromosome partitioning in Escherichia coli. To study the function of this protein, we isolated from the temperature-sensitive mukB null mutant and characterized 56 suppressor mutants which could grow at 42 degrees C. Ten of the mutants also showed cold-sensitive growth at 22 degrees C. Using one of the cold-sensitive mutants as host, the wild type of the suppressor gene was cloned. The cloned suppressor gene complemented all of the 56 suppressor mutations. DNA sequencing revealed the presence of an open reading frame of 723 bp which could encode a protein of 25,953 Da. The gene product was indeed detected. The previously undiscovered gene, named smbA (suppressor of mukB), is located at 4 min on the E. coli chromosome, between the tsf and frr genes. The smbA gene is essential for cell proliferation in the range from 22 to 42 degrees C. Cells which lacked the SmbA protein ceased macromolecular synthesis. The smbA mutants are sensitive to a detergent, sodium dodecyl sulfate, and they show a novel morphological phenotype under nonpermissive conditions, suggesting a defect in specific membrane sites.

Synthesis of the cell surface during the division cycle of rod-shaped, gram-negative bacteria

When the growth of the gram-negative bacterial cell wall is considered in relation to the synthesis of the other components of the cell, a new understanding of the pattern of wall synthesis emerges. Rather than a switch in synthesis between the side wall and pole, there is a partitioning of synthesis such that the volume of the cell increases exponentially and thus perfectly encloses the exponentially increasing cytoplasm. This allows the density of the cell to remain constant during the division cycle. This model is explored at both the cellular and molecular levels to give a unified description of wall synthesis which has the following components: (i) there is no demonstrable turnover of peptidoglycan during cell growth, (ii) the side wall grows by diffuse intercalation, (iii) pole synthesis starts by some mechanism and is preferentially synthesized compared with side wall, and (iv) the combined side wall and pole syntheses enclose the newly synthesized cytoplasm at a constant cell density. The central role of the surface stress model in wall growth is distinguished from, and preferred to, models that propose cell-cycle-specific signals as triggers of changes in the rate of wall synthesis. The actual rate of wall synthesis during the division cycle is neither exponential nor linear, but is close to exponential when compared with protein synthesis during the division cycle.

Biosynthesis of a membrane adhesion zone fraction throughout the cell cycle of Escherichia coli

Synchronized cells of Escherichia coli were pulse-labeled with [3H]leucine and subjected to membrane fractionation to determine whether a fraction that is enriched for membrane-murein adhesion zones (fraction OML) was preferentially generated at specific times during the cell cycle, as previously suggested from studies of lkyD and cha mutants. Contrary to this prediction, the experiments showed that OML was formed continuously during the division cycle.

Transfer of preformed terminal C5b-9 complement complexes into the outer membrane of viable gram-negative bacteria: effect on viability and integrity

An efficient fusion system between Gram-negative bacteria and liposomes incorporating detergent-extracted C5b-9 complexes has been developed that allows delivery of preformed terminal complexes to the cell envelope (Tomlinson et al., 1989b). Fusion of Salmonella minnesota Re595 and Escherichia coli 17 with C5b-9-incorporated liposomes resulted in the transfer of 1900 C5b-9 complexes to each target bacterial cell. No loss in viability of bacteria was observed following fusion, even though the deposotion of 900 complexes onto the envelope following exposure to lysozyme-free serum effected a greater than 99% loss of viability. Increased sensitivity to antibiotics normally excluded from the cell by an integral outer membrane (OM), as well as the ability of the chromogenic substrate PADAC to gain access to periplasmically located beta-lactamase, indicated that transferred C5b-9 complexes functioned as water-filled channels through the OM. A similar conclusion was drawn from measurements demonstrating the uptake by cells of the lipophilic cation tetraphenylphosphonium (bromide), a result further indicating that the membrane potential across the cytoplasmic membrane was maintained following C5b-9 transfer to the OM. Examination of S. minnesota Re595 by electron microscopy revealed no obvious difference between cells exposed to lethal concentrations of lysozyme-free serum and cells following fusion with C5b-9-incorporated liposomes. These data suggest either that there are critical sites in the OM to which liposome-delivered C5b-9 complexes are unable to gain access or that bacterial cell death is related to events occurring during polymerization of C9 on the cell surface.

Positioning of replicated chromosomes in Escherichia coli

The positioning of replicated chromosomes at one-fourth and three-fourths of the cell length was inhibited when protein synthesis was inhibited by chloramphenicol or rifampin or by starvation for amino acids. Under these conditions, the progress of chromosome replication continued and replicated chromosomes were located close to each other as one nucleoid mass at midcell. Cells which already had two separate daughter chromosomes located at the cell quarters divided into two daughter cells under these conditions. When protein synthesis resumed, daughter chromosomes moved from midcell to the cell quarters, respectively, before any detectable increase in cell length was observed. The chromosome positioning occurred even under inhibition of the initiation of chromosome replication and under inactivation of DNA gyrase. The chromosome positioning presumably requires new synthesis of a particular protein(s) or translation itself.

A lacZ-pbpB gene fusion coding for an inducible hybrid protein that recognizes localized sites in the inner membrane of Escherichia coli

An in-phase gene fusion consisting of the 5'-terminal 1,314 base pairs (bp) of the structural gene for beta-galactosidase (lacZ) and the 3'-terminal 1,644 bp of the structural gene coding for penicillin-binding protein 3 (pbpB) of Escherichia coli was constructed and cloned in the plasmid pDIAM64. The product of the fusion gene was a remarkably stable protein with an apparent molecular weight of 110,000 (p110) that retained the ability to covalently interact with beta-lactam antibiotics. The fusion protein was found associated with the membrane at low levels of induction, but it accumulated in the cytoplasm of cells induced for a long time as inclusion bodies of high density. Inclusion bodies were localized at defined positions corresponding to septal sites in all of the pDIAM64-containing strains tested except PAT84 and GD113 (which carry the ftsZ84 mutant allele). These findings indicate a possible role of the FtsZ protein in the integration of Pbp3 into the membrane and in septum localization during the cell division cycle.