Membrane-murein attachment at the leading edge of the division septum: a second membrane-murein structure associated with morphogenesis of the gram-negative bacterial division septum

Molecular Cytology of 'Little Animals': Personal Recollections of Escherichia coli (and Bacillus subtilis)

This article relates personal recollections and starts with the origin of electron microscopy in the sixties of the previous century at the University of Amsterdam. Novel fixation and embedding techniques marked the discovery of the internal bacterial structures not visible by light microscopy. A special status became reserved for the freeze-fracture technique. By freeze-fracturing chemically fixed cells, it proved possible to examine the morphological effects of fixation. From there on, the focus switched from bacterial structure as such to their cell cycle. This invoked bacterial physiology and steady-state growth combined with electron microscopy. Electron-microscopic autoradiography with pulses of [3H] Dap revealed that segregation of replicating DNA cannot proceed according to a model of zonal growth (with envelope-attached DNA). This stimulated us to further investigate the sacculus, the peptidoglycan macromolecule. In particular, we focused on the involvement of penicillin-binding proteins such as PBP2 and PBP3, and their role in division. Adding aztreonam (an inhibitor of PBP3) blocked ongoing divisions but not the initiation of new ones. A PBP3-independent peptidoglycan synthesis (PIPS) appeared to precede a PBP3-dependent step. The possible chemical nature of PIPS is discussed.

Coordination of peptidoglycan synthesis and outer membrane constriction during Escherichia coli cell division

To maintain cellular structure and integrity during division, Gram-negative bacteria must carefully coordinate constriction of a tripartite cell envelope of inner membrane, peptidoglycan (PG), and outer membrane (OM). It has remained enigmatic how this is accomplished. Here, we show that envelope machines facilitating septal PG synthesis (PBP1B-LpoB complex) and OM constriction (Tol system) are physically and functionally coordinated via YbgF, renamed CpoB (Coordinator of PG synthesis and OM constriction, associated with PBP1B). CpoB localizes to the septum concurrent with PBP1B-LpoB and Tol at the onset of constriction, interacts with both complexes, and regulates PBP1B activity in response to Tol energy state. This coordination links PG synthesis with OM invagination and imparts a unique mode of bifunctional PG synthase regulation by selectively modulating PBP1B cross-linking activity. Coordination of the PBP1B and Tol machines by CpoB contributes to effective PBP1B function in vivo and maintenance of cell envelope integrity during division.

DOI:http://dx.doi.org/10.7554/eLife.07118.001

All bacterial cells are surrounded by a membrane, which forms a protective barrier around the cell. Most bacteria also have a wall surrounding the membrane, which provides structural support. When a bacterial cell divides to produce two daughter cells, it produces a belt-like structure around the middle of the cell. This brings the membrane and cell wall on each side together to a ‘pinch-point’ until the two halves of the cell have been separated. This process must be carefully controlled to ensure that the cell does not burst open at any point.

Some bacteria known as ‘Gram-negative’ bacteria have a second membrane on the other side of the cell wall. These cells divide in the same way as other bacteria, but the need to coordinate the movement of three structures instead of two makes it more complicated. Many proteins are known to be involved. For example, one group (or ‘complex’) of proteins—which includes a protein called PBP1B—helps to produce new cell wall material. Another complex called the Tol system provides the energy needed for the outer membrane to be pulled inwards towards the pinch point. However, it has not been clear how these complexes work together to allow the cell to divide.

Here, Gray, Egan et al. searched for proteins that can interact with PBP1B during cell division in the Gram-negative bacterium E. coli. The experiments found that a protein called CpoB interacts with both PBP1B and the Tol system. CpoB is found in a band around the middle of the cell, and it regulates the activity of PBP1B in response to signals from the Tol system. If the activity of CpoB is disrupted, cell wall production and the movement of the outer membrane are no longer coordinated, and the membrane falls apart, leading to the death of the bacteria.

Gray, Egan et al.'s findings show how the production of new cell wall material can be linked to the inwards movement of the outer membrane during cell division. The next challenges are to understand the precise details of how these processes are coordinated by CpoB and to find out whether CpoB also plays the same role in other bacteria.

DOI:http://dx.doi.org/10.7554/eLife.07118.002

A protein critical for cell constriction in the Gram-negative bacterium Caulobacter crescentus localizes at the division site through its peptidoglycan-binding LysM domains

During division of Gram-negative bacteria, invagination of the cytoplasmic membrane and inward growth of the peptidoglycan (PG) are followed by the cleavage of connective septal PG to allow cell separation. This PG splitting process requires temporal and spatial regulation of cell wall hydrolases. In Escherichia coli, LytM factors play an important role in PG splitting. Here we identify and characterize a member of this family (DipM) in Caulobacter crescentus. Unlike its E. coli counterparts, DipM is essential for viability under fast-growth conditions. Under slow-growth conditions, the DeltadipM mutant displays severe defects in cell division and FtsZ constriction. Consistent with its function in division, DipM colocalizes with the FtsZ ring during the cell cycle. Mutagenesis suggests that the LytM domain of DipM is essential for protein function, despite being non-canonical. DipM also carries two tandems of the PG-binding LysM domain that are sufficient for FtsZ ring localization. Localization and fluorescence recovery after photobleaching microscopy experiments suggest that DipM localization is mediated, at least in part, by the ability of the LysM tandems to distinguish septal, multilayered PG from non-septal, monolayered PG.

The asymmetric flagellar distribution and motility of Escherichia coli

Rod-shaped bacteria such as Escherichia coli divide by binary fission. They inherit an old pole from the parent cell. The new pole is recently derived from the septum. Because the chemoreceptor accumulates linearly with time on the cell pole, the old pole carries more receptors than does the new pole. Here, further evidence is provided that the old pole appears more frequently at the rear when bacteria swim. This phenomenon had been observed, yet not extensively explored in the literature. The biased swimming orientation is the consequence of the asymmetric distribution of flagella over the cell surface. On about 75% of cells, there are more flagella on the old-pole half of the cell than on the new-pole half, regardless of growth conditions. Most flagella are lateral, and few were found on the cell pole per se. The asymmetric flagellar distribution makes cells more efficient in chemotaxis. Both swimming orientation and receptor localization are components of chemotaxis, by which bacteria follow environmental stimuli. If unipolarly flagellated cells, such as the swarmer cells of Caulobacter crescentus, are regarded as 100% polar with respect to chemotaxis, E. coli is about 75%. The difference is quantitative. The peritrichous flagellation might enhance the motility and chemotaxis in the viscous environment of enteric bacteria.

The dynamic interplay between a cell fate determinant and a lysozyme homolog drives the asymmetric division cycle of Caulobacter crescentus

Caulobacter crescentus divides asymmetrically into a swarmer cell and a stalked cell, a process that is governed by the imbalance in phosphorylated levels of the DivK cell fate determinant in the two cellular compartments. The asymmetric polar localization of the DivJ kinase results in its specific inheritance in the stalked daughter cell where it phosphorylates DivK. The mechanism for the polar positioning of DivJ is poorly understood. SpmX, an uncharacterized lysozyme homolog, is shown here to control DivJ localization and activation. In the absence of SpmX, DivJ is delocalized and dysfunctional, resulting in developmental defects caused by an insufficiency in phospho-DivK. While SpmX stimulates DivK phosphorylation in the stalked cell, unphosphorylated DivK in the swarmer cell activates an intricate transcriptional cascade that leads to the production of the spmX message. This event primes the swarmer cell for the impending transition into a stalked cell, a transition that is sparked by the abrupt accumulation and localization of SpmX to the future stalked cell pole. Localized SpmX then recruits and stimulates DivJ, and the resulting phospho-DivK implements the stalked cell fate. The dynamic interplay between SpmX and DivK is at the heart of the molecular circuitry that sustains the Caulobacter developmental cycle.

Exploration into the spatial and temporal mechanisms of bacterial polarity

The recognition of bacterial asymmetry is not new: the first high-resolution microscopy studies revealed that bacteria come in a multitude of shapes and sometimes carry asymmetrically localized external structures such as flagella on the cell surface. Even so, the idea that bacteria could have an inherent overall polarity, which affects not only their outer appearance but also many of their vital processes, has only recently been appreciated. In this review, we focus on recent advances in our understanding of the molecular mechanisms underlying the establishment of polarized functions and cell polarity in bacteria.

The trans-envelope Tol-Pal complex is part of the cell division machinery and required for proper outer-membrane invagination during cell constriction in E. coli

Fission of bacterial cells involves the co-ordinated invagination of the envelope layers. Invagination of the cytoplasmic membrane (IM) and peptidoglycan (PG) layer is likely driven by the septal ring organelle. Invagination of the outer membrane (OM) in Gram-negative species is thought to occur passively via its tethering to the underlying PG layer with generally distributed PG-binding OM (lipo)proteins. The Tol-Pal system is energized by proton motive force and is well conserved in Gram-negative bacteria. It consists of five proteins that can connect the OM to both the PG and IM layers via protein-PG and protein-protein interactions. Although the system is needed to maintain full OM integrity, and for class A colicins and filamentous phages to enter cells, its precise role has remained unclear. We show that all five components accumulate at constriction sites in Escherichia coli and that mutants lacking an intact system suffer delayed OM invagination and contain large OM blebs at constriction sites and cell poles. We propose that Tol-Pal constitutes a dynamic subcomplex of the division apparatus in Gram-negative bacteria that consumes energy to establish transient trans-envelope connections at/near the septal ring to draw the OM onto the invaginating PG and IM layers during constriction.

The bacterial cytoskeleton

In recent years it has been shown that bacteria contain a number of cytoskeletal structures. The bacterial cytoplasmic elements include homologs of the three major types of eukaryotic cytoskeletal proteins (actin, tubulin, and intermediate filament proteins) and a fourth group, the MinD-ParA group, that appears to be unique to bacteria. The cytoskeletal structures play important roles in cell division, cell polarity, cell shape regulation, plasmid partition, and other functions. The proteins self-assemble into filamentous structures in vitro and form intracellular ordered structures in vivo. In addition, there are a number of filamentous bacterial elements that may turn out to be cytoskeletal in nature. This review attempts to summarize and integrate the in vivo and in vitro aspects of these systems and to evaluate the probable future directions of this active research field.

Bacterial Birth Scar Proteins Mark Future Flagellum Assembly Site

Many prokaryotic protein complexes underlie polar asymmetry. In Caulobacter crescentus, a flagellum is built exclusively at the pole that arose from the previous cell division. The basis for this pole specificity is unclear but could involve a cytokinetic birth scar that marks the newborn pole as the flagellum assembly site. We identified two developmental proteins, TipN and TipF, which localize to the division septum and the newborn pole after division. We show that septal localization of TipN/F depends on cytokinesis. Moreover, TipF, a c-di-GMP phosphodiesterase homolog, is a flagellum assembly factor that relies on TipN for proper positioning. In the absence of TipN, flagella are assembled at ectopic locations, and TipF is mislocalized to such sites. Thus TipN and TipF establish a link between bacterial cytokinesis and polar asymmetry, demonstrating that division does indeed leave a positional mark in its wake to direct the biogenesis of a polar organelle.

Identification of an antigen localized to an apparent septum within dividing chlamydiae

The process of chlamydial cell division has not been thoroughly investigated. The lack of detectable peptidoglycan and the absence of an FtsZ homolog within chlamydiae suggest an unusual mechanism for the division process. Our laboratory has identified an antigen (SEP antigen) localized to a ring-like structure at the apparent septum within dividing chlamydial reticulate bodies (RB). Antisera directed against SEP show similar patterns of antigen distribution in Chlamydia trachomatis and Chlamydia psittaci RB. In contrast to localization in RB, SEP in elementary bodies appears diffuse and irregular, suggesting that the distribution of the antigen is developmental-stage specific. Treatment of chlamydiae with inhibitors of peptidoglycan synthesis or culture of chlamydiae in medium lacking tryptophan leads to the formation of nondividing, aberrant RB. Staining of aberrant RB with anti-SEP reveals a marked redistribution of the antigen. Within C. trachomatis-infected cells, ampicillin treatment leads to high levels of SEP accumulation at the periphery of aberrant RB, while in C. psittaci, treatment causes SEP to localize to distinct punctate sites within the bacteria. Aberrancy produced via tryptophan depletion results in a different pattern of SEP distribution. In either case, the reversal of aberrant formation results in the production of normal RB and a redistribution of SEP to the apparent plane of bacterial division. Collectively these studies identify a unique chlamydial-genus-common and developmental-stage-specific antigen that may be associated with RB division.

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.

Morphogenesis of Escherichia coli

The shape of Escherichia coli is strikingly simple compared to those of higher eukaryotes. In fact, the end result of E. coli morphogenesis is a cylindrical tube with hemispherical caps. It is argued that physical principles affect biological forms. In this view, genes code for products that contribute to the production of suitable structures for physical factors to act upon. After introduction of a physical model, the discussion is focused on the shape-maintaining (peptidoglycan) layer of E. coli. This is followed by a detailed analysis of the structural relationship of the cellular interior to the cytoplasmic membrane. A basic theme of this review is that the transcriptionally active nucleoid and the cytoplasmic translation machinery form a structural continuity with the growing cellular envelope. An attempt has been made to show how this dynamic relationship during the cell cycle affects cell polarity and how it leads to cell division.

Murein segregation in Escherichia coli

Peptidoglycan (murein) segregation has been studied by means of a new labeling method. The method relies on the ability of Escherichia coli cells to incorporate D-Cys into macromolecular murein. The incorporation depends on a periplasmic amino acid exchange reaction. At low concentrations, D-Cys is innocuous to the cell. The distribution of modified murein in purified sacculi can be traced and visualized by immunodetection of the -SH groups by fluorescence and electron microscopy techniques. Analysis of murein segregation in wild-type and cell division mutant strains revealed that murein in polar caps is metabolically inert and is segregated in a conservative fashion. Elongation of the sacculus apparently occurs by diffuse insertion of precursors over the cylindrical part of the cell surface. At the initiation of cell division, there is a FtsZ-dependent localized activation of murein synthesis at the potential division sites. Penicillin-binding protein 3 and the products of the division genes ftsA and ftsQ are dispensable for the activation of division sites. As a consequence, under restrictive conditions ftsA,ftsI,or ftsQ mutants generate filamentous sacculi with rings of all-new murein at the positions where septa would otherwise develop.

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.

Characterization of translucent segments observed in an smbA mutant of Escherichia coli

The smbA gene of Escherichia coli is essential for cell proliferation. The smbA2 mutant shows cold-sensitive colony formation at 22 degrees C. A novel morphological phenotype, formation of a translucent segment at midcell or at a cell pole, was observed by phase-contrast microscopy at a high frequency in the smbA2 mutant cells incubated in L medium lacking NaCl at 22 degrees C, but not observed in L medium containing 1% NaCl or 20% sucrose at the same temperature. No translucent segment was observed in the wild-type cells in any of the media used. Electron microscopic observation revealed that the translucent segments resulted from the enlargement of a periplasmic space by separation of the inner membrane from the peptidoglycan layer and the outer membrane.

Polarized cells, polar actions

The recognition of polar bacterial organization is just emerging. The examples of polar localization given here are from a variety of bacterial species and concern a disparate array of cellular functions. A number of well-characterized instances of polar localization of bacterial proteins, including the chemoreceptor complex in both C. crescentus and E. coli, the maltose-binding protein in E. coli, the B. japonicum surface attachment proteins, and the actin tail of L. monocytogenes within a mammalian cell, involve proteins or protein complexes that facilitate bacterial interaction with the environment, either the extracellular milieux or that within a plant or mammalian host. The significance of this observation remains unclear. Polarity in bacteria poses many problems, including the necessity for a mechanism for asymmetrically distributing proteins as well as a mechanism by which polar localization is maintained. Large structures, such as a flagellum, are anchored at the pole by means of the basal body that traverses the peptidoglycan wall. But for proteins and small complexes, whether in the periplasm or the membrane, one must invoke a mechanism that prevents the diffusion of these proteins away from the cell pole. Perhaps the periplasmic proteins are retained at the pole by the presence of the periseptal annulus (35). The constraining features for membrane components are not known. For large aggregates, such as the clusters of MCP, CheA, and CheW complexes, perhaps the size of the aggregate alone prevents displacement. In most cases of cellular asymmetry, bacteria are able to discriminate between the new pole and the old pole and to utilize this information for localization specificity. The maturation of new pole to old pole appears to be a common theme as well. Given numerous examples reported thus far, we propose that bacterial polarity displays specific rules and is a more general phenomenon than has been previously recognized.

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.

Dynamics of PhiX174 protein E-mediated lysis of Escherichia coli

Expression of cloned gene E of bacteriophage PhiX174 induces lysis by formation of a transmembrane tunnel structure in the cell envelope of Escherichia coli. Ultrastructural studies of the location of the lysis tunnel indicate that it is preferentially located at the septum or at polar regions of the cell. Furthermore, the diameter and shape of individual tunnel structures vary greatly indicating that its structure is not rigid. Apparently, the contours of individual lysis tunnels are determined by enlarged meshes in the peptidoglycan net and the force produced at its orifice, by the outflow of cytoplasmic content. Once the tunnel is formed the driving force for the lysis process is the osmotic pressure difference between cytoplasm and medium. During the lysis process areas of the cytoplasmic membrane which are not tightly attached to the envelope are extended inward by the negative pressure produced during lysis. After cell lysis external medium can diffuse through the lysis tunnel filling the inner cell space of the still rigid bacterial ghosts.

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.

Amount of peptidoglycan in cell walls of gram-negative bacteria

The amount of diaminopimelic acid (Dap) in the cell wall of Escherichia coli was measured in two ways. A radiochemical method first described by us in 1985 (F. B. Wientjes, E. Pas, P. E. M. Taschner, and C. L. Woldringh, J. Bacteriol. 164:331-337, 1985) is based on the steady-state incorporation of [3H]Dap during several generations. Knowing the cell concentration and the specific activity of the [3H]Dap, one can calculate the number of Dap molecules per sacculus. The second method measures the Dap content chemically in sacculi isolated from a known number of cells. With both methods, a value of 3.5 x 10(6) Dap molecules per sacculus was obtained. Combined with electron microscopic measurements of the surface area of the cells, the data indicate an average surface area per disaccharide unit of ca. 2.5 nm2. This finding suggests that the peptidoglycan is basically a monolayered structure.

Cell division and peptidoglycan assembly in Escherichia coli

Research on bacterial cell division has recently gained renewed impetus because of new information about peptidoglycan assembly and about specific cell-division genes and their products. This paper concerns aspects of cell division that specifically concern the peptidoglycan. It is shown that upon division, peptidoglycan assembly switches from lateral wall location to the cell centre, that assembly takes place at the leading edge of the invaginating constriction, that the mode of glycan strand insertion changes from a single-stranded mode to a multi-stranded mode, and that the initiation of division (in contrast to its continuation) requires penicillin-insensitive peptidoglycan synthesis (PIPS). A membrane component X (possibly FtsQ) is proposed to coordinate PIPS with the cell division-initiating protein FtsZ. It is suggested that a largely proteinaceous macromolecular complex (divisome) at the leading edge of constriction encompasses three compartments (cytoplasm, membrane and periplasm). The composition of this complex is proposed to vary depending on whether division is being initiated or completed.

To shape a cell: an inquiry into the causes of morphogenesis of microorganisms

We recognize organisms first and foremost by their forms, but how they grow and shape themselves still largely passes understanding. The objective of this article is to survey what has been learned of morphogenesis of walled eucaryotic microorganisms as a set of problems in cellular heredity, biochemistry, physiology, and organization. Despite the diversity of microbial forms and habits, some common principles can be discerned. (i) That the form of each organism represents the expression of a genetic program is almost universally taken for granted. However, reflection on the findings with morphologically aberrant mutants suggests that the metaphor of a genetic program is misleading. Cellular form is generated by a web of interacting chemical and physical processes, whose every strand is woven of multiple gene products. The relationship between genes and form is indirect and cumulative; therefore, morphogenesis must be addressed as a problem not of molecular genetics but of cellular physiology. (ii) The shape of walled cells is determined by the manner in which the wall is laid down during growth and development. Turgor pressure commonly, perhaps always, supplies the driving force for surface enlargement. Cells yield to this scalar force by localized, controlled wall synthesis; their forms represent variations on the theme of local compliance with global force. (iii) Growth and division in bacteria display most immediately the interplay of hydrostatic pressure, localized wall synthesis, and structural constraints. Koch's surface stress theory provides a comprehensive and quantitative framework for understanding bacterial shapes. (iv) In the larger and more versatile eucaryotic cells, expansion is mediated by the secretion of vesicles. Secretion and ancillary processes, such as cytoplasmic transport, are spatially organized on the micrometer scale. The diversity of vectorial physiology and of the forms it generates is illustrated by examples: apical growth of fungal hyphae, bud formation in yeasts, germination of fucoid zygotes, and development of cells of Nitella, Closterium, and other unicellular algae. (v) Unicellular organisms, no less than embryos, have a remarkable capacity to impose spatial order upon themselves with or without the help of directional cues. Self-organization is reviewed here from two perspectives: the theoretical exploration of morphogens, gradients, and fields, and experimental study of polarization in Fucus cells, extension of hyphal tips, and pattern formation in ciliates. Here is the heart of the matter, yet self-organization remains nearly as mysterious as it was a century ago, a subject in search of a paradigm.

The 'Bayer bridges' confronted with results from improved electron microscopy methods

In electron micrographs of conventionally prepared thin sections of Escherichia coli one observes (i) a wavy appearance of the two membranes showing frequent appositions (named adhesion sites) and (ii) intermembrane bridges after plasmolysis which, it is claimed, occur at the adhesion sites and are related to intermembrane protein transport (transmigration). When chemical fixation is replaced by cryofixation, the observations are very different. (a) The two membranes are equally spaced and no contacts, adhesions or other sorts of connections are visible. (b) After plasmolysis the protoplast is shrunken, but the typical bridges are no longer produced. (c) In addition, when peptidoglycan is stained on conventionally prepared sections, it is revealed as a 7-nm-thick sacculus which is not interrupted at the sites of apposition. In view of the new observations, the structural concepts derived from conventionally prepared material must be revised. It is proposed that the intermembrane space is entirely filled by a gel, the outer part of which is the 7 nm thick, very stable, chemically resistant peptidoglycan (or murein). The inner part is much less stable and is proposed to undergo rapid autolytic changes upon cell death. The large 'Bayer bridges' might then tentatively be explained as an artificial post-mortem enhancement of either a stream of proteins transmigrating across the periplasm or of a pre-existing, but not yet resolved, structure. This enhancement probably occurs during the 7-10 min between plasmolysis and fixation that are prescribed for the procedure necessary for revealing 'Bayer bridges'.

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.

Rate and topography of peptidoglycan synthesis during cell division in Escherichia coli: concept of a leading edge

The rate at which the peptidoglycan of Escherichia coli is synthesized during the division cycle was studied with two methods. One method involved synchronization of E. coli MC4100 lysA cultures by centrifugal elutriation and subsequent pulse-labeling of the synchronously growing cultures with [meso-3H]diaminopimelic acid ([3H]Dap). The second method was autoradiography of cells pulse-labeled with [3H]Dap. It was found that the peptidoglycan is synthesized at a more or less exponentially increasing rate during the division cycle with a slight acceleration in this rate as the cells start to constrict. Apparently, polar cap formation requires synthesis of extra surface components, presumably to accommodate for a change in the surface-to-volume ratio. Furthermore, it was found that the pool size of Dap was constant during the division cycle. Close analysis of the topography of [3H]Dap incorporation at the constriction site revealed that constriction proceeded by synthesis of peptidoglycan at the leading edge of the invaginating cell envelope. During constriction, no reallocation of incorporation occurred, i.e., the incorporation at the leading edge remained high throughout the process of constriction. Impairment of penicillin-binding protein 3 by mutation or by the specific beta-lactam antibiotic furazlocillin did not affect [3H]Dap incorporation during initiation of constriction. However, the incorporation at the constriction site was inhibited in later stages of the constriction process. It is concluded that during division at least two peptidoglycan-synthesizing systems are operating sequentially.

The envelope structure of Branhamella catarrhalis as studied by transmission electron microscopy

The envelope structure of Branhamella catarrhalis was studied by electron microscopy and compared with that of other bacteria of the family Neisseriaceae, such as Moraxella lacunata subsp. liquefaciens and Neisseria gonorrhoeae. Negative staining of B. catarrhalis showed a mamilliform surface similar to that of Moraxella. On thin sections, the cell wall appeared to be made up of a wavy outer membrane tightly linked to a straight peptidoglycan layer. Spicule-like structures protruded from the cell surface. Ruthenium red staining revealed that they contained polysaccharides. While the outer polysaccharide layer of N. gonorrhoeae was unstable after repeated subcultures in vitro, this layer remained stable in B. catarrhalis and in Moraxella lacunata subsp. liquefaciens.

Studies with FtsA-LacZ protein fusions reveal FtsA located inner-outer membrane junctions

Four FtsA-LacZ translational gene fusions were constructed using a mini-Mu transposon (MudII 1734). FtsA-LacZ fusions and FtsA protein that were radioactively labelled using maxicell technique fractionated identically into membranes and cytoplasm. The FtsA-LacZ fusion proteins were also localized in wild type dividing cells using beta-galactosidase activity. Fractions from a modified sucrose equilibrium gradient exhibited beta-galactosidase activity in fractions corresponding to outer membrane-heavy (OMH) and outer membrane light (OML). The data are consistent with a model in which FtsA protein is incorporated into septal adhesion sites associated with cell division.