Polarized cells, polar actions

MinD-RNase E interplay controls localization of polar mRNAs in E. coli

The E. coli transcriptome at the cell's poles (polar transcriptome) is unique compared to the membrane and cytosol. Several factors have been suggested to mediate mRNA localization to the membrane, but the mechanism underlying polar localization of mRNAs remains unknown. Here, we combined a candidate system approach with proteomics to identify factors that mediate mRNAs localization to the cell poles. We identified the pole-to-pole oscillating protein MinD as an essential factor regulating polar mRNA localization, although it is not able to bind RNA directly. We demonstrate that RNase E, previously shown to interact with MinD, is required for proper localization of polar mRNAs. Using in silico modeling followed by experimental validation, the membrane-binding site in RNase E was found to mediate binding to MinD. Intriguingly, not only does MinD affect RNase E interaction with the membrane, but it also affects its mode of action and dynamics. Polar accumulation of RNase E in ΔminCDE cells resulted in destabilization and depletion of mRNAs from poles. Finally, we show that mislocalization of polar mRNAs may prevent polar localization of their protein products. Taken together, our findings show that the interplay between MinD and RNase E determines the composition of the polar transcriptome, thus assigning previously unknown roles for both proteins.

Complete structure of the chemosensory array core signalling unit in an E. coli minicell strain

Motile bacteria sense chemical gradients with transmembrane receptors organised in supramolecular signalling arrays. Understanding stimulus detection and transmission at the molecular level requires precise structural characterisation of the array building block known as a core signalling unit. Here we introduce an Escherichia coli strain that forms small minicells possessing extended and highly ordered chemosensory arrays. We use cryo-electron tomography and subtomogram averaging to provide a three-dimensional map of a complete core signalling unit, with visible densities corresponding to the HAMP and periplasmic domains. This map, combined with previously determined high resolution structures and molecular dynamics simulations, yields a molecular model of the transmembrane core signalling unit and enables spatial localisation of its individual domains. Our work thus offers a solid structural basis for the interpretation of a wide range of existing data and the design of further experiments to elucidate signalling mechanisms within the core signalling unit and larger array.

Asymmetric polar localization dynamics of the serine chemoreceptor protein Tsr in Escherichia coli

The spatial location of proteins in living cells can be critical for their function. For example, the E. coli chemotaxis machinery is localized to the cell poles. Here we describe the polar localization of the serine chemoreceptor Tsr using a strain synthesizing a fluorescent Tsr-Venus fusion at a low level from a single-copy chromosomal construct. Using photobleaching and imaging during recovery by new synthesis, we observed distinct asymmetry between a bright (old) pole and a dim (new) pole. The old pole was shown to be a more stable cluster and to recover after photobleaching faster, which is consistent with the hypothesis that newly synthesized Tsr proteins are inserted directly at or near the old pole. The new pole was shown to be a less stable cluster and to exchange proteins freely with highly mobile Tsr-Venus proteins diffusing in the membrane. We propose that the new pole arises from molecules escaping from the old pole and diffusing to the new pole where a more stable cluster forms over time. Our localization imaging data support a model in which a nascent new pole forms prior to stable cluster formation.

Genome-scale quantitative characterization of bacterial protein localization dynamics throughout the cell cycle

Bacterial cells display both spatial and temporal organization, and this complex structure is known to play a central role in cellular function. Although nearly one-fifth of all proteins in Escherichia coli localize to specific subcellular locations, fundamental questions remain about how cellular-scale structure is encoded at the level of molecular-scale interactions. One significant limitation to our understanding is that the localization behavior of only a small subset of proteins has been characterized in detail. As an essential step toward a global model of protein localization in bacteria, we capture and quantitatively analyze spatial and temporal protein localization patterns throughout the cell cycle for nearly every protein in E. coli that exhibits nondiffuse localization. This genome-scale analysis reveals significant complexity in patterning, notably in the behavior of DNA-binding proteins. Complete cell-cycle imaging also facilitates analysis of protein partitioning to daughter cells at division, revealing a broad and robust assortment of asymmetric partitioning behaviors.

Subcellular localization of bacteriophage PRD1 proteins in Escherichia coli

Bacteria possess an intricate internal organization resembling that of the eukaryotes. The complexity is especially prominent at the bacterial cell poles, which are also known to be the preferable sites for some bacteriophages to infect. Bacteriophage PRD1 is a well-known model serving as an ideal system to study structures and functions of icosahedral internal membrane-containing viruses. Our aim was to analyze the localization and interactions of individual PRD1 proteins in its native host Escherichia coli. This was accomplished by constructing a vector library for production of fluorescent fusion proteins. Analysis of solubility and multimericity of the fusion proteins, as well as their localization in living cells by confocal microscopy, indicated that multimeric PRD1 proteins were prone to localize in the cell poles. Furthermore, PRD1 spike complex proteins P5 and P31, as fusion proteins, were shown to be functional in the virion assembly. In addition, they were shown to co-localize in the specific polar area of the cells, which might have a role in the multimerization and formation of viral protein complexes.

Flexibility of syntrophic enzyme systems in Desulfovibrio species ensures their adaptation capability to environmental changes

The mineralization of organic matter in anoxic environments relies on the cooperative activities of hydrogen producers and consumers obligately linked by interspecies metabolite exchange in syntrophic consortia that may include sulfate reducing species such as Desulfovibrio. To evaluate the metabolic flexibility of syntrophic Desulfovibrio to adapt to naturally fluctuating methanogenic environments, we studied Desulfovibrio alaskensis strain G20 grown in chemostats under respiratory and syntrophic conditions with alternative methanogenic partners, Methanococcus maripaludis and Methanospirillum hungatei, at different growth rates. Comparative whole-genome transcriptional analyses, complemented by G20 mutant strain growth experiments and physiological data, revealed a significant influence of both energy source availability (as controlled by dilution rate) and methanogen on the electron transfer systems, ratios of interspecies electron carriers, energy generating systems, and interspecies physical associations. A total of 68 genes were commonly differentially expressed under syntrophic versus respiratory lifestyle. Under low-energy (low-growth-rate) conditions, strain G20 further had the capacity to adapt to the metabolism of its methanogenic partners, as shown by its differing gene expression of enzymes involved in the direct metabolic interactions (e.g., periplasmic hydrogenases) and the ratio shift in electron carriers used for interspecies metabolite exchange (hydrogen/formate). A putative monomeric [Fe-Fe] hydrogenase and Hmc (high-molecular-weight-cytochrome c3) complex-linked reverse menaquinone (MQ) redox loop become increasingly important for the reoxidation of the lactate-/pyruvate oxidation-derived redox pair, DsrC(red) and Fd(red), relative to the Qmo-MQ-Qrc (quinone-interacting membrane-bound oxidoreductase; quinone-reducing complex) loop. Together, these data underscore the high enzymatic and metabolic adaptive flexibility that likely sustains Desulfovibrio in naturally fluctuating methanogenic environments.

Vital dye reaction and granule localization in periplasm of Escherichia coli

Background

Tetrazolium salts are widely used in biology as indicators of metabolic activity – hence termed vital dyes – but their reduction site is still debated despite decades of intensive research. The prototype, 2,3,5- triphenyl tetrazolium chloride, which was first synthesized a century ago, often generates a single formazan granule at the old pole of Escherichia coli cells after reduction. So far, no explanation for their pole localization has been proposed.

Method/Principal Findings

Here we provide evidence that the granules form in the periplasm of bacterial cells. A source of reducing power is deduced to be thiol groups destined to become disulfides, since deletion of dsbA, coding for thiol-oxidase, enhances the formation of reduced formazan. However, pervasive reduction did not result in a random distribution of formazan aggregates. In filamentous cells, large granules appear at regular intervals of about four normal cell-lengths, consistent with a diffusion-to-capture model. Computer simulations of a minimal biophysical model showed that the pole localization of granules is a spontaneous process, i.e. small granules in a normal size bacterium have lower energy at the poles. This biased their diffusion to the poles. They kept growing there and eventually became fixed.

Conclusions

We observed that formazan granules formed in the periplasm after reduction of tetrazolium, which calls for re-evaluation of previous studies using cell-free systems that liberate inaccessible intracellular reductant and potentially generate artifacts. The localization of formazan granules in E. coli cells can now be understood. In living bacteria, the seeds formed at or migrated to the new pole would become visible only when that new pole already became an old pole, because of the relatively slow growth rate of granules relative to cell division.

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.

Molecular mapping of lipoarabinomannans on mycobacteria

Although the chemical composition of mycobacterial cell walls is well known, the 3D organization of the various constituents is not fully understood. In particular, it is unclear whether the major wall component lipoarabinomannan (LAM) is exposed on the outermost surface or hindered by other constituents such as mycolic acids. To address this pertinent question, we used atomic force microscopy (AFM) with tips bearing anti-LAM antibodies to detect single LAM molecules on Mycobacterium bovis BCG cells. First, we showed the ability of anti-LAM tips to detect isolated, purified LAM molecules. We then mapped the distribution of LAM on mycobacteria, prior to and after treatment with the drug isoniazid. We found that LAM was not exposed on the surface of native cells, pointing to the presence of a homogeneous layer of mycolic acids, whereas it was greatly exposed on isoniazid-treated cells, in agreement with the action mode of the drug. This single-molecule study provides novel insight into the architecture of mycobacterial cell walls and offers promising perspectives for understanding the action modes of antimycobacterial drugs.

Towards nanomicrobiology using atomic force microscopy

At the cross-roads of nanoscience and microbiology, the nanoscale analysis of microbial cells using atomic force microscopy (AFM) is an exciting, rapidly evolving research field. Over the past decade, there has been tremendous progress in our use of AFM to observe membrane proteins and live cells at high resolution. Remarkable advances have also been made in applying force spectroscopy to manipulate single membrane proteins, to map surface properties and receptor sites on cells and to measure cellular interactions at the single-cell and single-molecule levels. In addition, recent developments in cantilever nanosensors have opened up new avenues for the label-free detection of microorganisms and bioanalytes.

Localization of SpoVAD to the inner membrane of spores of Bacillus subtilis

The products of the hexacistronic spoVA operon of Bacillus subtilis may be involved in the transport of dipicolinic acid into the forespore during sporulation and its release during spore germination. The major hydrophilic coding region of B. subtilis spoVAD was cloned, the protein was expressed in Escherichia coli as a His tag fusion protein, and a rabbit antiserum was raised against the purified protein. Western blot analyses of fractions from B. subtilis spores showed that SpoVAD is an integral inner membrane protein present at levels >50-fold higher than those of the spore's nutrient germinant receptors that are also present in the inner membrane. SpoVAD also persisted in outgrowing spores.

The elastic properties of the caulobacter crescentus adhesive holdfast are dependent on oligomers of N-acetylglucosamine

The aquatic bacterium Caulobacter crescentus attaches to solid surfaces through an adhesive holdfast located at the tip of its polar stalk, a thin cylindrical extension of the cell membrane. In this paper, the elastic properties of the C. crescentus stalk and holdfast assembly were studied by using video light microscopy. In particular, the contribution of oligomers of N-acetylglucosamine (GlcNAc) to the elasticity of holdfast was examined by lysozyme digestion. C. crescentus cells attached to a surface undergo Brownian motion while confined effectively in a harmonic potential. Mathematical analysis of such motion enabled us to determine the force constant of the stalk-holdfast assembly, which quantifies its elastic properties. The measured force constant exhibits no dependence on stalk length, consistent with the theoretical estimate showing that the stalk can be treated as a rigid rod with respect to fluctuations of the attached cells. Therefore, the force constant of the stalk-holdfast assembly can be attributed to the elasticity of the holdfast. Motions of cells in a rosette were found to be correlated, consistent with the elastic characteristics of the holdfast. Atomic force microscopy analysis indicates that the height of a dried (in air) holdfast is approximately one-third of that of a wet (in water) holdfast, consistent with the gel-like nature of the holdfast. Lysozyme, which cleaves oligomers of GlcNAc, reduced the force constant to less than 10% of its original value, consistent with the polysaccharide gel-like nature of the holdfast. These results also indicate that GlcNAc polymers play an important role in the strength of the holdfast.

Single-cell microbiology: tools, technologies, and applications

The field of microbiology has traditionally been concerned with and focused on studies at the population level. Information on how cells respond to their environment, interact with each other, or undergo complex processes such as cellular differentiation or gene expression has been obtained mostly by inference from population-level data. Individual microorganisms, even those in supposedly "clonal" populations, may differ widely from each other in terms of their genetic composition, physiology, biochemistry, or behavior. This genetic and phenotypic heterogeneity has important practical consequences for a number of human interests, including antibiotic or biocide resistance, the productivity and stability of industrial fermentations, the efficacy of food preservatives, and the potential of pathogens to cause disease. New appreciation of the importance of cellular heterogeneity, coupled with recent advances in technology, has driven the development of new tools and techniques for the study of individual microbial cells. Because observations made at the single-cell level are not subject to the "averaging" effects characteristic of bulk-phase, population-level methods, they offer the unique capacity to observe discrete microbiological phenomena unavailable using traditional approaches. As a result, scientists have been able to characterize microorganisms, their activities, and their interactions at unprecedented levels of detail.

Restricted Mobility of Cell Surface Proteins in the Polar Regions of Escherichia coli

The polar regions of the Escherichia coli murein sacculus are metabolically inert and stable in time. Because the sacculus and the outer membrane are tightly associated, we investigated whether polar inert murein could restrict the mobility of other cell envelope elements. Cells were covalently labeled with a fluorescent reagent, chased in dye-free medium, and observed by microscopy. Fluorescent material was more efficiently retained at the cell poles than at any other location. The boundary between high and low fluorescence intensity areas was rather sharp. Labeled material consisted mostly of cell envelope proteins, among them the free and murein-bound forms of Braun's lipoprotein. Our results indicate that the mobility of at least some cell envelope proteins is restrained at regions in correspondence with underlying areas of inert murein.

Translocation of jellyfish green fluorescent protein via the Tat system of Escherichia coli and change of its periplasmic localization in response to osmotic up-shock

The bacterial twin arginine translocation (Tat) pathway is capable of exporting cofactor-containing enzymes into the periplasm. To assess the capacity of the Tat pathway to export heterologous proteins and to gain information about the property of the periplasm, we fused the twin arginine signal peptide of the trimethylamine N-oxide reductase to the jellyfish green fluorescent protein (GFP). Unlike the Sec pathway, the Tat system successfully exported correctly folded GFP into the periplasm of Escherichia coli. Interestingly, GFP appeared as a halo in most cells and occasionally showed a polar localization in wild type strains. When subjected to a mild osmotic up-shock, GFP relocalized very quickly at the two poles of the cells. The conversion from the halo structure to a periplasmic gathering at particular locations was also observed with spherical cells of the DeltarodA-pbpA mutant or of the wild type strain treated with lysozyme. Therefore, the periplasm is not a uniform compartment and the polarization of GFP is unlikely to be caused by simple invagination of the cytoplasmic membrane at the poles. Moreover, the polar gathering of GFP is reversible; the reversion was accelerated by glucose and inhibited by azide and carbonyl cyanide m-chlorophenylhydrazone, indicating an active adaptation of the bacteria to the osmolarity in the medium. These results strongly suggest a relocalization of periplasmic substances in response to environmental changes. The polar area might be the preferential zone where bacteria sense the change in the environment.

Principles of macromolecular organization and cell function in bacteria and archaea

Structural organization of the cytoplasm by compartmentation is a well established fact for the eukaryotic cell. In prokaryotes, compartmentation is less obvious. Most prokaryotes do not need intracytoplasmic membranes to maintain their vital functions. This review, especially dealing with prokaryotes, will point out that compartmentation in prokaryotes is present, but not only achieved by membranes. Besides membranes, the nucleoid, multienzyme complexes and metabolons, storage granules, and cytoskeletal elements are involved in compartmentation. In this respect, the organization of the cytoplasm of prokaryotes is similar to that in the eukaryotic cell. Compartmentation influences properties of water in cells.

Localization of the histidine kinase PilS to the poles of Pseudomonas aeruginosa and identification of a localization domain

Transcription of the type IV pilus subunit gene of Pseudomonas aeruginosa is controlled by a two-component signal transduction system. PilS, the histidine kinase, is membrane bound and PilR, its cognate response regulator, is cytoplasmic. The signal that activates PilS is unknown. PilS has three domains: (i) The N-terminus, predicted to form six transmembrane (TM) helices; (ii) a central linker domain; and (iii) the C-terminal transmitter domain containing all the conserved residues of sensor kinases. A translational fusion of the gfp gene (green fluorescent protein) to the 3' end of pilS was used to determine the position of PilS in the bacterial cell. Epifluorescence microscopy revealed that PilS is retained to the poles of P. aeruginosa but is distributed evenly about the membrane of Escherichia coli. Deletions of the PilS-GFP fusion revealed that the TM domain was sufficient and necessary to bring GFP to the membrane of P. aeruginosa and E. coli but was not sufficient to confine GFP to the poles. Retention to the poles of P. aeruginosa required both the TM and linker domains. Replacement of the PilS TM domain with an E. coli membrane protein, MalG, still allowed polar localization. Therefore, the PilS TM domain positions the linker domain close to the membrane allowing it to interact with the putative polar anchor which is specific to P. aeruginosa.

Hypothesis: hyperstructures regulate bacterial structure and the cell cycle

A myriad different constituents or elements (genes, proteins, lipids, ions, small molecules etc.) participate in numerous physico-chemical processes to create bacteria that can adapt to their environments to survive, grow and, via the cell cycle, reproduce. We explore the possibility that it is too difficult to explain cell cycle progression in terms of these elements and that an intermediate level of explanation is needed. This level is that of hyperstructures. A hyperstructure is large, has usually one particular function, and contains many elements. Non-equilibrium, or even dissipative, hyperstructures that, for example, assemble to transport and metabolize nutrients may comprise membrane domains of transporters plus cytoplasmic metabolons plus the genes that encode the hyperstructure's enzymes. The processes involved in the putative formation of hyperstructures include: metabolite-induced changes to protein affinities that result in metabolon formation, lipid-organizing forces that result in lateral and transverse asymmetries, post-translational modifications, equilibration of water structures that may alter distributions of other molecules, transertion, ion currents, emission of electromagnetic radiation and long range mechanical vibrations. Equilibrium hyperstructures may also exist such as topological arrays of DNA in the form of cholesteric liquid crystals. We present here the beginning of a picture of the bacterial cell in which hyperstructures form to maximize efficiency and in which the properties of hyperstructures drive the cell cycle.

Bacterial cells: The migrating kinase and the master regulator

It is becoming clear that, as in eukaryotes, proteins in bacterial cells are targeted to specific cellular locations. The most recently discovered example is a remarkable histidine kinase that oscillates between polar and global distributions while temporally regulating transcription and DNA replication in Caulobacter.

Establishment of unipolar localization of IcsA in Shigella flexneri 2a is not dependent on virulence plasmid determinants

Unipolar localization of IcsA on the surface of Shigella flexneri is required for efficient formation of actin tails and protrusions in infected eucaryotic cells. Lipopolysaccharide (LPS) mutations have been demonstrated to affect either the establishment or the maintenance of IcsA in a unipolar location, although the mechanism is unknown. In order to analyze the contribution of virulence plasmid determinants on the unipolar localization of IcsA, we examined the localization of IcsA expressed from a cloned plasmid copy in two different genetic backgrounds. The localization of IcsA was first examined in a virulence plasmid-cured derivative of the wild-type S. flexneri 2a isolate 2457T. This approach examined the contribution of virulence plasmid-borne factors, including the previously identified virulence plasmid-borne protease that is responsible for cleaving IcsA in the outer membrane and releasing the 95-kDa secreted form from the cell surface. IcsA localization in a related but nonpathogenic Escherichia coli strain expressing LPS of the O8 serotype was also examined. IcsA surface presentation in both of these genetic backgrounds continued to be unipolar, demonstrating that virulence plasmid-borne determinants are not responsible for unipolar localization of IcsA. The unipolar localization of IcsA in the E. coli background suggests that a common pathway that allows IcsA to be spatially restricted to one pole on the bacterial cell surface exists in Shigella and E. coli.

Chemotaxis receptors: a progress report on structure and function

Recent biochemical and structural studies have provided many new insights into the structure and function of bacterial chemoreceptors. Aspects of their ligand binding, conformational changes, and interactions with other members of the signaling pathway are being defined at the structural level. It is anticipated that the combined effort will soon provide a detailed, unified view of an entire response system.

Spatial and Temporal Deposition of Adhesive Extracellular Polysaccharide Capsule and Fimbriae by Hyphomonas Strain MHS-3

Hyphomonas strain MHS-3, a member of a genus of primary colonizers of surfaces immersed in marine water, synthesizes two structures that mediate adhesion to solid substrata, namely, capsular exopolysaccharide and fimbriae. Specific stains, gold-labelled lectins, and monoclonal antibodies, along with transmission electron microscopy of synchronized populations, revealed that both structures are polarly and temporally expressed. The timed synthesis and placement of the fimbriae and capsule correlated with the timing and locus of MHS-3 adhesion.

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.

Effect of O side-chain length and composition on the virulence of Shigella flexneri 2a

IcsA of Shigella flexneri is required for intercellular spread and is located in the outer membrane at one pole of the bacterium, where it catalyses the polymerization of host-cell actin. The formation of the a tin tail provides the force to move the bacterium in a unidirectional manner through the host-cell cytoplasm. We have previously demonstrated that rough lipopolysaccharide (LPS) mutants of S. flexneri 2a are avirulent and cannot form plaques in tissue-culture monolayers. This inability to form plaques is associated with non-polar localization of IcsA and loss of host-cell membrane-protrusion formation ("fireworks'). To define the minimal LPS structure required for fireworks formation, we constructed a strain of S. flexneri (BS497) that contains a mutation in rfc, encoding the O side-chain polymerase, and a strain, BS520, that possesses a defective O side-chain ligase due to a mutation in rfaL. BS497 produces a LPS that consists of a core with one repeat unit of the O side-chain, while BS520 produces a LPS consisting of a complete core with no O side-chain. BS497 remained invasive but did not form fireworks or plaques in tissue-culture monolayers and was negative in the Serény test. BS520 was invasive, generated reduced numbers of short fireworks, and made tiny plaques, but it was negative in the Serény test. Analysis of BS497 with anti-IcsA antibody demonstrated that IcsA was distributed over the entire cell surface. The distribution of IcsA on the surface of BS520 was predominantly unipolar, with some trail-back of IcsA label along the sides of the bacterium. A similar pattern was seen when infected monolayers were stained for polymerized actin. These results suggest that both the presence and the length of the O side-chain are important in the proper localization or maintenance of IcsA at the pole which subsequently affects the ability to form actin tails and produce fireworks. This reduced ability to form actin tails and fireworks results in a decreased ability of Shigella to move into adjacent host cells. To determine if the sugar composition of the O side-chain is important in the ability to form fireworks, the rfb region of S. flexneri 2a was replaced with the rfb region from Escherichia coli serotype O8 or O25. Both hybrids were invasive, formed plaques, and gave positive Serény reactions. These results suggest that, unlike LPS length, the sugar composition of the O side-chain is not a critical requirement for the proper localization of IcsA and efficient intercellular movement.

Cell division. Bud-site selection is only skin deep

Yeast cells that divide by budding place new buds in predetermined locations. Recent studies of the subcellular localization of the Bud3 protein help to explain how this occurs.

Hypothesis: chromosome separation in Escherichia coli involves autocatalytic gene expression, transertion and membrane-domain formation

To explain how daughter chromosomes are separated into discrete nucleoids and why chromosomes are partitioned with pole preferences, I propose that differential gene expression occurs during DNA replication in Escherichia coli. This differential gene expression means that the daughter chromosomes have different patterns of gene expression and that cell division is not a simple process of binary fission. Differential gene expression arises from autocatalytic gene expression and creates a separate proteolipid domain around each developing chromosome via the coupled transcription-translation-insertion of proteins into membranes (transertion). As these domains are immiscible, daughter chromosomes are simultaneously replicated and separated into discrete nucleoids. I also propose that the partitioning relationship between chromosome age and cell age arises because the poles of cells have a proteolipid composition that favours transertion from one nucleoid rather than from the other. This hypothesis forms part of an ensemble of related hypotheses which attempt to explain cell division, differentiation and wall growth in bacteria in terms of the physical properties and interactions of the principal constituents of cells.

Avirulence of rough mutants of Shigella flexneri: requirement of O antigen for correct unipolar localization of IcsA in the bacterial outer membrane

Mutations in the lipopolysaccharide (LPS) of Shigella spp. result in attenuation of the bacteria in both in vitro and in vivo models of virulence, although the precise block in pathogenesis is not known. We isolated defined mutations in two genes, galU and rfe, which directly affect synthesis of the LPS of S. flexneri 2a, in order to determine more precisely the step in virulence at which LPS mutants are blocked. The galU and rfe mutants invaded HeLa cells but failed to generate the membrane protrusions (fireworks) characteristic of intracellular motility displayed by wild-type shigellae. Furthermore, the galU mutant was unable to form plaques on a confluent monolayer of eucaryotic cells and the rfe mutant generated only tiny plaques. These observations indicated that the mutants were blocked in their ability to spread from cell to cell. Western immunoblot analysis of expression of IcsA, the protein essential for intracellular motility and intercellular spread, demonstrated that both mutants synthesized IcsA, although they secreted less of the protein to the extracellular medium than did the wild-type parent. More strikingly, the LPS mutants showed aberrant surface localization of IcsA. Unlike the unipolar localization of IcsA seen in the wild-type parent, the galU mutant expressed the protein in a circumferential fashion. The rfe mutant had an intermediate phenotype in that it displayed some localization of IcsA at one pole while also showing diffuse localization around the bacterium. Given the known structures of the LPS of wild-type S. flexneri 2a, the rfe mutant, and the galU mutant, we hypothesize that the core and O-antigen components of LPS are critical elements in the correct unipolar localization of IcsA. These observations indicate a more precise role for LPS in Shigella pathogenesis.

The actin-based motility of the facultative intracellular pathogen Listeria monocytogenes

The Gram-positive bacterium Listeria monocytogenes is a facultative intracellular parasite that invades and multiplies within diverse eukaryotic cell types. An essential pathogenicity determinant is its ability to move in the host cell cytoplasm and to spread within tissues by directly passing from one cell to another. The propulsive force for intracellular movement is thought to be generated by continuous actin assembly at the rear end of the bacterium. Moving bacteria that reach the plasma membrane induce the formation of long membranous protrusions that are internalized by neighbouring cells, thus mediating the spread of infection. The unrelated pathogens Shigella and Rickettsia use a similar process of actin-based motility to disseminate in infected tissues. This review focuses on the bacterial and cellular factors involved in the actin-based motility of L. monocytogenes.