Role of the nucleoid in the toporegulation of division

Glycogen phase separation drives macromolecular rearrangement and asymmetric division in E. coli

Bacteria often experience nutrient limitation in nature and the laboratory. While exponential and stationary growth phases are well characterized in the model bacterium Escherichia coli, little is known about what transpires inside individual cells during the transition between these two phases. Through quantitative cell imaging, we found that the position of nucleoids and cell division sites becomes increasingly asymmetric during transition phase. These asymmetries were coupled with spatial reorganization of proteins, ribosomes, and RNAs to nucleoid-centric localizations. Results from live-cell imaging experiments, complemented with genetic and 13C whole-cell nuclear magnetic resonance spectroscopy studies, show that preferential accumulation of the storage polymer glycogen at the old cell pole leads to the observed rearrangements and asymmetric divisions. In vitro experiments suggest that these phenotypes are likely due to the propensity of glycogen to phase separate in crowded environments, as glycogen condensates exclude fluorescent proteins under physiological crowding conditions. Glycogen-associated differences in cell sizes between strains and future daughter cells suggest that glycogen phase separation allows cells to store large glucose reserves without counting them as cytoplasmic space.

Tight coupling of cell width to nucleoid structure in Escherichia coli

Cell dimensions of rod-shaped bacteria such as Escherichia coli are connected to mass growth and chromosome replication. During their interdivision cycle (τ min), cells enlarge by elongation only, but at faster growth in richer media, they are also wider. Changes in width W upon nutritional shift-up (shortening τ) occur during the division process. The elusive signal directing the mechanism for W determination is likely related to the tightly linked duplications of the nucleoid (DNA) and the sacculus (peptidoglycan), the only two structures (macromolecules) existing in a single copy that are coupled, temporally and spatially. Six known parameters related to the nucleoid structure and replication are reasonable candidates to convey such a signal, all simple functions of the key number of replication positions n(=C/τ), the ratio between the rates of growth (τ-1) and of replication (C-1). The current analysis of available literature-recorded data discovered that, of these, nucleoid complexity NC[=(2n-1)/(n×ln2)] is by far the most likely parameter affecting cell width W. The exceedingly high correlations found between these two seemingly unrelated measures (NC and W) indicate that coupling between them is of major importance to the species' survival. As an exciting corollary, to the best of our knowledge, a new, indirect approach to estimate DNA replication rate is revealed. Potential involvement of DNA topoisomerases in W determination is also proposed and discussed.

Apparent simplicity and emergent robustness in the control of the Escherichia coli cell cycle

To examine how bacteria achieve robust cell proliferation across diverse conditions, we developed a method that quantifies 77 cell morphological, cell cycle, and growth phenotypes of a fluorescently labeled Escherichia coli strain and >800 gene deletion derivatives under multiple nutrient conditions. This approach revealed extensive phenotypic plasticity and deviating mutant phenotypes were often nutrient dependent. From this broad phenotypic landscape emerged simple and robust unifying rules (laws) that connect DNA replication initiation, nucleoid segregation, FtsZ ring formation, and cell constriction to specific aspects of cell size (volume, length, or added length) at the population level. Furthermore, completion of cell division followed the initiation of cell constriction after a constant time delay across strains and nutrient conditions, identifying cell constriction as a key control point for cell size determination. Our work provides a population-level description of the governing principles by which E. coli integrates cell cycle processes and growth rate with cell size to achieve its robust proliferative capability. A record of this paper's transparent peer review process is included in the supplemental information.

Extending Validity of the Bacterial Cell Cycle Model through Thymine Limitation: A Personal View

The contemporary view of bacterial physiology was established in 1958 at the "Copenhagen School", culminating a decade later in a detailed description of the cell cycle based on four parameters. This model has been subsequently supported by numerous studies, nicknamed BCD (The Bacterial Cell-Cycle Dogma). It readily explains, quantitatively, the coupling between chromosome replication and cell division, size and DNA content. An important derivative is the number of replication positions n, the ratio between the time C to complete a round of replication and the cell mass doubling time τ; the former is constant at any temperature and the latter is determined by the medium composition. Changes in cell width W are highly correlated to n through the equation for so-called nucleoid complexity NC (=(2ⁿ - 1)/(ln2 × n)), the amount of DNA per terC (i.e., chromosome) in genome equivalents. The narrow range of potential n can be dramatically extended using the method of thymine limitation of thymine-requiring mutants, which allows a more rigorous testing of the hypothesis that the nucleoid structure is the primary source of the signal that determines W during cell division. How this putative signal is relayed from the nucleoid to the divisome is still highly enigmatic. The aim of this Opinion article is to suggest the possibility of a new signaling function for nucleoid DNA.

The Bacterial Nucleoid: From Electron Microscopy to Polymer Physics—A Personal Recollection

In the 1960s, electron microscopy did not provide a clear answer regarding the compact or dispersed organization of the bacterial nucleoid. This was due to the necessary preparation steps of fixation and dehydration (for embedding) and freezing (for freeze-fracturing). Nevertheless, it was possible to measure the lengths of nucleoids in thin sections of slow-growing Escherichia coli cells, showing their gradual increase along with cell elongation. Later, through application of the so-called agar filtration method for electron microscopy, we were able to perform accurate measurements of cell size and shape. The introduction of confocal and fluorescence light microscopy enabled measurements of size and position of the bacterial nucleoid in living cells, inducing the concepts of “nucleoid occlusion” for localizing cell division and of “transertion” for the final step of nucleoid segregation. The question of why the DNA does not spread throughout the cytoplasm was approached by applying polymer-physical concepts of interactions between DNA and proteins. This gave a mechanistic insight in the depletion of proteins from the nucleoid, in accordance with its low refractive index observed by phase-contrast microscopy. Although in most bacterial species, the widely conserved proteins of the ParABS-system play a role in directing the segregation of newly replicated DNA strands, the basis for the separation and opposing movement of the chromosome arms was proposed to lie in preventing intermingling of nascent daughter strands already in the early replication bubble. E. coli, lacking the ParABS system, may be suitable for investigating this basic mechanism of DNA strand separation and segregation.

Repurposing Peptide Nanomaterials as Synthetic Biomolecular Condensates in Bacteria

Peptide nanomaterials exhibit diverse applications in vitro, such as drug delivery. Here, we consider the utility of de novo peptide nanomaterials to organize biochemistry within the bacterial cytoplasm. Toward this goal, we discovered that ABC coiled-coil triblock peptides form gel-like biomolecular condensates with a c_sat of 10 μM in addition to their well-known hydrogel-forming capabilities. Expression of the coiled-coil triblock peptides in bacteria leads to cell pole accumulation via a nucleoid occlusion mechanism. We then provide a proof of principle that these synthetic biomolecular condensates could sequester clients at the cell pole. Finally, we demonstrate that triblock peptides and another biomolecular condensate, RNase E, phase-separate as distinct protein-rich assemblies in vitro and in vivo. These results reveal the potential of using peptide nanomaterials to divide the bacterial cytoplasm into distinct subcellular zones with future metabolic engineering and synthetic biology applications.

The Division Defect of a Bacillus subtilis minD noc Double Mutant Can Be Suppressed by Spx-Dependent and Spx-Independent Mechanisms

During growth, bacteria increase in size and divide. Division is initiated by the formation of the Z-ring, a ring-like cytoskeletal structure formed by treadmilling protofilaments of the tubulin homolog FtsZ. FtsZ localization is thought to be controlled by the Min and Noc systems, and here we explore why cell division fails at high temperature when the Min and Noc systems are simultaneously mutated. Microfluidic analysis of a minD noc double mutant indicated that FtsZ formed proto-Z-rings at periodic interchromosome locations but that the rings failed to mature and become functional. Extragenic suppressor analysis indicated that a variety of mutations restored high temperature growth to the minD noc double mutant, and while many were likely pleiotropic, others implicated the proteolysis of the transcription factor Spx. Further analysis indicated that a Spx-dependent pathway activated the expression of ZapA, a protein that primarily compensates for the absence of Noc. In addition, an Spx-independent pathway reduced the length of the cytokinetic period, perhaps by increasing divisome activity. Finally, we provide evidence of an as-yet-unidentified protein that is activated by Spx and governs the frequency of polar division and minicell formation. IMPORTANCE Bacteria must properly position the location of the cell division machinery in order to grow, divide, and ensure each daughter cell receives one copy of the chromosome. In Bacillus subtilis, cell division site selection depends on the Min and Noc systems, and while neither is individually essential, cells fail to grow at high temperature when both are mutated. Here, we show that cell division fails in the absence of Min and Noc, due not to a defect in FtsZ localization but rather to a failure in the maturation of the cell division machinery. Suppressor mutations that restored growth were selected, and while some activated the expression of ZapA via the Spx stress response pathway, others appeared to directly enhance divisome activity.

Mechanisms for Chromosome Segregation in Bacteria

The process of DNA segregation, the redistribution of newly replicated genomic material to daughter cells, is a crucial step in the life cycle of all living systems. Here, we review DNA segregation in bacteria which evolved a variety of mechanisms for partitioning newly replicated DNA. Bacterial species such as Caulobacter crescentus and Bacillus subtilis contain pushing and pulling mechanisms that exert forces and directionality to mediate the moving of newly synthesized chromosomes to the bacterial poles. Other bacteria such as Escherichia coli lack such active segregation systems, yet exhibit a spontaneous de-mixing of chromosomes due to entropic forces as DNA is being replicated under the confinement of the cell wall. Furthermore, we present a synopsis of the main players that contribute to prokaryotic genome segregation. We finish with emphasizing the importance of bottom-up approaches for the investigation of the various factors that contribute to genome segregation.

Cohesion of Sister Chromosome Termini during the Early Stages of Sporulation in Bacillus subtilis

During sporulation of Bacillus subtilis, the cell cycle is reorganized to generate separated prespore and mother cell compartments, each containing a single fully replicated chromosome. The process begins with reorganization of the nucleoid to form an elongated structure, the axial filament, in which the two chromosome origins are attached to opposite cell poles, with the remainder of the DNA stretched between these sites. When the cell then divides asymmetrically, the division septum closes around the chromosome destined for the smaller prespore, trapping the origin-proximal third of the chromosome in the prespore. A translocation pore is assembled through which a DNA transporter, SpoIIIE/FtsK, transfers the bulk of the chromosome to complete the segregation process. Although the mechanisms involved in attaching origin regions to the cell poles are quite well understood, little is known about other aspects of axial filament morphology. We have studied the behavior of the terminus region of the chromosome during sporulation using time-lapse imaging of wild-type and mutant cells. The results suggest that the elongated structure involves cohesion of the terminus regions of the sister chromosomes and that this cohesion is resolved when the termini reach the asymmetric septum or translocation pore. Possible mechanisms and roles of cohesion and resolution are discussed.IMPORTANCE Endospore formation in Firmicutes bacteria provides one of the most highly resistant life forms on earth. During the early stages of endospore formation, the cell cycle is reorganized so that exactly two fully replicated chromosomes are generated, before the cell divides asymmetrically to generate the prespore and mother cell compartments that are critical for the developmental process. Decades ago, it was discovered that just prior to asymmetrical division the two chromosomes enter an unusual elongated configuration called the axial filament. This paper provides new insights into the nature of the axial filament structure and suggests that cohesion of the normally separated sister chromosome termini plays an important role in axial filament formation.

Geometric principles underlying the proliferation of a model cell system

Many bacteria can form wall-deficient variants, or L-forms, that divide by a simple mechanism that does not require the FtsZ-based cell division machinery. Here, we use microfluidic systems to probe the growth, chromosome cycle and division mechanism of Bacillus subtilis L-forms. We find that forcing cells into a narrow linear configuration greatly improves the efficiency of cell growth and chromosome segregation. This reinforces the view that L-form division is driven by an excess accumulation of surface area over volume. Cell geometry also plays a dominant role in controlling the relative positions and movement of segregating chromosomes. Furthermore, the presence of the nucleoid appears to influence division both via a cell volume effect and by nucleoid occlusion, even in the absence of FtsZ. Our results emphasise the importance of geometric effects for a range of crucial cell functions, and are of relevance for efforts to develop artificial or minimal cell systems.

Cell division protein FtsZ: from structure and mechanism to antibiotic target

Antimicrobial resistance to virtually all clinically applied antibiotic classes severely limits the available options to treat bacterial infections. Hence, there is an urgent need to develop and evaluate new antibiotics and targets with resistance-breaking properties. Bacterial cell division has emerged as a new antibiotic target pathway to counteract multidrug-resistant pathogens. New approaches in antibiotic discovery and bacterial cell biology helped to identify compounds that either directly interact with the major cell division protein FtsZ, thereby perturbing the function and dynamics of the cell division machinery, or affect the structural integrity of FtsZ by inducing its degradation. The impressive antimicrobial activities and resistance-breaking properties of certain compounds validate the inhibition of bacterial cell division as a promising strategy for antibiotic intervention.

Does the Nucleoid Determine Cell Dimensions in Escherichia coli?

Bacillary, Gram-negative bacteria grow by elongation with no discernible change in width, but during faster growth in richer media the cells are also wider. The mechanism regulating the change in cell width W during transitions from slow to fast growth is a fundamental, unanswered question in molecular biology. The value of W that changes in the divisome and during the division process only, is related to the nucleoid complexity, determined by the rates of growth and of chromosome replication; the former is manipulated by nutritional conditions and the latter-by thymine limitation of thyA mutants. Such spatio-temporal regulation is supported by existence of a minimal possible distance between successive replisomes, so-called eclipse that limits the number of replisomes to a maximum. Breaching this limit by slowing replication in fast growing cells results in maximal nucleoid complexity that is associated with maximum cell width, supporting the notion of Nucleoid-to-Divisome signal transmission. Physical signal(s) may be delivered from the nucleoid to assemble the divisome and to fix the value of W in the nascent cell pole.

The ParB homologs, Spo0J and Noc, together prevent premature midcell Z ring assembly when the early stages of replication are blocked in Bacillus subtilis

Precise cell division in coordination with DNA replication and segregation is of utmost importance for all organisms. The earliest stage of cell division is the assembly of a division protein FtsZ into a ring, known as the Z ring, at midcell. What still eludes us, however, is how bacteria precisely position the Z ring at midcell. Work in B. subtilis over the last two decades has identified a link between the early stages of DNA replication and cell division. A recent model proposed that the progression of the early stages of DNA replication leads to an increased ability for the Z ring to form at midcell. This model arose through studies examining Z ring position in mutants blocked at different steps of the early stages of DNA replication. Here, we show that this model is unlikely to be correct and the mutants previously studied generate nucleoids with different capacity for blocking midcell Z ring assembly. Importantly, our data suggest that two proteins of the widespread ParB family, Noc and Spo0J are required to prevent Z ring assembly over the bacterial nucleoid and help fine tune the assembly of the Z ring at midcell during the cell cycle.

Does the eclipse limit bacterial nucleoid complexity and cell width?

Cell size of bacteria M is related to 3 temporal parameters: chromosome replication time C, period from replication-termination to subsequent division D, and doubling time τ. Steady-state, bacillary cells grow exponentially by extending length L only, but their constant width W is larger at shorter τ's or longer C's, in proportion to the number of chromosome replication positions n (= C/τ), at least in Escherichia coli and Salmonella typhimurium. Extending C by thymine limitation of fast-growing thyA mutants result in continuous increase of M, associated with rising W, up to a limit before branching. A set of such puzzling observations is qualitatively consistent with the view that the actual cell mass (or volume) at the time of replication-initiation Mi (or Vi), usually relatively constant in growth at varying τ's, rises with time under thymine limitation of fast-growing, thymine-requiring E. coli strains. The hypothesis will be tested that presumes existence of a minimal distance l_min between successive moving replisomes, translated into the time needed for a replisome to reach l_min before a new replication-initiation at oriC is allowed, termed Eclipse E. Preliminary analysis of currently available data is inconsistent with a constant E under all conditions, hence other explanations and ways to test them are proposed in an attempt to elucidate these and other results. The complex hypothesis takes into account much of what is currently known about Bacterial Physiology: the relationships between cell dimensions, growth and cycle parameters, particularly nucleoid structure, replication and position, and the mode of peptidoglycan biosynthesis. Further experiments are mentioned that are necessary to test the discussed ideas and hypotheses.

Dissecting the role of conformational change and membrane binding by the bacterial cell division regulator MinE in the stimulation of MinD ATPase activity

The bacterial cell division regulators MinD and MinE together with the division inhibitor MinC localize to the membrane in concentrated zones undergoing coordinated pole-to-pole oscillation to help ensure that the cytokinetic division septum forms only at the mid-cell position. This dynamic localization is driven by MinD-catalyzed ATP hydrolysis, stimulated by interactions with MinE's anti-MinCD domain. This domain is buried in the 6-β-stranded MinE "closed" structure, but is liberated for interactions with MinD, giving rise to a 4-β-stranded "open" structure through an unknown mechanism. Here we show that MinE-membrane interactions induce a structural change into a state resembling the open conformation. However, MinE mutants lacking the MinE membrane-targeting sequence stimulated higher ATP hydrolysis rates than the full-length protein, indicating that binding to MinD is sufficient to trigger this conformational transition in MinE. In contrast, conformational change between the open and closed states did not affect stimulation of ATP hydrolysis rates in the absence of membrane binding, although the MinD-binding residue Ile-25 is critical for this conformational transition. We therefore propose an updated model where MinE is brought to the membrane through interactions with MinD. After stimulation of ATP hydrolysis, MinE remains bound to the membrane in a state that does not catalyze additional rounds of ATP hydrolysis. Although the molecular basis for this inhibited state is unknown, previous observations of higher-order MinE self-association may explain this inhibition. Overall, our findings have general implications for Min protein oscillation cycles, including those that regulate cell division in bacterial pathogens.

Threshold effect of growth rate on population variability of Escherichia coli cell lengths

A long-standing question in biology is the effect of growth on cell size. Here, we estimate the effect of Escherichia coli growth rate (r) on population cell size distributions by estimating the coefficient of variation of cell lengths (CV_L) from image analysis of fixed cells in DIC microscopy. We find that the CV_L is constant at growth rates less than one division per hour, whereas above this threshold, CV_L increases with an increase in the growth rate. We hypothesize that stochastic inhibition of cell division owing to replication stalling by a RecA-dependent mechanism, combined with the growth rate threshold of multi-fork replication (according to Cooper and Helmstetter), could form the basis of such a threshold effect. We proceed to test our hypothesis by increasing the frequency of stochastic stalling of replication forks with hydroxyurea (HU) treatment and find that cell length variability increases only when the growth rate exceeds this threshold. The population effect is also reproduced in single-cell studies using agar-pad cultures and 'mother machine'-based experiments to achieve synchrony. To test the role of RecA, critical for the repair of stalled replication forks, we examine the CV_L of E. coli ΔrecA cells. We find cell length variability in the mutant to be greater than wild-type, a phenotype that is rescued by plasmid-based RecA expression. Additionally, we find that RecA-GFP protein recruitment to nucleoids is more frequent at growth rates exceeding the growth rate threshold and is further enhanced on HU treatment. Thus, we find growth rates greater than a threshold result in increased E. coli cell lengths in the population, and this effect is, at least in part, mediated by RecA recruitment to the nucleoid and stochastic inhibition of division.

Threshold effect of growth rate on population variability of Escherichia coli cell lengths

A long-standing question in biology is the effect of growth on cell size. Here, we estimate the effect of Escherichia coli growth rate (r) on population cell size distributions by estimating the coefficient of variation of cell lengths (CV_L) from image analysis of fixed cells in DIC microscopy. We find that the CV_L is constant at growth rates less than one division per hour, whereas above this threshold, CV_L increases with an increase in the growth rate. We hypothesize that stochastic inhibition of cell division owing to replication stalling by a RecA-dependent mechanism, combined with the growth rate threshold of multi-fork replication (according to Cooper and Helmstetter), could form the basis of such a threshold effect. We proceed to test our hypothesis by increasing the frequency of stochastic stalling of replication forks with hydroxyurea (HU) treatment and find that cell length variability increases only when the growth rate exceeds this threshold. The population effect is also reproduced in single-cell studies using agar-pad cultures and 'mother machine'-based experiments to achieve synchrony. To test the role of RecA, critical for the repair of stalled replication forks, we examine the CV_L of E. coli ΔrecA cells. We find cell length variability in the mutant to be greater than wild-type, a phenotype that is rescued by plasmid-based RecA expression. Additionally, we find that RecA-GFP protein recruitment to nucleoids is more frequent at growth rates exceeding the growth rate threshold and is further enhanced on HU treatment. Thus, we find growth rates greater than a threshold result in increased E. coli cell lengths in the population, and this effect is, at least in part, mediated by RecA recruitment to the nucleoid and stochastic inhibition of division.

E. coli Cell Cycle Machinery

Cytokinesis in E. coli is organized by a cytoskeletal element designated the Z ring. The Z ring is formed at midcell by the coalescence of FtsZ filaments tethered to the membrane by interaction of FtsZ's conserved C-terminal peptide (CCTP) with two membrane-associated proteins, FtsA and ZipA. Although interaction between an FtsZ monomer and either of these proteins is of low affinity, high affinity is achieved through avidity - polymerization linked CCTPs interacting with the membrane tethers. The placement of the Z ring at midcell is ensured by antagonists of FtsZ polymerization that are positioned within the cell and target FtsZ filaments through the CCTP. The placement of the ring is reinforced by a protein network that extends from the terminus (Ter) region of the chromosome to the Z ring. Once the Z ring is established, additional proteins are recruited through interaction with FtsA, to form the divisome. The assembled divisome is then activated by FtsN to carry out septal peptidoglycan synthesis, with a dynamic Z ring serving as a guide for septum formation. As the septum forms, the cell wall is split by spatially regulated hydrolases and the outer membrane invaginates in step with the aid of a transenvelope complex to yield progeny cells.

Bacterial Nucleoid Occlusion: Multiple Mechanisms for Preventing Chromosome Bisection During Cell Division

In most bacteria cell division is driven by the prokaryotic tubulin homolog, FtsZ, which forms the cytokinetic Z ring. Cell survival demands both the spatial and temporal accuracy of this process to ensure that equal progeny are produced with intact genomes. While mechanisms preventing septum formation at the cell poles have been known for decades, the means by which the bacterial nucleoid is spared from bisection during cell division, called nucleoid exclusion (NO), have only recently been deduced. The NO theory was originally posited decades ago based on the key observation that the cell division machinery appeared to be inhibited from forming near the bacterial nucleoid. However, what might drive the NO process was unclear. Within the last 10 years specific proteins have been identified as important mediators of NO. Arguably the best studied NO mechanisms are those employed by the Escherichia coli SlmA and Bacillus subtilis Noc proteins. Both proteins bind specific DNA sequences within selected chromosomal regions to act as timing devices. However, Noc and SlmA contain completely different structural folds and utilize distinct NO mechanisms. Recent studies have identified additional processes and factors that participate in preventing nucleoid septation during cell division. These combined data show multiple levels of redundancy as well as a striking diversity of mechanisms have evolved to protect cells against catastrophic bisection of the nucleoid. Here we discuss these recent findings with particular emphasis on what is known about the molecular underpinnings of specific NO machinery and processes.

Bacterial cell biology outside the streetlight

As much as vertical transmission of microbial symbionts requires their deep integration into the host reproductive and developmental biology, symbiotic lifestyle might profoundly affect bacterial growth and proliferation. This review describes the reproductive oddities displayed by bacteria associated - more or less intimately - with multicellular eukaryotes.

Cell-shape homeostasis in Escherichia coli is driven by growth, division, and nucleoid complexity

Analysis of recently published high-throughput measurements of wild-type Escherichia coli cells growing at a wide range of rates demonstrates that cell width W, which is constant at any particular growth rate, is related (with a CV = 2.4%) to the level of nucleoid complexity, expressed as the amount of DNA in genome equivalents that is associated with chromosome terminus (G/terC). The relatively constant (CV = 7.3%) aspect ratio of newborn cells (Lb/W) in populations growing at different rates indicates existence of cell-shape homeostasis. Enlarged W of thymine-limited thyA mutants growing at identical rates support the hypothesis that nucleoid complexity actively affects W. Nucleoid dynamics is proposed to transmit a primary signal to the peptidoglycan-synthesizing system through the transertion mechanism, i.e., coupled transcription/translation of genes encoding membrane proteins and inserting these proteins into the membrane.

Structures of the nucleoid occlusion protein SlmA bound to DNA and the C-terminal domain of the cytoskeletal protein FtsZ

Cell division in most prokaryotes is mediated by FtsZ, which polymerizes to create the cytokinetic Z ring. Multiple FtsZ-binding proteins regulate FtsZ polymerization to ensure the proper spatiotemporal formation of the Z ring at the division site. The DNA-binding protein SlmA binds to FtsZ and prevents Z-ring formation through the nucleoid in a process called "nucleoid occlusion" (NO). As do most FtsZ-accessory proteins, SlmA interacts with the conserved C-terminal domain (CTD) that is connected to the FtsZ core by a long, flexible linker. However, SlmA is distinct from other regulatory factors in that it must be DNA-bound to interact with the FtsZ CTD. Few structures of FtsZ regulator-CTD complexes are available, but all reveal the CTD bound as a helix. To deduce the molecular basis for the unique SlmA-DNA-FtsZ CTD regulatory interaction and provide insight into FtsZ-regulator protein complex formation, we determined structures of Escherichia coli, Vibrio cholera, and Klebsiella pneumonia SlmA-DNA-FtsZ CTD ternary complexes. Strikingly, the FtsZ CTD does not interact with SlmA as a helix but binds as an extended conformation in a narrow, surface-exposed pocket formed only in the DNA-bound state of SlmA and located at the junction between the DNA-binding and C-terminal dimer domains. Binding studies are consistent with the structure and underscore key interactions in complex formation. Combined, these data reveal the molecular basis for the SlmA-DNA-FtsZ interaction with implications for SlmA's NO function and underscore the ability of the FtsZ CTD to adopt a wide range of conformations, explaining its ability to bind diverse regulatory proteins.

Cell cycle regulation by the bacterial nucleoid

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Nucleoid occlusion prevents cell division over the bacterial chromosome.

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Nucleoid occlusion factors identified in B. subtilis, E. coli and S. aureus.

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Noc and SlmA are sequence specific DNA-binding proteins.

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They both act as spatial and temporal regulators of cell division.

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Using some basic general principles bacteria employ diverse regulatory mechanisms.

Division site selection presents a fundamental challenge to all organisms. Bacterial cells are small and the chromosome (nucleoid) often fills most of the cell volume. Thus, in order to maximise fitness and avoid damaging the genetic material, cell division must be tightly co-ordinated with chromosome replication and segregation. To achieve this, bacteria employ a number of different mechanisms to regulate division site selection. One such mechanism, termed nucleoid occlusion, allows the nucleoid to protect itself by acting as a template for nucleoid occlusion factors, which prevent Z-ring assembly over the DNA. These factors are sequence-specific DNA-binding proteins that exploit the precise organisation of the nucleoid, allowing them to act as both spatial and temporal regulators of bacterial cell division. The identification of proteins responsible for this process has provided a molecular understanding of nucleoid occlusion but it has also prompted the realisation that substantial levels of redundancy exist between the diverse systems that bacteria employ to ensure that division occurs in the right place, at the right time.

Chromosome replication, cell growth, division and shape: a personal perspective

The origins of Molecular Biology and Bacterial Physiology are reviewed, from our personal standpoints, emphasizing the coupling between bacterial growth, chromosome replication and cell division, dimensions and shape. Current knowledge is discussed with historical perspective, summarizing past and present achievements and enlightening ideas for future studies. An interactive simulation program of the bacterial cell division cycle (BCD), described as "The Central Dogma in Bacteriology," is briefly represented. The coupled process of transcription/translation of genes encoding membrane proteins and insertion into the membrane (so-called transertion) is invoked as the functional relationship between the only two unique macromolecules in the cell, DNA and peptidoglycan embodying the nucleoid and the sacculus respectively. We envision that the total amount of DNA associated with the replication terminus, so called "nucleoid complexity," is directly related to cell size and shape through the transertion process. Accordingly, the primary signal for cell division transmitted by DNA dynamics (replication, transcription and segregation) to the peptidoglycan biosynthetic machinery is of a physico-chemical nature, e.g., stress in the plasma membrane, relieving nucleoid occlusion in the cell's center hence enabling the divisome to assemble and function between segregated daughter nucleoids.

Bacterial growth and form under mechanical compression

A combination of physical and chemical processes is involved in determining the bacterial cell shape. In standard medium, Escherichia coli cells are rod-shaped, and maintain a constant diameter during exponential growth. Here, we demonstrate that by applying compressive forces to growing E. coli, cells no longer retain their rod-like shapes but grow and divide with a flat pancake-like geometry. The deformation is reversible: deformed cells can recover back to rod-like shapes in several generations after compressive forces are removed. During compression, the cell elongation rate, proliferation rate, DNA replication rate, and protein synthesis are not significantly altered from those of the normal rod-shaped cells. Quantifying the rate of cell wall growth under compression reveals that the cell wall growth rate depends on the local cell curvature. MreB not only influences the rate of cell wall growth, but also influences how the growth rate scales with cell geometry. The result is consistent with predictions of a mechanochemical model, and suggests an active mechanical role for MreB during cell wall growth. The developed compressive device is also useful for studying a variety of cells in unique geometries.

Spatial coordination between chromosomes and cell division proteins in Escherichia coli

To successfully propagate, cells need to coordinate chromosomal replication and segregation with cell division to prevent formation of DNA-less cells and cells with damaged DNA. Here, we review molecular systems in Escherichia coli that are known to be involved in positioning the divisome and chromosome relative to each other. Interestingly, this well-studied micro-organism has several partially redundant mechanisms to achieve this task; none of which are essential. Some of these systems determine the localization of the divisome relative to chromosomes such as SlmA-dependent nucleoid occlusion, some localize the chromosome relative to the divisome such as DNA translocation by FtsK, and some are likely to act on both systems such as the Min system and newly described Ter linkage. Moreover, there is evidence that E. coli harbors other divisome-chromosome coordination systems in addition to those known. The review also discusses the minimal requirements of coordination between chromosomes and cell division proteins needed for cell viability. Arguments are presented that cells can propagate without any dedicated coordination between their chromosomes and cell division machinery at the expense of lowered fitness.

Guiding divisome assembly and controlling its activity

Cell division in bacteria requires the construction of two new polar caps for the daughter cells. To constrict the cell membrane and build these new surface layers, bacteria employ a multiprotein machine called the divisome. Over the years, most of the essential division proteins have been identified and localized to the ring-like divisome apparatus. The challenge now is to determine the molecular function of these factors, how they cooperate to bring about the dramatic transformation of the mother cell envelope, and what coordinates their activity with other major cell cycle events. In this review, we discuss recent progress in these areas with an emphasis on results from the model organisms Escherichia coli and Bacillus subtilis.

Nucleoid occlusion protein Noc recruits DNA to the bacterial cell membrane

To proliferate efficiently, cells must co-ordinate division with chromosome segregation. In Bacillus subtilis, the nucleoid occlusion protein Noc binds to specific DNA sequences (NBSs) scattered around the chromosome and helps to protect genomic integrity by coupling the initiation of division to the progression of chromosome replication and segregation. However, how it inhibits division has remained unclear. Here, we demonstrate that Noc associates with the cell membrane via an N-terminal amphipathic helix, which is necessary for function. Importantly, the membrane-binding affinity of this helix is weak and requires the assembly of nucleoprotein complexes, thus establishing a mechanism for DNA-dependent activation of Noc. Furthermore, division inhibition by Noc requires recruitment of NBS DNA to the cell membrane and is dependent on its ability to bind DNA and membrane simultaneously. Indeed, Noc production in a heterologous system is sufficient for recruitment of chromosomal DNA to the membrane. Our results suggest a simple model in which the formation of large membrane-associated nucleoprotein complexes physically occludes assembly of the division machinery.

Effect of the Min system on timing of cell division in Escherichia coli

In Escherichia coli the Min protein system plays an important role in positioning the division site. We show that this system also has an effect on timing of cell division. We do this in a quantitative way by measuring the cell division waiting time (defined as time difference between appearance of a division site and the division event) and the Z-ring existence time. Both quantities are found to be different in WT and cells without functional Min system. We develop a series of theoretical models whose predictions are compared with the experimental findings. Continuous improvement leads to a final model that is able to explain all relevant experimental observations. In particular, it shows that the chromosome segregation defect caused by the absence of Min proteins has an important influence on timing of cell division. Our results indicate that the Min system affects the septum formation rate. In the absence of the Min proteins this rate is reduced, leading to the observed strongly randomized cell division events and the longer division waiting times.

Highly Deviated Asymmetric Division in Very Low Proportion of Mycobacterial Mid-log Phase Cells

In this study, we show that about 20% of the septating Mycobacterium smegmatis and Mycobacterium xenopi cells in the exponential phase populationdivideasymmetrically, with an unusually high deviation (17 ± 4%) in the division site from the median, to generate short cells and long cells, thereby generating population heterogeneity. This mode of division is very different from the symmetric division of themajority (about 80%) of the septating cells in the Mycobacterium smegmatis, Mycobacterium marinum, and Mycobacterium bovis BCG exponential phase population, with 5-10% deviation in the division site from the mid-cell site, as reported by recent studies. The short cells and the long cells further grew and divided to generate a population. We speculate that the generation of the short cells and the long cells through the highly deviated asymmetric divisionin the low proportions of mycobacterial population may have a role in stress tolerance.

Robustness and accuracy of cell division in Escherichia coli in diverse cell shapes

Cell division in typical rod-shaped bacteria such as Escherichia coli shows a remarkable plasticity in being able to adapt to a variety of irregular cell shapes. Here, we investigate the roles of the Min system and the nucleoid-occlusion factor SlmA in supporting this adaptation. We study "squeezed" E. coli in narrow nanofabricated channels where these bacteria exhibit highly irregular shapes and large volumes. Despite the severely anomalous morphologies we find that most of these bacteria maintain their ability to divide into two equally sized daughters with an accuracy comparable to that of normal rod-shaped cells (about 4%). Deletion of either slmA or minC shows that the molecular systems associated with these genes are largely dispensable for accurate cell division in these irregular cell shapes. Using fluorescence time-lapse microscopy, we determine that the functionality of the Min system is affected by the cell shape, whereas the localization of a nucleoid relative to the cell division proteins (the divisome) remains unperturbed in a broad spectrum of morphologies, consistent with nucleoid occlusion. The observed positioning of the nucleoid relative to the divisome appears not to be affected by the nucleoid-occlusion factor SlmA. The current study underscores the importance of nucleoid occlusion in positioning the divisome and shows that it is robust against shape irregularities.

The Min system and nucleoid occlusion are not required for identifying the division site in Bacillus subtilis but ensure its efficient utilization

Precise temporal and spatial control of cell division is essential for progeny survival. The current general view is that precise positioning of the division site at midcell in rod-shaped bacteria is a result of the combined action of the Min system and nucleoid (chromosome) occlusion. Both systems prevent assembly of the cytokinetic Z ring at inappropriate places in the cell, restricting Z rings to the correct site at midcell. Here we show that in the bacterium Bacillus subtilis Z rings are positioned precisely at midcell in the complete absence of both these systems, revealing the existence of a mechanism independent of Min and nucleoid occlusion that identifies midcell in this organism. We further show that Z ring assembly at midcell is delayed in the absence of Min and Noc proteins, while at the same time FtsZ accumulates at other potential division sites. This suggests that a major role for Min and Noc is to ensure efficient utilization of the midcell division site by preventing Z ring assembly at potential division sites, including the cell poles. Our data lead us to propose a model in which spatial regulation of division in B. subtilis involves identification of the division site at midcell that requires Min and nucleoid occlusion to ensure efficient Z ring assembly there and only there, at the right time in the cell cycle.

How organisms regulate biological processes so that they occur at the correct place within the cell is a fundamental question in research. Spatial regulation of cell division is vital to ensure equal partitioning of DNA into newborn cells. Correct positioning of the division site at the cell centre in rod-shaped bacteria is generally believed to occur via the combined action of two factors: (i) nucleoid (chromosome) occlusion and (ii) a set of proteins known collectively as the Min system. The earliest stage in bacterial cell division is the assembly of a ring, called the Z ring, at the division site. Nucleoid occlusion and Min work by preventing Z ring assembly at all sites along the cell other than the cell centre. Here we make the surprising discovery that, in the absence of both these factors, Z rings are positioned correctly at the division site, but there is a delay in this process and it is less efficient. We propose that a separate mechanism identifies the division site at midcell in rod-shaped bacteria, and nucleoid occlusion and Min ensure that the Z ring forms there and only there, at the right time and every time.

Polar flagellar biosynthesis and a regulator of flagellar number influence spatial parameters of cell division in Campylobacter jejuni

Spatial and numerical regulation of flagellar biosynthesis results in different flagellation patterns specific for each bacterial species. Campylobacter jejuni produces amphitrichous (bipolar) flagella to result in a single flagellum at both poles. These flagella confer swimming motility and a distinctive darting motility necessary for infection of humans to cause diarrheal disease and animals to promote commensalism. In addition to flagellation, symmetrical cell division is spatially regulated so that the divisome forms near the cellular midpoint. We have identified an unprecedented system for spatially regulating cell division in C. jejuni composed by FlhG, a regulator of flagellar number in polar flagellates, and components of amphitrichous flagella. Similar to its role in other polarly-flagellated bacteria, we found that FlhG regulates flagellar biosynthesis to limit poles of C. jejuni to one flagellum. Furthermore, we discovered that FlhG negatively influences the ability of FtsZ to initiate cell division. Through analysis of specific flagellar mutants, we discovered that components of the motor and switch complex of amphitrichous flagella are required with FlhG to specifically inhibit division at poles. Without FlhG or specific motor and switch complex proteins, cell division occurs more often at polar regions to form minicells. Our findings suggest a new understanding for the biological requirement of the amphitrichous flagellation pattern in bacteria that extend beyond motility, virulence, and colonization. We propose that amphitrichous bacteria such as Campylobacter species advantageously exploit placement of flagella at both poles to spatially regulate an FlhG-dependent mechanism to inhibit polar cell division, thereby encouraging symmetrical cell division to generate the greatest number of viable offspring. Furthermore, we found that other polarly-flagellated bacteria produce FlhG proteins that influence cell division, suggesting that FlhG and polar flagella may function together in a broad range of bacteria to spatially regulate division.

Campylobacter jejuni is a leading cause of gastroenteritis in humans and requires amphitrichous (bipolar) flagella to promote infection of hosts. This pattern of flagellation results in a single flagellum at both poles, which is characteristic of many Campylobacter species, but fairly unusual amongst other motile bacteria. In this work, we discovered an unprecedented system to spatially regulate cell division that relies on the FlhG ATPase and amphitrichous flagellar biosynthesis. In addition to its role in other polar flagellates in controlling flagellar number, we discovered that FlhG influences spatial regulation of cell division in C. jejuni. Further analysis revealed that components of the flagellar motor and switch are required with FlhG to inhibit cell division specifically at the poles of the bacterium. These findings indicate that flagella have an additional function in C. jejuni beyond promoting motility, virulence, and colonization in functioning in a mechanism with FlhG to inhibit cell division specifically at poles. Furthermore, our findings suggest that the specific amphitrichous pattern of flagellar biosynthesis in this pathogen is an essential determinant for inhibiting cell division at both bacterial poles so that symmetrical cell division occurs and generates viable progenitors.

Molecular mechanism by which the nucleoid occlusion factor, SlmA, keeps cytokinesis in check

Nucleoid occlusion (NO) restricts bacterial cell division to prevent chromosome guillotining in the cell midzone when replication or segregation is delayed. Structural work suggests that the NO factor SlmA (synthetic lethal with a defective Min system) interferes with formation of the cytokinetic Z-ring by altering associations between FtsZ protofilaments.

In Escherichia coli, cytokinesis is orchestrated by FtsZ, which forms a Z-ring to drive septation. Spatial and temporal control of Z-ring formation is achieved by the Min and nucleoid occlusion (NO) systems. Unlike the well-studied Min system, less is known about the anti-DNA guillotining NO process. Here, we describe studies addressing the molecular mechanism of SlmA (synthetic lethal with a defective Min system)-mediated NO. SlmA contains a TetR-like DNA-binding fold, and chromatin immunoprecipitation analyses show that SlmA-binding sites are dispersed on the chromosome except the Ter region, which segregates immediately before septation. SlmA binds DNA and FtsZ simultaneously, and the SlmA–FtsZ structure reveals that two FtsZ molecules sandwich a SlmA dimer. In this complex, FtsZ can still bind GTP and form protofilaments, but the separated protofilaments are forced into an anti-parallel arrangement. This suggests that SlmA may alter FtsZ polymer assembly. Indeed, electron microscopy data, showing that SlmA–DNA disrupts the formation of normal FtsZ polymers and induces distinct spiral structures, supports this. Thus, the combined data reveal how SlmA derails Z-ring formation at the correct place and time to effect NO.

Synchronization of chromosome dynamics and cell division in bacteria

Bacterial cells have evolved a variety of regulatory circuits that tightly synchronize their chromosome replication and cell division cycles, thereby ensuring faithful transmission of genetic information to their offspring. Complex multicomponent signaling cascades are used to monitor the progress of cytokinesis and couple replication initiation to the separation of the two daughter cells. Moreover, the cell-division apparatus actively participates in chromosome partitioning and, particularly, in the resolution of topological problems that impede the segregation process, thus coordinating chromosome dynamics with cell constriction. Finally, bacteria have developed mechanisms that harness the cell-cycle-dependent positioning of individual chromosomal loci or the nucleoid to define the cell-division site and control the timing of divisome assembly. Each of these systems manages to integrate a complex set of spatial and temporal cues to regulate and execute critical steps in the bacterial cell cycle.

Reaction-diffusion systems in intracellular molecular transport and control

Chemical reactions make cells work only if the participating chemicals are delivered to desired locations in a timely and precise fashion. Most research to date has focused on active-transport mechanisms, although passive diffusion is often equally rapid and energetically less costly. Capitalizing on these advantages, cells have developed sophisticated reaction-diffusion (RD) systems that control a wide range of cellular functions-from chemotaxis and cell division, through signaling cascades and oscillations, to cell motility. These apparently diverse systems share many common features and are "wired" according to "generic" motifs such as nonlinear kinetics, autocatalysis, and feedback loops. Understanding the operation of these complex (bio)chemical systems requires the analysis of pertinent transport-kinetic equations or, at least on a qualitative level, of the characteristic times of the constituent subprocesses. Therefore, in reviewing the manifestations of cellular RD, we also describe basic theory of reaction-diffusion phenomena.

A new multicompartmental reaction-diffusion modeling method links transient membrane attachment of E. coli MinE to E-ring formation

Many important cellular processes are regulated by reaction-diffusion (RD) of molecules that takes place both in the cytoplasm and on the membrane. To model and analyze such multicompartmental processes, we developed a lattice-based Monte Carlo method, Spatiocyte that supports RD in volume and surface compartments at single molecule resolution. Stochasticity in RD and the excluded volume effect brought by intracellular molecular crowding, both of which can significantly affect RD and thus, cellular processes, are also supported. We verified the method by comparing simulation results of diffusion, irreversible and reversible reactions with the predicted analytical and best available numerical solutions. Moreover, to directly compare the localization patterns of molecules in fluorescence microscopy images with simulation, we devised a visualization method that mimics the microphotography process by showing the trajectory of simulated molecules averaged according to the camera exposure time. In the rod-shaped bacterium Escherichia coli, the division site is suppressed at the cell poles by periodic pole-to-pole oscillations of the Min proteins (MinC, MinD and MinE) arising from carefully orchestrated RD in both cytoplasm and membrane compartments. Using Spatiocyte we could model and reproduce the in vivo MinDE localization dynamics by accounting for the previously reported properties of MinE. Our results suggest that the MinE ring, which is essential in preventing polar septation, is largely composed of MinE that is transiently attached to the membrane independently after recruited by MinD. Overall, Spatiocyte allows simulation and visualization of complex spatial and reaction-diffusion mediated cellular processes in volumes and surfaces. As we showed, it can potentially provide mechanistic insights otherwise difficult to obtain experimentally.

Expression of Escherichia coli Min system in Bacillus subtilis and its effect on cell division

In both rod-shaped Bacillus subtilis and Escherichia coli cells, Min proteins are involved in the regulation of division septa formation. In E. coli, dynamic oscillation of MinCD inhibitory complex and MinE, a topological specificity protein, prevents improper polar septation. However, in B. subtilis no MinE is present and no oscillation of Min proteins can be observed. The function of MinE is substituted by that of an unrelated DivIVA protein, which targets MinCD to division sites and retains them at the cell poles. We inspected cell division when the E. coli Min system was introduced into B. subtilis cells. Expression of these heterologous Min proteins resulted in cell elongation. We demonstrate here that E. coli MinD can partially substitute for the function of its B. subtilis protein counterpart. Moreover, E. coli MinD was observed to have similar helical localization as B. subtilis MinD.

Noc protein binds to specific DNA sequences to coordinate cell division with chromosome segregation

Coordination of chromosome segregation and cytokinesis is crucial for efficient cell proliferation. In Bacillus subtilis, the nucleoid occlusion protein Noc protects the chromosomes by associating with the chromosome and preventing cell division in its vicinity. Using protein localization, ChAP-on-Chip and bioinformatics, we have identified a consensus Noc-binding DNA sequence (NBS), and have shown that Noc is targeted to about 70 discrete regions scattered around the chromosome, though absent from a large region around the replication terminus. Purified Noc bound specifically to an NBS in vitro. NBSs inserted near the replication terminus bound Noc-YFP and caused a delay in cell division. An autonomous plasmid carrying an NBS array recruited Noc-YFP and conferred a severe Noc-dependent inhibition of cell division. This shows that Noc is a potent inhibitor of division, but that its activity is strictly localized by the interaction with NBS sites in vivo. We propose that Noc serves not only as a spatial regulator of cell division to protect the nucleoid, but also as a timing device with an important role in the coordination of chromosome segregation and cell division.

SepF increases the assembly and bundling of FtsZ polymers and stabilizes FtsZ protofilaments by binding along its length

SepF (Septum Forming) protein has been recently identified through genetic studies, and it has been suggested to be involved in the division of Bacillus subtilis cells. We have purified functional B. subtilis SepF from the inclusion bodies overexpressed in Escherichia coli. Far-UV circular dichroism and fluorescence spectroscopic analysis involving the extrinsic fluorescent probe 1-anilinonaphthalene-8-sulfonic acid suggested that the purified SepF had characteristics of folded proteins. SepF was found to promote the assembly and bundling of FtsZ protofilaments using three complimentary techniques, namely 90 degrees light scattering, sedimentation, and transmission electron microscopy. SepF also decreased the critical concentration of FtsZ assembly, prevented the dilution-induced disassembly of FtsZ protofilaments, and suppressed the GTPase activity of FtsZ. Further, thick bundles of FtsZ protofilaments were observed using fluorescein isothiocyanate-labeled SepF (FITC-SepF). Interestingly, FITC-SepF was found to be uniformly distributed along the length of the FtsZ protofilaments, suggesting that SepF copolymerizes with FtsZ. SepF formed a stable complex with FtsZ, as evident from the gel filtration analysis. Using a C-terminal tail truncated FtsZ (FtsZDelta16) and a C-terminal synthetic peptide of B. subtilis FtsZ (366-382); we provided evidence indicating that SepF binds primarily to the C-terminal tail of FtsZ. The present work in concert with the available in vivo data support a model in which SepF plays an important role in regulating the assembly dynamics of the divisome complex; therefore, it may have an important role in bacterial cell division.

Shape of nonseptated Escherichia coli is asymmetric

The shape of Escherichia coli is approximately that of a cylinder with hemispherical caps. Since its size is not much larger than optical resolution, it has been difficult to quantify deviations from this approximation. We show that one can bypass this limitation and obtain the cell shape with subpixel accuracy. The resulting contours are shown to deviate from the hemisphere-cylinder-hemisphere shape. In particular, the cell is weakly asymmetric. Its two caps are different from each other and the sides are slightly curved. Most cells have convex sides. We discuss our results in light of several mechanisms that are involved in determining the shape of cells.

Lipid spirals in Bacillus subtilis and their role in cell division

The fluid mosaic model of membrane structure has been revised in recent years as it has become evident that domains of different lipid composition are present in eukaryotic and prokaryotic cells. Using membrane binding fluorescent dyes, we demonstrate the presence of lipid spirals extending along the long axis of cells of the rod-shaped bacterium Bacillus subtilis. These spiral structures are absent from cells in which the synthesis of phosphatidylglycerol is disrupted, suggesting an enrichment in anionic phospholipids. Green fluorescent protein fusions of the cell division protein MinD also form spiral structures and these were shown by fluorescence resonance energy transfer to be coincident with the lipid spirals. These data indicate a higher level of membrane lipid organization than previously observed and a primary role for lipid spirals in determining the site of cell division in bacterial cells.

Skin and bones: the bacterial cytoskeleton, cell wall, and cell morphogenesis

The bacterial world is full of varying cell shapes and sizes, and individual species perpetuate a defined morphology generation after generation. We review recent findings and ideas about how bacteria use the cytoskeleton and other strategies to regulate cell growth in time and space to produce different shapes and sizes.

Antigen 84, an effector of pleiomorphism in Mycobacterium smegmatis

While in most rod-shaped bacteria, morphology is based on MreB-like proteins that form an actin-like cytoskeletal scaffold for cell wall biosynthesis, the factors that determine the more flexible rod-like shape in actinobacteria such as Mycobacterium species are unknown. Here we show that a Mycobacterium smegmatis protein homologous to eubacterial DivIVA-like proteins, including M. tuberculosis antigen 84 (Ag84), localized symmetrically to centers of peptidoglycan biosynthesis at the poles and septa. Controlled gene disruption experiments indicated that the gene encoding Ag84, wag31, was essential; when overexpressed, cells became longer and wider, with Ag84 asymmetrically distributed at one pole. Many became grossly enlarged, bowling-pin-shaped cells having up to 80-fold-increased volume. In these cells, Ag84 accumulated predominantly at a bulbous pole that was apparently generated by uncontrolled cell wall expansion. In some cells, Ag84 was associated with exceptional sites of cell wall expansion (buds) that evolved into branches. M. bovis BCG Ag84 was able to form oligomers in vitro, perhaps reflecting its superstructure in vivo. These data suggested a role for Ag84 in cell division and modulating cell shape in pleiomorphic actinobacteria.

Modeling partitioning of Min proteins between daughter cells after septation in Escherichia coli

Ongoing sub-cellular oscillation of Min proteins is required to block minicelling in Escherichia coli. Experimentally, Min oscillations are seen in newly divided cells and no minicells are produced. In model Min systems many daughter cells do not oscillate following septation because of unequal partitioning of Min proteins between the daughter cells. Using the 3D model of Huang et al, we investigate the septation process in detail to determine the cause of the asymmetric partitioning of Min proteins between daughter cells. We find that this partitioning problem arises at certain phases of the MinD and MinE oscillations with respect to septal closure and it persists independently of parameter variation. At most 85% of the daughter cells exhibit Min oscillation following septation. Enhanced MinD binding at the static polar and dynamic septal regions, consistent with cardiolipin domains, does not substantially increase this fraction of oscillating daughters. We believe that this problem will be shared among all existing Min models and discuss possible biological mechanisms that may minimize partitioning errors of Min proteins following septation.

Division site recognition inEscherichia coliandBacillus subtilis

The process of cell division has been intensively studied at the molecular level for decades but some basic questions remain unanswered. The mechanisms of cell division are probably best characterized in the rod-shaped bacteria Escherichia coli and Bacillus subtilis. Many of the key players are known, but detailed descriptions of the molecular mechanisms which determine where, how and when cells form the division septum are lacking. Different models have been proposed to account for the high precision with which the septum is constructed at the midcell and these models have been evaluated and refined against new data emerging from the fast improving methodologies of cell biology. This review summarizes important advances in our understanding of how the cell positions the division septum, whether it be vegetative or asymmetric. It also describes how the asymmetric septum forms and how this septation event is linked to chromosome segregation and subsequent asymmetric gene expression during spore formation in B. subtilis.