Assembly of the FtsZ ring at the central division site in the absence of the chromosome

An integrative view of cell cycle control in Escherichia coli

Bacterial proliferation depends on the cells' capability to proceed through consecutive rounds of the cell cycle. The cell cycle consists of a series of events during which cells grow, copy their genome, partition the duplicated DNA into different cell halves and, ultimately, divide to produce two newly formed daughter cells. Cell cycle control is of the utmost importance to maintain the correct order of events and safeguard the integrity of the cell and its genomic information. This review covers insights into the regulation of individual key cell cycle events in Escherichia coli. The control of initiation of DNA replication, chromosome segregation and cell division is discussed. Furthermore, we highlight connections between these processes. Although detailed mechanistic insight into these connections is largely still emerging, it is clear that the different processes of the bacterial cell cycle are coordinated to one another. This careful coordination of events ensures that every daughter cell ends up with one complete and intact copy of the genome, which is vital for bacterial survival.

Evidence that Listeria innocua modulates its membrane's stored curvature elastic stress, but not fluidity, through the cell cycle

This paper reports that the abundances of endogenous cardiolipin and phosphatidylethanolamine halve during elongation of the Gram-positive bacterium Listeria innocua. The lyotropic phase behaviour of model lipid systems that describe these modulations in lipid composition indicate that the average stored curvature elastic stress of the membrane is reduced on elongation of the cell, while the fluidity appears to be maintained. These findings suggest that phospholipid metabolism is linked to the cell cycle and that changes in membrane composition can facilitate passage to the succeding stage of the cell cycle. This therefore suggests a means by which bacteria can manage the physical properties of their membranes through the cell cycle.

ZapA and ZapB form an FtsZ-independent structure at midcell

Cell division in Escherichia coli begins with the polymerization of FtsZ into a ring-like structure, the Z-ring, at midcell. All other division proteins are thought to require the Z-ring for recruitment to the future division site. Here, it is reported that the Z-ring associated proteins ZapA and ZapB form FtsZ-independent structures at midcell. Upon Z-ring disruption by the FtsZ polymerization antagonist SulA, ZapA remained at midcell as a cloud-like accumulation. Using ZapA(N60Y), a variant defective for interaction with FtsZ, it was established that these ZapA structures form without a connection to the Z-ring. Furthermore, midcell accumulations of GFP-ZapA(N60Y) often preceded Z-rings at midcell and required ZapB to assemble, suggesting that ZapB polymers form the foundation of these structures. In the absence of MatP, a DNA-binding protein that links ZapB to the chromosomal terminus region, cloud-like ZapA structures still formed but failed to track with the chromosome terminus and did not consistently precede FtsZ at midcell. Taken together, the results suggest that FtsZ-independent structures of ZapA-ZapB provide additional positional cues for Z-ring formation and may help coordinate its assembly with chromosome replication and segregation.

Kinetics of large-scale chromosomal movement during asymmetric cell division in Escherichia coli

Coordination between cell division and chromosome replication is essential for a cell to produce viable progeny. In the commonly accepted view, Escherichia coli realize this coordination via the accurate positioning of its cell division apparatus relative to the nucleoids. However, E. coli lacking proper positioning of its cell division planes can still successfully propagate. Here, we characterize how these cells partition their chromosomes into daughters during such asymmetric divisions. Using quantitative time-lapse imaging, we show that DNA translocase, FtsK, can pump as much as 80% (3.7 Mb) of the chromosome between daughters at an average rate of 1700±800 bp/s. Pauses in DNA translocation are rare, and in no occasions did we observe reversals at experimental time scales of a few minutes. The majority of DNA movement occurs at the latest stages of cell division when the cell division protein ZipA has already dissociated from the septum, and the septum has closed to a narrow channel with a diameter much smaller than the resolution limit of the microscope (~250 nm). Our data suggest that the narrow constriction is necessary for effective translocation of DNA by FtsK.

DNA translocases are conserved throughout bacteria. While at atomic and molecular levels they have been well characterized, their ability to partition DNA in vegetatively growing cells has remained less clear. Here we show that E. coli translocase, FtsK, can move as much as 80% (3.7 Mb) of the chromosomal DNA across the closing septum in asymmetrically dividing cells at an average rate of 1700 bp/s. The majority of DNA movement occurs at the latest stages of cell division when the septum has closed to a narrow channel. Our data implies that a narrow septal opening is needed for effective translocation of DNA by FtsK.

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.

The bacterial divisome: ready for its close-up

Bacterial cells divide by targeting a transmembrane protein machine to the division site and regulating its assembly and disassembly so that cytokinesis occurs at the correct time in the cell cycle. The structure and dynamics of this machine (divisome) in bacterial model systems are coming more clearly into focus, thanks to incisive cell biology methods in combination with biochemical and genetic approaches. The main conserved structural element of the machine is the tubulin homologue FtsZ, which assembles into a circumferential ring at the division site that is stabilized and anchored to the inner surface of the cytoplasmic membrane by FtsZ-binding proteins. Once this ring is in place, it recruits a series of transmembrane proteins that ultimately trigger cytokinesis. This review will survey the methods used to characterize the structure of the bacterial divisome, focusing mainly on the Escherichia coli model system, as well as the challenges that remain. These methods include recent super-resolution microscopy, cryo-electron tomography and synthetic reconstitution.

Detection of uracil within DNA using a sensitive labeling method for in vitro and cellular applications

The role of uracil in genomic DNA has been recently re-evaluated. It is now widely accepted to be a physiologically important DNA element in diverse systems from specific phages to antibody maturation and Drosophila development. Further relevant investigations would largely benefit from a novel reliable and fast method to gain quantitative and qualitative information on uracil levels in DNA both in vitro and in situ, especially since current techniques does not allow in situ cellular detection. Here, starting from a catalytically inactive uracil-DNA glycosylase protein, we have designed several uracil sensor fusion proteins. The designed constructs can be applied as molecular recognition tools that can be detected with conventional antibodies in dot-blot applications and may also serve as in situ uracil-DNA sensors in cellular techniques. Our method is verified on numerous prokaryotic and eukaryotic cellular systems. The method is easy to use and can be applied in a high-throughput manner. It does not require expensive equipment or complex know-how, facilitating its easy implementation in any basic molecular biology laboratory. Elevated genomic uracil levels from cells of diverse genetic backgrounds and/or treated with different drugs can be demonstrated also in situ, within the cell.

A novel Drosophila mitochondrial carrier protein acts as a Mg(2+) exporter in fine-tuning mitochondrial Mg(2+) homeostasis

The homeostasis of magnesium (Mg(2+)), an abundant divalent cation indispensable for many biological processes including mitochondrial functions, is underexplored. In yeast, the mitochondrial Mg(2+) homeostasis is accurately controlled through the combined effects of importers, Mrs2 and Lpe10, and an exporter, Mme1. However, little is known about this Mg(2+) homeostatic process in multicellular organisms. Here, we identified the first mitochondrial Mg(2+) transporter in Drosophila, the orthologue of yeast Mme1, dMme1, by homologous comparison and functional complementation. dMme1 can mediate the exportation of mitochondrial Mg(2+) when heterologously expressed in yeast. Altering the expression of dMme1, although only resulting in about a 10% change in mitochondrial Mg(2+) levels in either direction, led to a significant survival reduction in Drosophila. Furthermore, the reduced survival resulting from dMme1 expression changes could be completely rescued by feeding the dMME1-RNAi flies Mg(2+)-restricted food or the dMME1-over-expressing flies the Mg(2+)-supplemented diet. Our studies therefore identified the first Drosophila mitochondrial Mg(2+) exporter, which is involved in the precise control of mitochondrial Mg(2+) homeostasis to ensure an optimal state for survival.

A mutation in Escherichia coli ftsZ bypasses the requirement for the essential division gene zipA and confers resistance to FtsZ assembly inhibitors by stabilizing protofilament bundling

The earliest step in Escherichia coli cell division consists of the assembly of FtsZ protein into a proto-ring structure, tethered to the cytoplasmic membrane by FtsA and ZipA. The proto-ring then recruits additional cell division proteins to form the divisome. Previously we described an ftsZ allele, ftsZL169R , which maps to the side of the FtsZ subunit and confers resistance to FtsZ assembly inhibitory factors including Kil of bacteriophage λ. Here we further characterize this allele and its mechanism of resistance. We found that FtsZL169R permits the bypass of the normally essential ZipA, a property previously observed for FtsA gain-of-function mutants such as FtsA* or increased levels of the FtsA-interacting protein FtsN. Similar to FtsA*, FtsZL169R also can partially suppress thermosensitive mutants of ftsQ or ftsK, which encode additional divisome proteins, and confers strong resistance to excess levels of FtsA, which normally inhibit FtsZ ring function. Additional genetic and biochemical assays provide further evidence that FtsZL169R enhances FtsZ protofilament bundling, thereby conferring resistance to assembly inhibitors and bypassing the normal requirement for ZipA. This work highlights the importance of FtsZ protofilament bundling during cell division and its likely role in regulating additional divisome activities.

The Min system and other nucleoid-independent regulators of Z ring positioning

Rod-shaped bacteria such as E. coli have mechanisms to position their cell division plane at the precise center of the cell, to ensure that the daughter cells are equal in size. The two main mechanisms are the Min system and nucleoid occlusion (NO), both of which work by inhibiting assembly of FtsZ, the tubulin-like scaffold that forms the cytokinetic Z ring. Whereas NO prevents Z rings from constricting over unsegregated nucleoids, the Min system is nucleoid-independent and even functions in cells lacking nucleoids and thus NO. The Min proteins of E. coli and B. subtilis form bipolar gradients that inhibit Z ring formation most at the cell poles and least at the nascent division plane. This article will outline the molecular mechanisms behind Min function in E. coli and B. subtilis, and discuss distinct Z ring positioning systems in other bacterial species.

Following the fate of bacterial cells experiencing sudden chromosome loss

Chromosomal DNA is a constant source of information, essential for any given cell to respond and adapt to changing conditions. Here, we investigated the fate of exponentially growing bacterial cells experiencing a sudden and rapid loss of their entire chromosome. Utilizing Bacillus subtilis cells harboring an inducible copy of the endogenous toxin yqcG, which encodes an endonuclease, we induced the formation of a population of cells that lost their genetic information simultaneously. Surprisingly, these DNA-less cells, termed DLCs, did not lyse immediately and exhibited normal cellular morphology for a period of at least 5 h after DNA loss. This cellular integrity was manifested by their capacity to maintain an intact membrane and membrane potential and cell wall architecture similar to those of wild-type cells. Unlike growing cells that exhibit a dynamic profile of macromolecules, DLCs displayed steady protein and RNA reservoirs. Remarkably, following DLCs by time lapse microscopy revealed that they succeeded in synthesizing proteins, elongating, and dividing, apparently forming de novo Z rings at the midcell position. Taken together, the persistence of key cellular events in DLCs indicates that the information to carry out lengthy processes is harbored within the remaining molecular components.

IMPORTANCE

Perturbing bacterial growth by the use of antibiotics targeting replication, transcription, or translation has been a subject of study for many years; however, the consequences of a more dramatic event, in which the entire bacterial chromosome is lost, have not been described. Here, we followed the fate of bacterial cells encountering an abrupt loss of their entire genome. Surprisingly, the cells preserved an intact envelope and functioning macromolecules. Furthermore, cells lacking their genome could still elongate and divide hours after the loss of DNA. Our data suggest that the information stored in the transient reservoir of macromolecules is sufficient to carry out complex and lengthy processes even in the absence of the chromosome. Based on our study, the formation of DNA-less bacteria could serve as a novel vaccination strategy, enabling an efficient induction of the immune system without the risk of bacterial propagation within the host.

Evidence for divisome localization mechanisms independent of the Min system and SlmA in Escherichia coli

Cell division in Escherichia coli starts with assembly of FtsZ protofilaments into a ring-like structure, the Z-ring. Positioning of the Z-ring at midcell is thought to be coordinated by two regulatory systems, nucleoid occlusion and the Min system. In E. coli, nucleoid occlusion is mediated by the SlmA proteins. Here, we address the question of whether there are additional positioning systems that are capable of localizing the E. coli divisome with respect to the cell center. Using quantitative fluorescence imaging we show that slow growing cells lacking functional Min and SlmA nucleoid occlusion systems continue to divide preferentially at midcell. We find that the initial Z-ring assembly occurs over the center of the nucleoid instead of nucleoid-free regions under these conditions. We determine that Z-ring formation begins shortly after the arrival of the Ter macrodomain at the nucleoid center. Removal of either the MatP, ZapB, or ZapA proteins significantly affects the accuracy and precision of Z-ring positioning relative to the nucleoid center in these cells in accordance with the idea that these proteins link the Ter macrodomain and the Z-ring. Interestingly, even in the absence of Min, SlmA, and the putative Ter macrodomain – Z-ring link, there remains a weak midcell positioning bias for the Z-ring. Our work demonstrates that additional Z-ring localization systems are present in E. coli than are known currently. In particular, we identify that the Ter macrodomain acts as a landmark for the Z-ring in the presence of MatP, ZapB and ZapA proteins.

Cell division in Escherichia coli begins with the assembly of FtsZ proteins into a ring-like structure, the Z-ring. Remarkably, the Z-ring localizes with very high precision at midcell. Currently, two molecular systems, nucleoid occlusion and the Min system, are known to localize the Z-ring. Here, we explore whether there are additional divisome localization systems in E. coli. Using quantitative fluorescence imaging, we show that slow growing cells lacking both known positioning systems continue to divide accurately at midcell. We find that the terminus region of the chromosome moves first to mid-cell where it functions as a positional landmark for the subsequent localization of the Z-ring. Furthermore, we provide evidence that this divisome positioning system involves MatP, ZapB, and ZapA proteins. Our work shows that E. coli can divide without the canonical mechanisms for localizing its cytokinetic ring. In particular, we identify that the Ter macrodomain acts as a landmark for the Z-ring in the presence of MatP, ZapB and ZapA proteins.

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.

The Cep63 paralogue Deup1 enables massive de novo centriole biogenesis for vertebrate multiciliogenesis

Dense multicilia in higher vertebrates are important for luminal flow and the removal of thick mucus. To generate hundreds of basal bodies for multiciliogenesis, specialized terminally differentiated epithelial cells undergo massive centriole amplification. In proliferating cells, however, centriole duplication occurs only once per cell cycle. How cells ensure proper regulation of centriole biogenesis in different contexts is poorly understood. We report that the centriole amplification is controlled by two duplicated genes, Cep63 and Deup1. Cep63 regulates mother-centriole-dependent centriole duplication. Deup1 governs deuterosome assembly to mediate large-scale de novo centriole biogenesis. Similarly to Cep63, Deup1 binds to Cep152 and then recruits Plk4 to activate centriole biogenesis. Phylogenetic analyses suggest that Deup1 diverged from Cep63 in a certain ancestor of lobe-finned fishes during vertebrate evolution and was subsequently adopted by tetrapods. Thus, the Cep63 gene duplication has enabled mother-centriole-independent assembly of the centriole duplication machinery to satisfy different requirements for centriole number.

Glutamate 83 and arginine 85 of helix H3 bend are key residues for FtsZ polymerization, GTPase activity and cellular viability of Escherichia coli: lateral mutations affect FtsZ polymerization and E. coli viability

Background

FtsZ is an essential cell division protein, which localizes at the middle of the bacterial cell to mediate cytokinesis. In vitro, FtsZ polymerizes and induces GTPase activity through longitudinal interactions to form the protofilaments, whilst lateral interactions result within formation of bundles. The interactions that participate in the protofilaments are similar to its eukaryotic homologue tubulin and are well characterized; however, lateral interactions between the inter protofilaments are less defined. FtsZ forms double protofilaments in vitro, though the key elements on the interface of the inter-protofilaments remain unclear as well as the structures involved in the lateral interactions in vivo and in vitro. In this study, we demonstrate that the highly conserved negative charge of glutamate 83 and the positive charge of arginine 85 located in the helix H3 bend of FtsZ are required for in vitro FtsZ lateral and longitudinal interactions, respectively and for in vivo cell division.

Results

The effect of mutation on the widely conserved glutamate-83 and arginine-85 residues located in the helix H3 (present in most of the tubulin family) was evaluated by in vitro and in situ experiments. The morphology of the cells expressing Escherichia coli FtsZ (E83Q) mutant at 42°C formed filamented cells while those expressing FtsZ(R85Q) formed shorter filamented cells. In situ immunofluorescence experiments showed that the FtsZ(E83Q) mutant formed rings within the filamented cells whereas those formed by the FtsZ(R85Q) mutant were less defined. The expression of the mutant proteins diminished cell viability as follows: wild type > E83Q > R85Q. In vitro, both, R85Q and E83Q reduced the rate of FtsZ polymerization (WT > E83Q >> R85Q) and GTPase activity (WT > E83Q >> R85Q). R85Q protein polymerized into shorter filaments compared to WT and E83Q, with a GTPase lag period that was inversely proportional to the protein concentration. In the presence of ZipA, R85Q GTPase activity increased two fold, but no bundles were formed suggesting that lateral interactions were affected.

Conclusions

We found that glutamate 83 and arginine 85 located in the bend of helix H3 at the lateral face are required for the protofilament lateral interaction and also affects the inter-protofilament lateral interactions that ultimately play a role in the functional localization of the FtsZ ring at the cell division site.

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.

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.

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.

Metabolism, cell growth and the bacterial cell cycle

Adaptation to fluctuations in nutrient availability is a fact of life for single-celled organisms in the 'wild'. A decade ago our understanding of how bacteria adjust cell cycle parameters to accommodate changes in nutrient availability stemmed almost entirely from elegant physiological studies completed in the 1960s. In this Opinion article we summarize recent groundbreaking work in this area and discuss potential mechanisms by which nutrient availability and metabolic status are coordinated with cell growth, chromosome replication and cell division.

ATP-binding site lesions in FtsE impair cell division

FtsE and FtsX of Escherichia coli constitute an apparent ABC transporter that localizes to the septal ring. In the absence of FtsEX, cells divide poorly and several membrane proteins essential for cell division are largely absent from the septal ring, including FtsK, FtsQ, FtsI, and FtsN. These observations, together with the fact that ftsE and ftsX are cotranscribed with ftsY, which helps to target some proteins for insertion into the cytoplasmic membrane, suggested that FtsEX might contribute to insertion of division proteins into the membrane. Here we show that this hypothesis is probably wrong, because cells depleted of FtsEX had normal amounts of FtsK, FtsQ, FtsI, and FtsN in the membrane fraction. We also show that FtsX localizes to septal rings in cells that lack FtsE, arguing that FtsX targets the FtsEX complex to the ring. Nevertheless, both proteins had to be present to recruit further Fts proteins to the ring. Mutant FtsE proteins with lesions in the ATP-binding site supported septal ring assembly (when produced together with FtsX), but these rings constricted poorly. This finding implies that FtsEX uses ATP to facilitate constriction rather than assembly of the septal ring. Finally, topology analysis revealed that FtsX has only four transmembrane segments, none of which contains a charged amino acid. This structure is not what one would expect of a substrate-specific transmembrane channel, leading us to suggest that FtsEX is not really a transporter even though it probably has to hydrolyze ATP to support cell division.

Force generation by a dynamic Z-ring in Escherichia coli cell division

FtsZ, a bacterial homologue of tubulin, plays a central role in bacterial cell division. It is the first of many proteins recruited to the division site to form the Z-ring, a dynamic structure that recycles on the time scale of seconds and is required for division to proceed. FtsZ has been recently shown to form rings inside tubular liposomes and to constrict the liposome membrane without the presence of other proteins, particularly molecular motors that appear to be absent from the bacterial proteome. Here, we propose a mathematical model for the dynamic turnover of the Z-ring and for its ability to generate a constriction force. Force generation is assumed to derive from GTP hydrolysis, which is known to induce curvature in FtsZ filaments. We find that this transition to a curved state is capable of generating a sufficient force to drive cell-wall invagination in vivo and can also explain the constriction seen in the in vitro liposome experiments. Our observations resolve the question of how FtsZ might accomplish cell division despite the highly dynamic nature of the Z-ring and the lack of molecular motors.

Are histones, tubulin, and actin derived from a common ancestral protein?

Histones and the cytoskeletal components tubulin and actin all act as thermal ratchets, using the energy present in Brownian motion to do work. All three also bind to nucleotides. Here we suggest that histones, tubulin, and actin derive from a common ancestral protein. There is some sequence similarity between histone 2A and the bacterial tubulin homologue FtsZ. Histones and actin also share some sequence similarity in the nucleotides and at phosphate-binding sites. Thus, actin and tubulin may also be related, although this is not obvious from sequence analysis. Indeed, actin and tubulin are closely functionally related and cooperate in many cellular processes. Interestingly, recent advances in nanotechnology suggest that thermal ratchets may be able to impart lifelike properties; thus, the evolution of the ancestral histone, tubulin, and actin thermal ratchet may have been crucial in the development of complexity in living organisms.

Cell shape dynamics in Escherichia coli

Bacteria are the simplest living organisms. In particular, Escherichia coli has been extensively studied and it has become one of the standard model systems in microbiology. However, optical microscopy studies of single E. coli have been limited by its small size, approximately 1 x 3 microm, not much larger than the optical resolution, approximately 0.25 microm. As a result, not enough quantitative dynamical information on the life cycle of single E. coli is presently available. We suggest that, by careful analysis of images from phase contrast and fluorescence time-lapse microscopy, this limitation can be bypassed. For example, we show that applying this approach to monitoring morphogenesis in individual E. coli leads to a simple, quantitative description of this process. First, we find the time when the formation of the septum starts, tau(c). It occurs much earlier than the time when the constriction can be directly observed by phase contrast. Second, we find that the growth law of single cells is more likely bilinear/trilinear than exponential. This is further supported by the relations that hold between the corresponding growth rates. These methods could be further extended to study the dynamics of cell components, e.g., the nucleoid and the Z-ring.

Behaviour of bacterial division protein FtsZ under a monolayer with phospholipid domains

Assembly of the tubulin-like protein FtsZ at or near the cytoplasmic membrane is one of the earliest steps in division of bacteria such as Escherichia coli. Exactly what constitutes the site at which FtsZ acts is less clear. To investigate the influence of the membrane phospholipids on FtsZ localization and assembly, we have elaborated with the Langmuir technique a two-lipid monolayer made of dilauryl-phosphatidylethanolamine (DLPE) and dipalmitoyl-phosphatidylglycerol (DPPG). This monolayer comprised stable condensed domains in an expanded continuous phase. In the presence of GTP, FtsZ assembly disrupts the condensed domains within 5 min. After several hours, with or without GTP, FtsZ assembled into large aggregates at the domain interface. We suggest that the GTP-induced polymerization of FtsZ is coupled to the association of FtsZ protofilaments with domain interfaces.

Assembly dynamics of the bacterial MinCDE system and spatial regulation of the Z ring

The positioning of a cytoskeletal element that dictates the division plane is a fundamental problem in biology. The assembly and positioning of this cytoskeletal element has to be coordinated with DNA segregation and cell growth to ensure that equal-sized progeny cells are produced, each with a copy of the chromosome. In most prokaryotes, cytokinesis involves positioning a Z ring assembled from FtsZ, the ancestral homologue of tubulin. The position of the Z ring is determined by a gradient of negative regulators of Z-ring assembly. In Escherichia coli, the Min system consists of three proteins that cooperate to position the Z ring through a fascinating oscillation, which inhibits the formation of the Z ring away from midcell. Additional gradients of negative regulators of FtsZ assembly are used by E. coli and other bacteria to achieve spatial control of Z-ring assembly.

Unlinking chromosome catenanes in vivo by site-specific recombination

A challenge for chromosome segregation in all domains of life is the formation of catenated progeny chromosomes, which arise during replication as a consequence of the interwound strands of the DNA double helix. Topoisomerases play a key role in DNA unlinking both during and at the completion of replication. Here we report that chromosome unlinking can instead be accomplished by multiple rounds of site-specific recombination. We show that step-wise, site-specific recombination by XerCD-dif or Cre-loxP can unlink bacterial chromosomes in vivo, in reactions that require KOPS-guided DNA translocation by FtsK. Furthermore, we show that overexpression of a cytoplasmic FtsK derivative is sufficient to allow chromosome unlinking by XerCD-dif recombination when either subunit of TopoIV is inactivated. We conclude that FtsK acts in vivo to simplify chromosomal topology as Xer recombination interconverts monomeric and dimeric chromosomes.

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.

Z ring as executor of bacterial cell division

It has become apparent that bacteria possess ancestors of the major eukaryotic cytoskeletal proteins. FtsZ, the ancestral homologue of tubulin, assembles into a cytoskeletal structure associated with cell division, designated the Z ring. Formation of the Z ring represents a major point of both spatial and temporal regulation of cell division. Here we discuss findings concerning the structure and the formation of the ring as well as its spatial and temporal regulation.

Bacterial cell division: the mechanism and its precison

The recent development of cell biology techniques for bacteria to allow visualization of fundamental processes in time and space, and their use in synchronous populations of cells, has resulted in a dramatic increase in our understanding of cell division and its regulation in these tiny cells. The first stage of cell division is the formation of a Z ring, composed of a polymerized tubulin-like protein, FtsZ, at the division site precisely at midcell. Several membrane-associated division proteins are then recruited to this ring to form a complex, the divisome, which causes invagination of the cell envelope layers to form a division septum. The Z ring marks the future division site, and the timing of assembly and positioning of this structure are important in determining where and when division will take place in the cell. Z ring assembly is controlled by many factors including negative regulatory mechanisms such as Min and nucleoid occlusion that influence Z ring positioning and FtsZ accessory proteins that bind to FtsZ directly and modulate its polymerization behavior. The replication status of the cell also influences the positioning of the Z ring, which may allow the tight coordination between DNA replication and cell division required to produce two identical newborn cells.

Division accuracy in a stochastic model of Min oscillations in Escherichia coli

Accurate cell division in Escherichia coli requires the Min proteins MinC, MinD, and MinE as well as the presence of nucleoids. MinD and MinE exhibit spatial oscillations, moving from pole to pole of the bacterium, resulting in an average MinD concentration that is low at the center of the cell and high at the poles. This concentration minimum is thought to signal the site of cell division. Deterministic models of the Min oscillations reproduce many observed features of the system, including the concentration minimum of MinD. However, there are only a few thousand Min proteins in a bacterium, so stochastic effects are likely to play an important role. Here, we show that Monte Carlo simulations with a large number of proteins agree well with the results from a deterministic treatment of the equations. The location of minimum local MinD concentration is too variable to account for cell division accuracy in wild-type, but is consistent with the accuracy of cell division in cells without nucleoids. This finding confirms the need to include additional mechanisms, such as reciprocal interactions with the cell division ring or positioning of the nucleoids, to explain wild-type accuracy.

SlmA, a nucleoid-associated, FtsZ binding protein required for blocking septal ring assembly over Chromosomes in E. coli

Cell division in Escherichia coli begins with assembly of the tubulin-like FtsZ protein into a ring structure just underneath the cell membrane. Spatial control over Z ring assembly is achieved by two partially redundant negative regulatory systems, the Min system and nucleoid occlusion (NO), which cooperate to position the division site at midcell. In contrast to the well-studied Min system, almost nothing is known about how Z ring assembly is blocked in the vicinity of nucleoids to effect NO. Reasoning that Min function might become essential in cells impaired for NO, we screened for mutations synthetically lethal with a defective Min system (slm mutants). By using this approach, we identified SlmA (Ttk) as the first NO factor in E. coli. Our combined genetic, cytological, and biochemical results suggest that SlmA is a DNA-associated division inhibitor that is directly involved in preventing Z ring assembly on portions of the membrane surrounding the nucleoid.

Transcription regulation of ezrA and its effect on cell division of Bacillus subtilis

The EzrA protein of Bacillus subtilis is a negative regulator for FtsZ (Z)-ring formation. It is able to modulate the frequency and position of Z-ring formation during cell division. The loss of this protein results in cells with multiple Z rings located at polar as well as medial sites; it also lowers the critical concentration of FtsZ required for ring formation (P. A. Levin, I. G. Kurster, and A. D. Grossman, Proc. Natl. Acad. Sci. USA 96:9642-9647, 1999). We have studied the regulation of ezrA expression during the growth of B. subtilis and its effects on the intracellular level of EzrA as well as the cell length of B. subtilis. With the aid of promoter probing, primer extension, in vitro transcription, and Western blotting analyses, two overlapping sigmaA-type promoters, P1 and P2, located about 100 bp upstream of the initiation codon of ezrA, have been identified. P1, supposed to be an extended -10 promoter, was responsible for most of the ezrA expression during the growth of B. subtilis. Disruption of this promoter reduced the intracellular level of EzrA very significantly compared with disruption of P2. Moreover, deletion of both promoters completely abolished EzrA in B. subtilis. More importantly, the cell length and percentage of filamentous cells of B. subtilis were significantly increased by disruption of the promoter(s). Thus, EzrA is required for efficient cell division during the growth of B. subtilis, despite serving as a negative regulator for Z-ring formation.

Screening for synthetic lethal mutants in Escherichia coli and identification of EnvC (YibP) as a periplasmic septal ring factor with murein hydrolase activity

Bacterial cytokinesis is driven by the septal ring apparatus, the assembly of which in Escherichia coli is directed to mid-cell by the Min system. Despite suffering aberrant divisions at the poles, cells lacking the minCDE operon (Min(-)) have an almost normal growth rate. We developed a generally applicable screening method for synthetic lethality in E. coli, and used it to select for transposon mutations (slm) that are synthetically lethal (or sick) in combination with DeltaminCDE. One of the slm insertions mapped to envC (yibP), proposed to encode a lysostaphin-like, metallo-endopeptidase that is exported to the periplasm by the general secretory (Sec) pathway. Min(-) EnvC(-) cells showed a severe division defect, supporting a role for EnvC in septal ring function. Accordingly, we show that an EnvC-green fluorescent protein fusion, when directed to the periplasm via the twin-arginine export system, is both functional and part of the septal ring apparatus. Using an in-gel assay, we also present evidence that EnvC possesses murein hydrolytic activity. Our results suggest that EnvC plays a direct role in septal murein cleavage to allow outer membrane constriction and daughter cell separation. By uncovering genetic interactions, the synthetic lethal screen described here provides an attractive new tool for studying gene function in E. coli.

Re-replication from non-sequesterable origins generates three-nucleoid cells which divide asymmetrically

In rapidly growing Escherichia coli cells replication cycles overlap and initiation occurs at multiple replication origins (oriCs). All origins within a cell are initiated essentially in synchrony and only once per cell cycle. Immediate re-initiation of new origins is avoided by sequestration, a mechanism dependent on the SeqA protein and Dam methylation of GATC sites in oriC. Here, GATC sites in oriC were changed to GTTC. This reduced the sequestration to essentially the level found in SeqA-less cells. The mutant origins underwent re-initiation, showing that the GATC sites in oriC are required for sequestration. Each re-initiation eventually gave rise to a cell containing an extra nucleoid. The three-nucleoid cells displayed one asymmetrically placed FtsZ-ring and divided into a two-nucleoid cell and a one-nucleoid cell. The three nucleoid-cells thus divided into three daughters by two consecutive divisions. The results show that extra rounds of replication cause extra daughter cells to be formed prematurely. The fairly normal mutant growth rate and size distribution show, however, that premature rounds of replication, chromosome segregation, and cell division are flexibly accommodated by the existing cell cycle controls.

Role of two essential domains of Escherichia coli FtsA in localization and progression of the division ring

The FtsA protein is a member of the actin superfamily that localizes to the bacterial septal ring during cell division. Deletions of domain 1C or the S12 and S13 beta-strands in domain 2B of the Escherichia coli FtsA, previously postulated to be involved in dimerization, result in partially active proteins that do not allow the normal progression of septation. The truncated FtsA protein lacking domain 1C (FtsADelta1C) localizes in correctly placed division rings, together with FtsZ and ZipA, but does not interact with other FtsA molecules in the yeast two-hybrid assay, and fails to recruit FtsQ and FtsN into the division ring. The rings containing FtsADelta1C are therefore incomplete and do not support division. The production of high levels of FtsADelta1C causes filamentation, an effect that has been reported to result as well from the imbalance between FtsA+ and FtsZ+ molecules. These data indicate that the domain 1C of FtsA participates in the interaction of the protein with other FtsA molecules and with the other proteins that are incorporated at later stages of ring assembly, and is not involved in the interaction with FtsZ and the localization of FtsA to the septal ring. The deletion of the S12-S13 strands of domain 2B generates a protein (FtsADeltaS12-13) that retains the ability to interact with FtsA+. When the mutated protein is expressed at wild-type levels, it localizes into division rings and recruits FtsQ and FtsN, but it fails to sustain septation at normal levels resulting in filamentation. A fivefold overexpression of FtsADeltaS12-13 produces short cells that have normal division rings, but also cells with polar localization of the mutated protein, and cells with rings at abnormal positions that result in the production of a fraction (15%) of small nucleoid-free cells. The S12-S13 strands of domain 2B are not essential for septation, but affect the localization of the division ring.

Effects of perturbing nucleoid structure on nucleoid occlusion-mediated toporegulation of FtsZ ring assembly

In Escherichia coli, assembly of the FtsZ ring (Z ring) at the cell division site is negatively regulated by the nucleoid in a phenomenon called nucleoid occlusion (NO). Previous studies have indicated that chromosome packing plays a role in NO, as mukB mutants grown in rich medium often exhibit FtsZ rings on top of diffuse, unsegregated nucleoids. To address the potential role of overall nucleoid structure on NO, we investigated the effects of disrupting chromosome structure on Z-ring positioning. We found that NO was mostly normal in cells with inactivated DNA gyrase or in mukB-null mutants lacking topA, although some suppression of NO was evident in the latter case. Previous reports suggesting that transcription, translation, and membrane insertion of proteins ("transertion") influence nucleoid structure prompted us to investigate whether disruption of these activities had effects on NO. Blocking transcription caused nucleoids to become diffuse, and FtsZ relocalized to multiple bands on top of these nucleoids, biased towards midcell. This suggested that these diffuse nucleoids were defective in NO. Blocking translation with chloramphenicol caused characteristic nucleoid compaction, but FtsZ rarely assembled on top of these centrally positioned nucleoids. This suggested that NO remained active upon translation inhibition. Blocking protein secretion by thermoinduction of a secA(Ts) strain caused a chromosome segregation defect similar to that in parC mutants, and NO was active. Although indirect effects are certainly possible with these experiments, the above data suggest that optimum NO activity may require specific organization and structure of the nucleoid.

Effects of high hydrostatic pressure on bacterial cytoskeleton FtsZ polymers in vivo and in vitro

Some rod-shaped bacteria, including Escherichia coli, exhibit cell filamentation without septum formation under high-hydrostatic-pressure conditions, indicating that the cell-division process is affected by hydrostatic pressure. The effects of elevated pressure on FtsZ-ring formation in E. coli cells were examined using indirect immunofluorescence microscopy. Elevated pressure of 40 MPa completely inhibited colony formation of E. coli cells under the cultivation conditions used, and the cells exhibited obviously filamentous shapes. In the elongated cells, normal cell-division processes appeared to be inhibited, because no FtsZ rings were observed by indirect immunofluorescent staining. In addition, it was observed that hydrostatic pressure dissociated the E. coli FtsZ polymers in vitro. These results suggest that high hydrostatic pressure directly affects cell survival and morphology through the dissociation of the cytoskeletal frameworks.

A hypothesis to explain division site selection in Escherichia coli by combining nucleoid occlusion and Min

The positioning of the site of cell division in Escherichia coli results, it is generally believed, from the operation of nucleoid occlusion in combination with the Min system. Nucleoid occlusion prevents division over the nucleoids and directs it by default to the mid-cell region between segregating nucleoids or to polar regions while the Min system prevents division in polar regions. Unresolved questions include how these systems interact to control the earliest known event in division, the assembly at the membrane of the tubulin-like protein, FtsZ, and, more importantly, what exactly constitutes a division site. Evidence exists that (1) the coupled transcription, translation and insertion of proteins into membrane (transertion), can structure the cytoplasmic membrane into phospholipid domains, (2) the MinD protein can convert vesicles into tubes and (3) a variety of membranous structures can be observed at mid-cell. These data support a model in which transertion from the segregating daughter chromosomes leads to the formation of a distinct proteolipid domain between them at mid-cell; the composition of this domain allows phospholipid tubes to extend like fingers into the cytoplasm; these tubes then become the substrate for the dynamic assembly and disassembly of FtsZ which converts them into the invaginating fold responsible for division; the Min system inhibits division at unwanted sites and times by removing these tubes especially at the cell poles.

A predicted ABC transporter, FtsEX, is needed for cell division in Escherichia coli

FtsE and FtsX have homology to the ABC transporter superfamily of proteins and appear to be widely conserved among bacteria. Early work implicated FtsEX in cell division in Escherichia coli, but this was subsequently challenged, in part because the division defects in ftsEX mutants are often salt remedial. Strain RG60 has an ftsE::kan null mutation that is polar onto ftsX. RG60 is mildly filamentous when grown in standard Luria-Bertani medium (LB), which contains 1% NaCl, but upon shift to LB with no NaCl growth and division stop. We found that FtsN localizes to potential division sites, albeit poorly, in RG60 grown in LB with 1% NaCl. We also found that in wild-type E. coli both FtsE and FtsX localize to the division site. Localization of FtsX was studied in detail and appeared to require FtsZ, FtsA, and ZipA, but not the downstream division proteins FtsK, FtsQ, FtsL, and FtsI. Consistent with this, in media lacking salt, FtsA and ZipA localized independently of FtsEX, but the downstream proteins did not. Finally, in the absence of salt, cells depleted of FtsEX stopped dividing before any change in growth rate (mass increase) was apparent. We conclude that FtsEX participates directly in the process of cell division and is important for assembly or stability of the septal ring, especially in salt-free media.

Assembly dynamics of the bacterial cell division protein FTSZ: poised at the edge of stability

FtsZ is a prokaryotic tubulin homolog that assembles into a ring at the future site of cell division. The resulting "Z ring" forms the framework for the division apparatus, and its assembly is regulated throughout the bacterial cell cycle. A highly dynamic structure, the Z ring exhibits continual subunit turnover and the ability to rapidly assemble, disassemble, and, under certain circumstances, relocalize. These in vivo properties are ultimately due to FtsZ's capacity for guanosine triphosphate (GTP)-dependent, reversible polymerization. FtsZ polymer stability appears to be fine-tuned such that subtle changes in its assembly kinetics result in large changes in the Z ring structure. Thus, regulatory proteins that modulate FtsZ's assembly dynamics can cause the ring to rapidly remodel in response to developmental and environmental cues.

Segregation of the Escherichia coli chromosome terminus

We studied the segregation of the replication terminus of the Escherichia coli chromosome by time-lapse and still photomicroscopy. The replicated termini lie together at the cell centre. They rapidly segregate away from each other immediately before cell division. At fast growth rate, the copies move progressively and quickly toward the centres of the new-born cells. At slow growth rate, the termini usually remain near the inner cell pole and migrate to the cell centre in the middle of the cell cycle. A terminus domain of about 160kb, roughly centred on the dif recombination site, segregated as a unit at cell division. Sequences outside this domain segregated before division, giving two separate foci in predivision cells. Resolution of chromosome dimers via the terminus dif site requires the XerC recombinase and an activity of the FtsK protein that is thought to align the dif sequences at the cell centre. We found that anchoring of the termini at the cell centre and proper segregation at cell division occurred normally in the absence of recombination via the XerC recombinase. Anchoring and proper segregation were, however, frequently disrupted when the C-terminal domain of FtsK was truncated.

Evidence for polar positional information independent of cell division and nucleoid occlusion

We present evidence that, in Escherichia coli, polar positional information is present at midcell independent of known cell division factors. In filamented cells, IcsA, which is normally polar, localizes at or near potential cell division sites. Because the cell pole is derived from the septum, the sites to which IcsA localizes in filaments correspond to future poles. IcsA localization to these sites is independent of FtsZ, MinCDE, septation, and nucleoid occlusion, indicating that positional information for the future pole is independent of cell division and chromosome positioning. Upon IcsA localization to these sites, septation is inhibited, suggesting that IcsA recognition of this polar positional information may influence cell division.

An intracellular actin motor in bacteria?

Actin performs structural as well as motor-like functions in eukaryotic cells. Orthologues of actin have also been identified in bacteria, where they perform an essential function during cell growth. Bacterial actins are implicated in the maintenance of rod-shaped cell morphology, and appear to form a cytoskeletal structure, localising as helical filaments underneath the cell membrane. Recently, a plasmid-borne actin orthologue has been shown to perform a mitotic-like function during segregation of a plasmid, and chromosomally encoded actin proteins were found to play an important role in chromosome segregation. Based on the findings that actin filaments are dynamic structures in two bacterial species, we propose that actins perform motor functions rather than a purely structural role in bacteria. We suggest that an intracellular motor exists in bacteria that could be derived from an ancestral actin motor that was present in cells early in evolution.

Growth rate-dependent regulation of medial FtsZ ring formation

FtsZ is an essential cell division protein conserved throughout the bacteria and archaea. In response to an unknown cell cycle signal, FtsZ polymerizes into a ring that establishes the future division site. We conducted a series of experiments examining the link between growth rate, medial FtsZ ring formation, and the intracellular concentration of FtsZ in the gram-positive bacterium Bacillus subtilis. We found that, although the frequency of cells with FtsZ rings varies as much as threefold in a growth rate-dependent manner, the average intracellular concentration of FtsZ remains constant irrespective of doubling time. Additionally, expressing ftsZ solely from a constitutive promoter, thereby eliminating normal transcriptional control, did not alter the growth rate regulation of medial FtsZ ring formation. Finally, our data indicate that overexpressing FtsZ does not dramatically increase the frequency of cells with medial FtsZ rings, suggesting that the mechanisms governing ring formation are refractile to increases in FtsZ concentration. These results support a model in which the timing of FtsZ assembly is governed primarily through cell cycle-dependent changes in FtsZ polymerization kinetics and not simply via oscillations in the intracellular concentration of FtsZ. Importantly, this model can be extended to the gram-negative bacterium Escherichia coli. Our data show that, like those in B. subtilis, average FtsZ levels in E. coli are constant irrespective of doubling time.

Pattern formation inside bacteria: fluctuations due to the low copy number of proteins

We examine fluctuation effects due to the low copy number of proteins involved in pattern-forming dynamics within a bacterium. We focus on a stochastic model of the oscillating MinCDE protein system regulating accurate cell division in E. coli. We find that, for some parameter regions, the protein concentrations are low enough that fluctuations are essential for the generation of patterns. We also examine the role of fluctuations in constraining protein concentration levels.