Role of the nucleoid in the toporegulation of division

Bacterial division: another way to box in the ring

Proper placement of the cell division site in some rod-shaped bacteria requires two different negative regulatory systems, nucleoid occlusion and the Min proteins. Caulobacter crescentus lacks these systems, but recent work has uncovered a novel regulator that achieves the same goals.

Cell biology of mitochondrial dynamics

Mitochondria are the product of an ancient endosymbiotic event between an alpha-proteobacterium and an archael host. An early barrier to overcome in this relationship was the control of the bacterium's proliferation within the host. Undoubtedly, the bacterium (or protomitochondrion) would have used its own cell division apparatus to divide at first and, today a remnant of this system remains in some "ancient" and diverse eukaryotes such as algae and amoebae, the most conserved and widespread of all bacterial division proteins, FtsZ. In many of the eukaryotes that still use FtsZ to constrict the mitochondria from the inside, the mitochondria still resemble bacteria in shape and size. Eukaryotes, however, have a mitochondrial morphology that is often highly fluid, and in their tubular networks of mitochondria, division is clearly complemented by mitochondrial fusion. FtsZ is no longer used by these complex eukaryotes, and may have been replaced by other proteins better suited to sustaining complex mitochondrial networks. Although proteins that divide mitochondria from the inside are just beginning to be characterized in higher eukaryotes, many division proteins are known to act on the outside of the organelle. The most widespread of these are the dynamin-like proteins, which appear to have been recruited very early in the evolution of mitochondria. The essential nature of mitochondria dictates that their loss is intolerable to human cells, and that mutations disrupting mitochondrial division are more likely to be fatal than result in disease. To date, only one disease (Charcot-Marie-Tooth disease 2A) has been mapped to a gene that is required for mitochondrial division, whereas two other diseases can be attributed to mutations in mitochondrial fusion genes. Apart from playing a role in regulating the morphology, which might be important for efficient ATP production, research has indicated that the mitochondrial division and fusion proteins can also be important during apoptosis; mitochondrial fragmentation is an early triggering (and under many stimuli, essential) step in the pathway to cell suicide.

FtsZ and the division of prokaryotic cells and organelles

Binary fission of many prokaryotes as well as some eukaryotic organelles depends on the FtsZ protein, which self-assembles into a membrane-associated ring structure early in the division process. FtsZ is homologous to tubulin, the building block of the microtubule cytoskeleton in eukaryotes. Recent advances in genomics and cell-imaging techniques have paved the way for the remarkable progress in our understanding of fission in bacteria and organelles.

The C-terminus of MinE from Neisseria gonorrhoeae acts as a topological specificity factor by modulating MinD activity in bacterial cell division

MinE regulates the proper placement of the cytokinetic FtsZ ring at midcell by inducing the pole-to-pole movement of MinCD complexes. While the N-terminus of MinE has been implicated in MinD binding, a clear functional role of the C-terminus has not been elucidated. We previously determined that MinE from Neisseria gonorrhoeae (Ng) was functional in Escherichia coli (Ec). Thus, using E. coli as a model organism, gonococcal MinE (MinE(Ng)) function was examined by generating amino acid substitutions of highly conserved MinE(Ng) residues and by testing the ability of the mutant proteins to interact with gonococcal MinD (MinD(Ng)), to induce a minicell phenotype upon overexpression, to initiate MinD(Ng) oscillation, and to stimulate MinD(Ng) ATPase activity. N-terminal MinE(Ng) mutants were unable to bind to MinD(Ng); thus, they did not induce a minicell phenotype, promote MinD(Ng) oscillation or stimulate MinD(Ng) ATPase activity. While C-terminal MinE(Ng) mutants exhibited reduced abilities to bind to MinD(Ng), we show that differences in MinD(Ng) binding to the C-terminus of MinE(Ng) alter the ability of MinE(Ng) to properly stimulate MinD(Ng) activity. We present four major findings from our studies of MinE(Ng): both the N- and C-termini of MinE(Ng) interact with MinD(Ng); interaction between MinD(Ng) and MinE(Ng) is required for the recruitment of MinD(Ng) to the coiled array; oscillation of MinD(Ng) does not require ATPase stimulation; and, the extent of MinD(Ng) ATPase stimulation depends on the binding strength between MinD(Ng) and the C-terminus of MinE(Ng.).

A conserved polar region in the cell division site determinant MinD is required for responding to MinE-induced oscillation but not for localization within coiled arrays

A region in the cell division site determinant MinD required for stimulation by MinE and which determines MinD topological specificity along coil-like structures has been identified. Structural modeling of dimeric MinD and sequence alignment of 24 MinD proteins revealed a conserved polar region in Gram-negative bacterial MinD proteins, corresponding to residues 92-94 of Neisseria gonorrhoeae MinD (MinD(Ng)). Using MinD(Ng) as a paradigm for MinD functionality in Gram-negative organisms, mutation of these conserved residues did not abrogate MinD(Ng) self-association, nor its interaction with MinE(Ng) and the cell division inhibitor MinC. Although the MinD(Ng) mutant dimerized in the presence of ATP, its ATPase activity was not stimulated by MinE(Ng), unlike wild-type MinD(Ng). GFP fusions to either MinD(Ng) or to Escherichia coli MinD bearing simultaneous or individual mutations to residues 92-94 localized within coiled arrays along the E. coli inner cell periphery, similar to wild-type GFP-MinD. However, unlike wild-type GFP-fusions, the mutant proteins were distributed uniformly throughout the array, despite the presence of MinE, which normally imparts topological specificity to MinD by inducing the latter to oscillate from pole-to-pole and away from midcell. Hence, despite localizing along the inner cell periphery as a polymeric structure, the mutant MinD proteins in this study have lost the ability to be efficiently stimulated by MinE(Ng), resulting in a loss of distinct pole-to-pole oscillation.

Min-protein oscillations in round bacteria

In rod-shaped Escherichia coli cells, the Min proteins, which are involved in division-site selection, oscillate from pole-to-pole. The homologs of the Min proteins from the round bacterium Neisseria gonorrhoeae also form a spatial oscillator when expressed in wild-type and round, rodA- mutants of E. coli, suggesting that the Min proteins form an oscillator in N. gonorrhoeae. Here we report that a numerical model for Min-protein oscillations in rod-shaped cells also produces oscillations in round cells (cocci). Our numerical results explain why the MinE-protein rings found in wild-type E. coli are absent in round mutants. In addition, we find that for round cells there is a minimum radius below which oscillations do not occur, and a maximum radius above which oscillations become mislocalized. Finally, we demonstrate that Min-protein oscillations can select the long axis in nearly round cells based solely on geometry, a potentially important factor in division-plane selection in cocci.

The N terminus of MinD contains determinants which affect its dynamic localization and enzymatic activity

MinD is involved in regulating the proper placement of the cytokinetic machinery in some bacteria, including Neisseria gonorrhoeae and Escherichia coli. Stimulation of the ATPase activity of MinD by MinE has been proposed to induce dynamic, pole-to-pole oscillations of MinD in E. coli. Here, we investigated the effects of deleting or mutating conserved residues within the N terminus of N. gonorrhoeae MinD (MinD(Ng)) on protein dynamism, localization, and interactions with MinD(Ng) and with MinE(Ng). Deletions or mutations were generated in the first five residues of MinD(Ng), and mutant proteins were evaluated by several functional assays. Truncation or mutation of N-terminal residues disrupted MinD(Ng) interactions with itself and with MinE. Although the majority of green fluorescent protein (GFP)-MinD(Ng) mutants could still oscillate from pole to pole in E. coli, the GFP-MinD(Ng) oscillation cycles were significantly faster and were accompanied by increased cytoplasmic localization. Interestingly, in vitro ATPase assays indicated that MinD(Ng) proteins lacking the first three residues or with an I5E substitution possessed higher MinE(Ng)-independent ATPase activities than the wild-type protein. These results indicate that determinants found within the extreme N terminus of MinD(Ng) are implicated in regulating the enzymatic activity and dynamic localization of the protein.

A network of net-workers: report of the Euresco conference on ‘Bacterial Neural Networks’ held at San Feliu (Spain) from 8 to 14 May 2004

In May 2004, over 100 bacteriologists from 19 different countries discussed recent progress in identification and understanding of individual signal transfer mechanisms in bacteria and in the mutual interactions between these systems to form a functional living cell. The meeting was held in San Feliu and supported by ESF and EMBO. In part through the extensive sequencing efforts of the past few years, the bulk of the bacterial signal transfer systems have been resolved and their detailed characterization is revealing such characteristics as signal specificity, signalling rate constants, molecular interaction affinities, subcellular localization, etc., which should provide a solid basis to a computational extension of this field of studies. In parallel, the new genomics techniques are providing tools to characterize the way a collection of such systems interact in an individual cell, to give rise to 'life'. Systems theory provides rational and convenient ways to bring order to the wide range of observables thus obtained. Ultimately, the performance of engineered design will have to prove whether or not we know enough about the processes involved.

Catching some Zs: a new protein for spatial regulation of bacterial cytokinesis

Prior to its duplication, the bacterial nucleoid exerts local negative control over assembly of the cytokinetic Z ring to prevent potential cutting of the chromosome. In this issue of Cell, Wu and Errington show that a specific nucleoid-associated protein mediates this nucleoid occlusion effect, providing the first mechanistic insight into this key spatial regulatory system.

Coordination of cell division and chromosome segregation by a nucleoid occlusion protein in Bacillus subtilis

A range of genetical and physiological experiments have established that diverse bacterial cells possess a function called nucleoid occlusion, which acts to prevent cell division in the vicinity of the nucleoid. We have identified a specific effector of nucleoid occlusion in Bacillus subtilis, Noc (YyaA), as an inhibitor of division that is also a nonspecific DNA binding protein. Under various conditions in which the cell cycle is perturbed, Noc prevents the division machinery from assembling in the vicinity of the nucleoid. Unexpectedly, cells lacking both Noc and the Min system (which prevents division close to the cell poles) are blocked for division, apparently because they establish multiple nonproductive accumulations of division proteins. The results help to explain how B. subtilis specifies the division site under a range of conditions and how it avoids catastrophic breakage of the chromosome by division through the nucleoid.

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.

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.

DNA–membrane interactions can localize bacterial cell center

In actively growing bacterial cells, the DNA exerts stress on the membrane, in addition to the turgor caused by osmotic pressure. This stress is applied through coupled transcription/translation and insertion of membrane proteins (so-called "transertion" process). In bacillary bacteria, the strength of this interaction varies along cell length with a minimum at its midpoint, and hence can locate the cell's equator for the assembly of the FtsZ-ring.

Dynamic structures in Escherichia coli: spontaneous formation of MinE rings and MinD polar zones

In Escherichia coli, division site selection is regulated in part by the Min-protein system. Oscillations of the Min proteins from pole to pole every approximately 40 sec have been revealed by in vivo studies of GFP fusions. The dynamic oscillatory structures produced by the Min proteins, including a ring of MinE protein, compact polar zones of MinD, and zebra-striped oscillations in filamentous cells, remain unexplained. We show that the Min oscillations, including mutant phenotypes, can be accounted for by in vitro-observed interactions involving MinD and MinE, with a crucial role played by the rate of nucleotide exchange. Recent discoveries suggest that protein oscillations may play a general role in proper chromosome and plasmid partitioning.

Localizing cell division in sphericalEscherichia coliby nucleoid occlusion

The spatial relationship between FtsZ localization and nucleoid segregation was followed in Escherichia coli thyA cells, made spheroidal by brief exposure to mecillinam and after manipulating chromosome replication time using changes ('steps') in thymine concentration [Zaritsky et al., Microbiology 145 (1999) 1015-1022]. In such cells, fluorescent FtsZ-GFP arcs did not overlap the DAPI-stained nucleoids. It is concluded that FtsZ rings are deposited between segregating nucleoids, consistent with the nucleoid occlusion model [Woldringh et al., J. Bacteriol. 176 (1994) 6030-6038].

Cytokinesis in bacteria

Work on two diverse rod-shaped bacteria, Escherichia coli and Bacillus subtilis, has defined a set of about 10 conserved proteins that are important for cell division in a wide range of eubacteria. These proteins are directed to the division site by the combination of two negative regulatory systems. Nucleoid occlusion is a poorly understood mechanism whereby the nucleoid prevents division in the cylindrical part of the cell, until chromosome segregation has occurred near midcell. The Min proteins prevent division in the nucleoid-free spaces near the cell poles in a manner that is beginning to be understood in cytological and biochemical terms. The hierarchy whereby the essential division proteins assemble at the midcell division site has been worked out for both E. coli and B. subtilis. They can be divided into essentially three classes depending on their position in the hierarchy and, to a certain extent, their subcellular localization. FtsZ is a cytosolic tubulin-like protein that polymerizes into an oligomeric structure that forms the initial ring at midcell. FtsA is another cytosolic protein that is related to actin, but its precise function is unclear. The cytoplasmic proteins are linked to the membrane by putative membrane anchor proteins, such as ZipA of E. coli and possibly EzrA of B. subtilis, which have a single membrane span but a cytoplasmic C-terminal domain. The remaining proteins are either integral membrane proteins or transmembrane proteins with their major domains outside the cell. The functions of most of these proteins are unclear with the exception of at least one penicillin-binding protein, which catalyzes a key step in cell wall synthesis in the division septum.

A widely conserved bacterial cell division protein that promotes assembly of the tubulin-like protein FtsZ

Cell division in bacteria is mediated by the tubulin-like protein FtsZ, which assembles into a structure known as the Z ring at the future site of cytokinesis. We report the discovery of a Z-ring-associated protein in Bacillus subtilis called ZapA. ZapA was found to colocalize with the Z ring in vivo and was capable of binding to FtsZ and stimulating the formation of higher-order assemblies of the cytokinetic protein in vitro. The absence of ZapA alone did not impair cell viability, but the absence of ZapA in combination with the absence of a second, dispensable division protein EzrA caused a severe block in cytokinesis. The absence of ZapA also caused lethality in cells producing lower than normal levels of FtsZ or lacking the division-site-selection protein DivIVA. Conversely, overproduction of ZapA reversed the toxicity of excess levels of the division inhibitor MinD. In toto, the evidence indicates that ZapA is part of the cytokinetic machinery of the cell and acts by promoting Z-ring formation. Finally, ZapA is widely conserved among bacteria with apparent orthologs in many species, including Escherichia coli, in which the orthologous protein exhibited a strikingly similar pattern of subcellular localization to that of ZapA. Members of the ZapA family of proteins are likely to be a common feature of the cytokinetic machinery in bacteria.

Effects of the Min system on nucleoid segregation in Escherichia coli

The Min system of Escherichia coli directs cell division to the mid-cell by a mechanism that involves the dynamic localization of all of its three constituent proteins, MinC, MinD and MinE. Both the Min system and the nucleoid regulate cell division negatively and strains of E. coli lacking a functional Min system can divide at nucleoid-free cell poles in addition to the nucleoid-free region between newly segregated nucleoids. Interestingly, E. coli strains with a defective Min system have disturbed nucleoid segregation and the cause for this disturbance is not known. It is reported here that growth conditions promoting a higher frequency of polar divisions also lead to a more pronounced disturbance in nucleoid segregation. In strains with an intact Min system, expression of MinE, but not of MinD, from an inducible promoter was followed by impaired nucleoid segregation. These results suggest that the disturbed nucleoid segregation in min mutants is not caused by polar divisions per se, nor by impaired resolution of chromosome dimers in min mutants, leaving open the possibility that the Min system has a direct effect on nucleoid segregation. It is also shown how the disturbed nucleoid segregation can explain in part the unexpected finding that the clear majority of cells in min mutant populations contain 2(n) (n=0, 1, 2.) origins of replication.

Conservation of dynamic localization among MinD and MinE orthologues: oscillation of Neisseria gonorrhoeae proteins in Escherichia coli

Min proteins are involved in the correct placement of division septa in many bacterial species. In Escherichia coli (Ec) cells, these proteins oscillate from pole to pole, ostensibly to prevent unwanted polar septation. Here, we show that Min proteins from the coccus Neisseria gonorrhoeae (Ng) also oscillate in E. coli. Green fluorescent protein (GFP) fusions to gonococcal MinD and MinE localized dynamically in different E. coli backgrounds. GFP-MinDNg moved from pole to pole in rod-shaped E. coli cells with a 70 +/- 25 s localization cycle when MinENg was expressed in cis. The oscillation time of GFP-MinDNg was reduced when wild-type MinENg was replaced with MinENg carrying a R30D mutation, but lengthened by 15 s when activated by MinEEc. Several mutations in the N-terminal domain of MinDNg, including K16Q and 4- and 19-amino acid truncations, prevented oscillation; these MinDNg mutants showed decreased or lost interaction with themselves and MinENg. Like MinEEc-GFP, MinENg-GFP formed MinE rings and oscillated in E. coli cells when MinDEc was expressed in cis. Finally, in round E. coli cells, GFP-MinDNg appeared to move in a plane parallel to completed septa. This pattern of movement is predicted to be similar in gonococcal cells, which also divide in alternating perpendicular planes.

DNA replication initiation is required for mid-cell positioning of FtsZ rings in Caulobacter crescentus

Polymerization of the GTPase FtsZ to form a structure called the Z-ring is the earliest known step in bacterial cell division. Mid-cell Z-ring assembly coincides with the beginning of the replication cycle in the differentiating bacterium Caulobacter crescentus. Z-ring disassembly occurs at the end of the division cycle, resulting in the complete degradation of FtsZ from both stalked and swarmer progeny cells. New Z-rings can only form in the replicative stalked cell. Conditional mutants in DNA replication were used to determine what role DNA replication events play in the process of Z-ring assembly at different stages in the cell cycle. Z-ring assembly occurred even when early stages of DNA replication were blocked; however, the Z-rings were localized at a subpolar region of the cell. Z-rings only assembled at the proper mid-cell location if DNA replication had initiated. Z-ring assembly coincided with areas containing little or no DNA, and Z-rings could not form over an unreplicated chromosome. Overexpressed FtsZ in the absence of DNA replication did not stimulate productive mid-cell Z-ring assembly but, instead, caused the ends of cells to constrict over an extended area away from the nucleoid. These results indicate that the state of chromosome replication is a major determinant of Z-ring localization in Caulobacter.

The tubulin ancestor, FtsZ, draughtsman, designer and driving force for bacterial cytokinesis

We discuss in this review the regulation of synthesis and action of FtsZ, its structure in relation to tubulin and microtubules, and the mechanism of polymerization and disassembly (contraction) of FtsZ rings from a specific nucleation site (NS) at mid cell. These topics are considered in the light of recent immunocytological studies, high resolution structures of some division proteins and results indicating how bacteria may measure their mid cell point.

Distribution of the Escherichia coli structural maintenance of chromosomes (SMC)-like protein MukB in the cell

Fluorescent polyclonal antibodies specific for MukB have been used to study its localization in Escherichia coli. In wild-type cells, the MukB protein appeared as a limited number of oblong shapes embracing the nucleoid. MukB remained associated with the nucleoid in the absence of DNA replication. The centre of gravity of the dispersed MukB signal initially localized near mid-cell, but moved to approximately quarter positions well before the termination of DNA replication and its subsequent reinitiation. Because MukB had been reported to bind to FtsZ and to its eukaryotic homologue tubulin in vitro, cells were co-labelled with MukB- and FtsZ-specific fluorophores. No co-localization of MukB with polymerized FtsZ (the FtsZ ring) was observed at any time during the cell cycle. A possible role for MukB in preventing premature FtsZ polymerization and in DNA folding that might assist DNA segregation is discussed.

Cytokinesis in prokaryotes and eukaryotes: common principles and different solutions

Cytokinesis requires duplication of cellular structures followed by bipolarization of the predivisional cell. As a common principle, this applies to prokaryotes as well as eukaryotes. With respect to eukaryotes, the discussion has focused mainly on Saccharomyces cerevisiae and on Schizosaccharomyces pombe. Escherichia coli and to a lesser extent Bacillus subtilis have been used as prokaryotic examples. To establish a bipolar cell, duplication of a eukaryotic origin of DNA replication as well as its genome is not sufficient. Duplication of the microtubule-organizing center is required as a prelude to mitosis, and it is here that the dynamic cytoskeleton with all its associated proteins comes to the fore. In prokaryotes, a cytoskeleton that pervades the cytoplasm appears to be absent. DNA replication and the concomitant DNA segregation seem to occur without help from extensive cytosolic supramacromolecular assemblies but with help from the elongating cellular envelope. Prokaryotic cytokinesis proceeds through a contracting ring, which has a roughly 100-fold-smaller circumference than its eukaryotic counterpart. Although the ring contains proteins that can be considered as predecessors of actin, tubulin, and microtubule-associated proteins, its macromolecular composition is essentially different.

Growth phase variation in cell and nucleoid morphology in a Bacillus subtilis recA mutant

The major role of RecA is thought to be in helping repair and restart stalled replication forks. During exponential growth, Bacillus subtilis recA cells exhibited few microscopically observable nucleoid defects. However, the efficiency of plating was about 12% of that of the parent strain. A substantial and additive defect in viability was also seen for addB and recF mutants, suggesting a role for the corresponding recombination paths during normal growth. Upon entry into stationary phase, a subpopulation (approximately 15%) of abnormally long cells and nucleoids developed in B. subtilis recA mutants. In addition, recA mutants showed a delay in, and a diminished capacity for, effecting prespore nucleoid condensation.

Co-ordinate regulation of the Escherichia coli cell cycle or The cloud of unknowing

A discussion of some aspects of the regulation of chromosome replication, segregation and cell division in Escherichia coli.

Cell cycle and positional constraints on FtsZ localization and the initiation of cell division in Caulobacter crescentus

Swarmer cells of Caulobacter crescentus are devoid of the cell division initiation protein FtsZ and do not replicate DNA. FtsZ is synthesized during the differentiation of swarmer cells into replicating stalked cells. We show that FtsZ first localizes at the incipient stalked pole in differentiating swarmer cells. FtsZ subsequently localizes at the mid-cell early in the cell cycle. In an effort to understand whether Z-ring formation and cell constriction are driven solely by the cell cycle-regulated increase in FtsZ concentration, FtsZ was artificially expressed in swarmer cells at a level equivalent to that found in predivisional cells. Immunofluorescence microscopy showed that, in these swarmer cells, simply increasing FtsZ concentration was not sufficient for Z-ring formation; Z-ring formation took place only in stalked cells. Expression of FtsZ in swarmer cells did not alter the timing of cell constriction initiation during the cell cycle but, instead, caused additional constrictions and a delay in cell separation. These additional constrictions were confined to sites close to the original mid-cell constriction. These results suggest that the timing and placement of Z-rings is tightly coupled to an early cell cycle event and that cell constriction is not solely dependent on a threshold level of FtsZ.

Influence of the nucleoid on placement of FtsZ and MinE rings in Escherichia coli

We previously presented evidence that replicating but unsegregated nucleoids, along with the Min system, act as topological inhibitors to restrict assembly of the FtsZ ring (Z ring) to discrete sites in the cell. To test if nonreplicating nucleoids have similar exclusion effects, we examined Z rings in dnaA (temperature sensitive) mutants. Z rings were excluded from centrally localized nucleoids and were often observed at nucleoid edges. Cells with nonreplicating nucleoids formed filaments, some of which contained large nucleoid-free areas in which Z rings were positioned at regular intervals. Because MinE may protect FtsZ from the action of the MinC inhibitor in these nucleoid-free zones, we examined the localization of a MinE-green fluorescent protein (GFP) fusion with respect to Z rings and nucleoids. Like Z rings, MinE-GFP appeared to localize independently of nucleoid position, forming rings at regular intervals in nucleoid-free regions. Unlike FtsZ, however, MinE-GFP often localized on top of nucleoids, replicating or not, suggesting that MinE is relatively insensitive to the nucleoid inhibition effect. These data suggest that both replicating and nonreplicating nucleoids are capable of topologically excluding Z rings but not MinE.

Hypothesis: membrane domains and hyperstructures control bacterial division

The mechanism responsible for creating the division site in the right place at the right time in bacteria is unknown. It has been attributed to the formation of proteolipid domains in the cytoplasmic membrane surrounding the nucleoids. We interpret the growing evidence for this hypothesis by invoking hyperstructures, which exist at a level of organization intermediate between macromolecules and genes. Non-equilibrium hyperstructures comprise the genes, mRNA proteins and lipids required for a particular function such as cell division, and assemble and disassemble according to the needs of the cell.

Dimensional regulation of cell-cycle events in Escherichia coli during steady-state growth

Two opposing models have been put forward in the literature to describe the changes in the shape of individual Escherichia coli cells in steady-state growth that take place during the cell cycle: the Length model, which maintains that the regulating dimension is cell length, and the Volume model, which asserts it to be cell volume. In addition, the former model envisages cell diameter as decreasing with length up to constriction whereas the latter sees it as being constrained by the rigid cell wall. These two models differ in the correlations they predict between the various cellular dimensions (diameter, length, volume) not only across the entire population of bacteria but also, and especially, within subpopulations that define specific cell-cycle events (division, for example, or onset of constriction); the coefficients of variation at these specific events are also expected to be very different. Observations from cells prepared for electron microscopy (air-dried) and for phase-contrast microscopy (hydrated) appeared qualitatively largely in accordance with the predictions of the Length model. To obtain a more quantitative comparison, simulations were carried out of populations defined by each of the models; again, the results favoured the Length model. Finally, in age-selected cells using membrane elution, the diameter-length and diameter-volume correlations were in complete agreement with the Length model, as were the coefficients of variation. It is concluded that, at least with respect to cell-cycle events such as onset of constriction and cell division, length rather than volume is the controlling dimension.

FtsZ ring formation without subsequent cell division after replication runout in Escherichia coli

In this report, we have investigated cell division after inhibition of initiation of chromosome replication in Escherichia coli. In a culture grown to the stationary phase, cells containing more than one chromosome were able to divide some time after restart of growth, under conditions not allowing initiation of chromosome replication. This shows that there is no requirement for cell division to take place within a certain time after initiation of chromosome replication. Continued growth without initiation of replication resulted in filamented cells that generally did not have any constrictions. Interestingly, FtsZ rings were formed in a majority of these cells as they reached a certain cell length. These rings appeared and were maintained for some time at the cell quarter positions on both sides of the centrally localized nucleoid. These results confirm previous findings that cell division sites are formed independently of chromosome replication and indicate that FtsZ ring assembly is dependent on cell size rather than on the capacity of the cell to divide. Disruption of the mukB gene caused a significant increase in the region occupied by DNA after the replication runout, consistent with a role of MukB in chromosome condensation. The aberrant nucleoid structure was accompanied by a shift in FtsZ ring positioning, indicating an effect of the nucleoid on the positioning of the FtsZ ring. A narrow cell length interval was found, under and over which primarily central and non-central FtsZ rings, respectively, were observed. This finding correlates well with the previously observed oscillatory movement of MinC and MinD in short and long cells.

MinDE-dependent pole-to-pole oscillation of division inhibitor MinC in Escherichia coli

By inhibiting FtsZ ring formation near the cell ends, the MinC protein plays a critical role in proper positioning of the division apparatus in Escherichia coli. MinC activity requires that of MinD, and the MinE peptide provides topological specificity by suppressing MinC-MinD-mediated division inhibition specifically at the middle of the cell. We recently presented evidence that MinE not only accumulates in an FtsZ-independent ring structure at the cell's middle but also imposes a unique dynamic localization pattern upon MinD in which the latter accumulates alternately in either one of the cell halves in what appears to be a rapidly oscillating membrane association-dissociation cycle. Here we show that functional green fluorescent protein-MinC displays a very similar oscillatory behavior which is dependent on both MinD and MinE and independent of FtsZ. The results support a model in which MinD recruits MinC to its site of action and in which FtsZ ring assembly at each of the cell ends is blocked in an intermittent and alternate fashion.

FtsZ ring clusters in min and partition mutants: role of both the Min system and the nucleoid in regulating FtsZ ring localization

To understand further the role of the nucleoid and the min system in selection of the cell division site, we examined FtsZ localization in Escherichia coli cells lacking MinCDE and in parC mutants defective in chromosome segregation. More than one FtsZ ring was sometimes found in the gaps between nucleoids in min mutant filaments. These multiple FtsZ rings were more apparent in longer cells; double or triple rings were often found in the nucleoid-free gaps in ftsI min and ftsA min double mutant filaments. Introducing a parC mutation into the ftsA min double mutant allowed the nucleoid-free gaps to become significantly longer. These gaps often contained dramatic clusters of FtsZ rings. In contrast, filaments of the ftsA parC double mutant, which contained active MinCDE, assembled only one or two rings in most of the large nucleoid-free gaps. These results suggest that all positions along the cell length are competent for FtsZ ring assembly, not just sites at mid-cell or at the poles. Consistent with previous results, unsegregated nucleoids also correlated with a lack of FtsZ localization. A model is proposed in which both the inhibitory effect of the nucleoid and the regulation by MinCDE ensure that cells divide precisely at the midpoint.

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

The FtsZ ring assembles between segregated daughter chromosomes in prokaryotic cells and is essential for cell division. To understand better how the FtsZ ring is influenced by chromosome positioning and structure in Escherichia coli, we investigated its localization in parC and mukB mutants that are defective for chromosome segregation. Cells of both mutants at non-permissive temperatures were either filamentous with unsegregated nucleoids or short and anucleate. In parC filaments, FtsZ rings tended to localize only to either side of the central unsegregated nucleoid and rarely to the cell midpoint; however, medial rings reappeared soon after switching back to the permissive temperature. Filamentous mukB cells were usually longer and lacked many potential rings. At temperatures permissive for mukB viability, medial FtsZ rings assembled despite the presence of apparently unsegregated nucleoids. However, a significant proportion of these FtsZ rings were mislocalized or structurally abnormal. The most surprising result of this study was revealed upon further examination of FtsZ ring positioning in anucleate cells generated by the parC and mukB mutants: many of these cells, despite having no chromosome, possessed FtsZ rings at their midpoints. This discovery strongly suggests that the chromosome itself is not required for the proper positioning and development of the medial division site.

Chromosome and low copy plasmid segregation in E. coli: visual evidence for distinct mechanisms

We have investigated DNA segregation in E. coli by inserting multiple lac operator sequences into the chromosome near the origin of replication (oriC), in the hisC gene, a terminus marker, and into plasmids P1 and F. Expression of a GFP-LacI fusion protein allowed visualization of lac operator localization. oriC was shown to be specifically localized at or near the cell poles, and when duplicated, one copy moved to the site of new pole formation near the site of cell division. In contrast, P1 and F localized to the cell center and on duplication appeared to move rapidly to the quarter positions in the cell. Our analysis suggests that different active processes are involved in movement and localization of the chromosome and of the two plasmids during segregation.

Division-inhibition capacity of penicillin in Escherichia coli is growth-rate dependent

Growing bacteria are sensitive to various beta-lactam derivatives due to their interference with peptidoglycan biosynthesis. At low concentrations, penicillin G (benzylpenicillin) blocks cell division without affecting mass growth rate. The MIC for division of Escherichia coli B/r (H266) was found to depend on the growth rate, which was modified by the nutritional conditions. Our hypothesis, that division sensitivity is proportional to the rate of peptidoglycan synthesis for septum formation, as well as to cell circumference, was thus confirmed.

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

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

Nucleoid partitioning and the division plane in Escherichia coli

Escherichia coli nucleoids were visualized after the DNA of OsO4-fixed but hydrated cells was stained with the fluorochrome DAPI (4',6-diamidino-2-phenylindole dihydrochloride hydrate). In slowly growing cells, the nucleoids are rod shaped and seem to move along the major cell axis, whereas in rapidly growing, wider cells they consist of two- to four-lobed structures that often appear to advance along axes lying perpendicular or oblique to the major axis of the cell. To test the idea that the increase in cell diameter following nutritional shift-up is caused by the increased amount of DNA in the nucleoid, the cells were subjected to DNA synthesis inhibition. In the absence of DNA replication, the nucleoids continued to move in the growing filaments and were pulled apart into small domains along the length of the cell. When these cells were then transferred to a richer medium, their diameters increased, especially in the region enclosing the nucleoid. It thus appears that the nucleoid motive force does not depend on DNA synthesis and that cell diameter is determined not by the amount of DNA per chromosome but rather by the synthetic activity surrounding the nucleoid. Under the non-steady-state but balanced growth conditions induced by thymine limitation, nucleoids become separated into small lobules, often lying in asymmetric configurations along the cell periphery, and oblique and asymmetric division planes occur in more than half of the constricting cells. We suggest that such irregular DNA movement affects both the angle of the division plane and its position.

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

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

Relationship between size of parent at cell division and relative size of its progeny in Escherichia coli

This article examines the empirical basis for the assumption of independence between the relative size (length or surface area) of a newborn cell w and the absolute size of its mother at cell division. Random samples from two strains of Escherichia coli B/r cells in steady-state exponential growth, covering a range of doubling times, were fixed in osmium tetroxide and prepared for electron microscopy by agar filtration. Length and diameter of over 3000 constricted cells were measured from the electron micrographs and cell surface area computed by assuming an idealized geometry of right circular cylinders with hemispherical polar caps. In general, these strains were found to divide into two daughter cells with a precision that is independent of the size of the mother. In addition, both a normal and a symmetrical beta-distribution were shown to fit the observed size distributions of w rather well; theoretical grounds for preferring the latter are discussed.

Involvement of FtsZ in coupling of nucleoid separation with septation

The cell-cycle parameters of an Escherichia coli strain expressing essential division gene ftsZ at one-fifth of its normal level, because of antisense regulation by DicF RNA, have been analysed. Inhibition of FtsZ expression affects neither the generation time nor the replication initiation mass, the C period, or the constriction period, but it does dramatically retard the initiation of constriction relative to replication termination. Separation of the nucleoids is equally postponed, indicating that division is not coupled to termination of replication, but to partitioning. The severe inhibition of nucleoid separation by DicF RNA, and its suppression by overproduction of FtsZ, suggest a role for FtsZ in the control of separation, and consequently in the coupling of separation and division. We suggest that the normal pattern of nucleoid separation previously found in cells deficient in ftsZ function was a consequence of the loss of a negative effect exerted by FtsZ on separation. In agreement with this view, we find that nucleoid separation is temporarily inhibited after arrest of FtsZ synthesis, but is later resumed as FtsZ is further diluted into the elongating filaments.

New minC mutations suggest different interactions of the same region of division inhibitor MinC with proteins specific for minD and dicB coinhibition pathways

Proper positioning of division sites in Escherichia coli requires balanced expression of minC, minD, and minE gene products. Previous genetic analysis has shown that either MinD or an apparently unrelated protein, DicB, cooperates with MinC to inhibit division. We have isolated and sequenced minC mutations that suppress division inhibition caused by overproduction of either DicB or MinD proteins. Most missense mutations were located in the amino acid 160 to 200 region of MinC (231 amino acids). Some mutations exhibited preferential resistance to one or the other coinhibitor, suggesting that two distinct proteins, possibly MinD and DicB themselves, interact in slightly different manners with the same region of MinC to promote division inhibition.

Autoradiographic analysis of diaminopimelic acid incorporation in filamentous cells of Escherichia coli: repression of peptidoglycan synthesis around the nucleoid

Peptidoglycan synthesis rate in nonconstricting filaments of Escherichia coli dnaX(Ts) has been studied by autoradiography of incorporated [3H]diaminopimelic acid. Analysis of autoradiograms of whole cells and sacculi showed that peptidoglycan is synthesized at a reduced rate in the nucleoid-containing parts of these filaments. The lower rate of peptidoglycan synthesis in the cell center coincides with a higher local rate of protein synthesis. DNA-less cell formation in dnaX(Ts), dnaX(Ts) sfiA, and the minB minicell-forming mutant is accompanied by a local increase in peptidoglycan synthesis at the constriction site.

Toporegulation of bacterial division according to the nucleoid occlusion model

A model for the toporegulation of division in Escherichia coli is presented in which cell constriction is initiated by the combined action of a biochemical and a structural event. It is proposed that the biochemical event of termination of DNA replication causes a transient change in the pool of deoxyribonucleotides, which serves as a localized trigger that is converted to a diffusible, cytoplasmic activator of peptidoglycan synthesis. The second event involves the segregation of the nucleoids. Evidence is presented that the nucleoid suppresses the activity of peptidoglycan synthesis in its vicinity. It is proposed that active transcription/translation around the nucleoids produces a strong but short-range inhibitor which prohibits division (nucleoid occlusion). The combined effects of the locally produced termination-activator and of the diminished occlusion as a result of nucleoid segregation, guarantee that division is normally placed between the separated nucleoids. The model can explain the pattern of division-recovery of filaments, the majority of which constrict at sites which produce polar daughter cells containing two nucleoids. In addition, the model offers an explanation for the occurrence of mini-cells under a variety of conditions.

Logic of the Escherichia coli cell cycle

The logic of Escherichia coli's responses to environmental changes gives hope that its cell cycle will be equally well designed. During growth in a constant environment, internal signals trigger cell-cycle events such as replication initiation and cell division. Internal signals must also provide the cell with information about its present state, enabling it to coordinate the synthesis of cytoplasm, DNA and cell wall and maintain proper cell shape and composition. How the cell regulates these aspects of its growth is a fascinating--and as yet unfinished--story.