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.     

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.     

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.     

The division of pleomorphic plastids with multiple FtsZ rings in tobacco BY-2 cells

Plastids, an essential group of plant cellular organelles, proliferate by division to maintain continuity through cell lineages in plants. In recent years, it was revealed that the bacterial cell division protein FtsZ is encoded in the nuclear genome of plant cells, and plays a major role in the plastid division process forming a ring along the center of plastids. Although the best-characterized type of plastid division so far is the division with a single FtsZ ring at the plastid midpoint, it was recently reported that in some plant organs and tissues, plastids are pleomorphic and form multiple FtsZ rings. However, the pleomorphic plastid division mechanism, such as the formation of multiple FtsZ rings, the constriction of plastids and the behavior of plastid (pt) nucleoids, remains totally unclear. To elucidate these points, we used the cultured cell line, tobacco (Nicotiana tabacum L.) Bright Yellow-2, in which plastids are pleomorphic and show dynamic morphological changes during culture. As a result, it was revealed that as the plastid elongates from an ellipsoid shape to a string shape after medium renewal, FtsZ rings are multiplied almost orderly and perpendicularly to the long axis of plastids. Active DNA synthesis of pt nucleoids is induced by medium transfer, and the division and the distribution of pt nucleoids occur along with plastid elongation. Although it was thought that the plastid divides with simultaneous multiple constrictions at all the FtsZ ring sites, giving rise to many small plastids, we found that the plastids generally divide constricting at only one FtsZ ring site. Moreover, using electron microscopy, we revealed that plastid-dividing (PD) rings are observed only at the constriction site, and not at swollen regions. These results indicate that in the pleomorphic plastid division with multiple FtsZ rings, the formation of PD rings occurs at a limited FtsZ ring site for one division. Multiplied FtsZ rings seem to localize in advance at the expected sites of division, and the formation of a PD ring at each FtsZ ring site occurs in a certain order, not simultaneously. Based on these results, a novel model for the pleomorphic plastid division with multiple FtsZ rings is proposed.     

PomZ,a ParA‐like protein,regulates Z‐ring formation and cell division in Myxococcus xanthus

Accurate positioning of the division site is essential to generate appropriately sized daughter cells with the correct chromosome number. In bacteria, division generally depends on assembly of the tubulin homologue FtsZ into the Z‐ring at the division site. Here, we show that lack of the ParA‐like protein PomZ in Myxococcus xanthus resulted in division defects with the formation of chromosome‐free minicells and filamentous cells. Lack of PomZ also caused reduced formation of Z‐rings and incorrect positioning of the few Z‐rings formed. PomZ localization is cell cycle regulated, and PomZ accumulates at the division site at midcell after chromosome segregation but prior to FtsZ as well as in the absence of FtsZ. FtsZ displayed cooperative GTP hydrolysis in vitro but did not form detectable filaments in vitro. PomZ interacted with FtsZ in M. xanthus cell extracts. These data show that PomZ is important for Z‐ring formation and is a spatial regulator of Z‐ring formation and cell division. The cell cycle‐dependent localization of PomZ at midcell provides a mechanism for coupling cell cycle progression and Z‐ring formation. Moreover, the data suggest that PomZ is part of a system that recruits FtsZ to midcell, thereby, restricting Z‐ring formation to this position.     

Perpendicular planes of FtsZ arcs in spheroidal Escherichia coli cells

Division planes in Escherichia coli, usually restricted to one dimension of the rod-shaped cell, were induced at all possible planes by transforming the cells to spheroids with mecillinam (inactivating PbpA). Such cells displayed many nucleoids and arcs of FtsZ, genetically tagged to green fluorescent protein, that developed to rings at constriction sites all around their surface. These observations are consistent with the view (Woldringh et al., J. Bacteriol. 176 (1994) 6030-6038) that nucleoids, forced during replication to segregate in the length axis of the cell by the rigid bacillary envelope, induce assembly of FtsZ to division rings in between them.     

RefZ Facilitates the Switch from Medial to Polar Division during Spore Formation in Bacillus subtilis

During sporulation, Bacillus subtilis redeploys the division protein FtsZ from midcell to the cell poles, ultimately generating an asymmetric septum. Here, we describe a sporulation-induced protein, RefZ, that facilitates the switch from a medial to a polar FtsZ ring placement. The artificial expression of RefZ during vegetative growth converts FtsZ rings into FtsZ spirals, arcs, and foci, leading to filamentation and lysis. Mutations in FtsZ specifically suppress RefZ-dependent division inhibition, suggesting that RefZ may target FtsZ. During sporulation, cells lacking RefZ are delayed in polar FtsZ ring formation, spending more time in the medial and transition stages of FtsZ ring assembly. A RefZ-green fluorescent protein (GFP) fusion localizes in weak polar foci at the onset of sporulation and as a brighter midcell focus at the time of polar division. RefZ has a TetR DNA binding motif, and point mutations in the putative recognition helix disrupt focus formation and abrogate cell division inhibition. Finally, chromatin immunoprecipitation assays identified sites of RefZ enrichment in the origin region and near the terminus. Collectively, these data support a model in which RefZ helps promote the switch from medial to polar division and is guided by the organization of the chromosome. Models in which RefZ acts as an activator of FtsZ ring assembly near the cell poles or as an inhibitor of the transient medial ring at midcell are discussed.     

FtsZ rings in mukB mutants with or without the Min system

The site of cell division in Escherichia coli is defined by formation of the Z ring between the two segregated daughter nucleoids. Positioning of the Z ring, composed of the highly conserved and tubulin-like FtsZ protein, appears to be negatively regulated by both the nucleoid and the oscillating MinCD inhibitor proteins. MukB protein is probably involved in nucleoid condensation, and in the absence of MukB, the negative effect of the nucleoid on Z rings appears to be partially suppressed. In this study, we examined the localization of Z rings in cells lacking both the Min system and MukB. In the Deltamin DeltamukB double null mutant, essentially all nucleoid-free zones, either at the cell poles or at non-polar sites between nucleoids, contained Z rings. However, a significant proportion of Z rings also formed on top of nucleoids. Interestingly, Z ring clusters often formed at gaps between nucleoids, and some of the rings within the clusters were clearly positioned on top of nucleoids. These results provide further evidence that the negative topological effect of nucleoids in cells lacking MukB is partially but not totally suppressed, and that the absence of the Min system allows more promiscuous Z ring formation.     

ROLE OF MULTIPLE FTSZ RINGS IN CHLOROPLAST DIVISION UNDER OLIGOTROPHIC AND EUTROPHIC CONDITIONS IN THE UNICELLULAR GREEN ALGA NANNOCHLORIS BACILLARIS (CHLOROPHYTA,TREBOUXIOPHYCEAE)1

Chloroplasts of the unicellular green alga Nannochloris bacillaris Naumann cultured under nutrient‐enriched conditions have multiple rings of FtsZ, a prokaryote‐derived chloroplast division protein. We previously reported that synthesis of excess chloroplast DNA and formation of multiple FtsZ rings occur simultaneously. To clarify the role of multiple FtsZ rings in chloroplast division, we investigated chloroplast DNA synthesis and ring formation in cells cultured under various culture conditions. Cells transferred from a nutrient‐enriched medium to an inorganic medium in the light showed a drop in cell division rate, a reduction in chloroplast DNA content, and changes in the shape of chloroplast nucleoids as cells divided. We then examined DNA synthesis by immunodetecting BrdU incorporated into DNA strands using the anti‐BrdU antibody. BrdU‐labeled nuclei were clearly observed in cells 48 h after transfer into the inorganic medium, while only weak punctate signals were visible in the chloroplasts. In parallel, the number of FtsZ rings decreased from 6 to only 1. When the cells were transferred from an inorganic medium to a nutrient‐enriched medium, the number of cells increased only slightly in the first 12 h after transfer; after this time, however, they started to divide more quickly and increased exponentially. Chloroplast nucleoids changed from punctate to rod‐like structures, and active chloroplast DNA synthesis and FtsZ ring formation were observed. On the basis of our results, we conclude that multiple FtsZ ring assembly and chloroplast DNA duplication under nutrient‐rich conditions facilitate chloroplast division after transfer to oligotrophic conditions without further duplication of chloroplast DNA and formation of new FtsZ rings.     

The new gene mukB codes for a 177 kd protein with coiled-coil domains involved in chromosome partitioning of E. coli.

An Escherichia coli temperature sensitive mutant which produces spontaneously normal size anucleate cells at low temperature was isolated. The mutant is defective in a previously undescribed gene, named mukB, located at 21 min on the chromosome. The mukB gene codes for a large protein (approximately 180 kd). A 1534 amino acid protein (176,826 daltons) was deduced from the nucleotide sequence of the mukB gene. Computer analysis revealed that the predicted MukB protein has distinct domains: an amino-terminal globular domain containing a nucleotide binding sequence, a central region containing two alpha-helical coiled-coil domains and one globular domain, and a carboxyl-terminal globular domain which is rich in Cys, Arg and Lys. A 180 kd protein detected in wild-type cell extracts by electrophoresis is absent in mukB null mutants. Although the null mutants are not lethal at low temperature, the absence of MukB leads to aberrant chromosome partitioning. At high temperature the mukB null mutants cannot form colonies and many nucleoids are distributed irregularly along elongated cells. We conclude that the MukB protein is required for chromosome partitioning in E. coli.     

The division inhibitor EzrA contains a seven-residue patch required for maintaining the dynamic nature of the medial FtsZ ring

The essential cytoskeletal protein FtsZ assembles into a ring-like structure at the nascent division site and serves as a scaffold for the assembly of the prokaryotic division machinery. We previously characterized EzrA as an inhibitor of FtsZ assembly in Bacillus subtilis. EzrA interacts directly with FtsZ to prevent aberrant FtsZ assembly and cytokinesis at cell poles. EzrA also concentrates at the cytokinetic ring in an FtsZ-dependent manner, although its precise role at this position is not known. Here, we identified a conserved patch of amino acids in the EzrA C terminus that is essential for localization to the FtsZ ring. Mutations in this patch (designated the “QNR patch”) abolish EzrA localization to midcell but do not significantly affect EzrA's ability to inhibit FtsZ assembly at cell poles. ezrA QNR patch mutant cells exhibit stabilized FtsZ assembly at midcell and are significantly longer than wild-type cells, despite lacking extra FtsZ rings. These results indicate that EzrA has two distinct activities in vivo: (i) preventing aberrant FtsZ ring formation at cell poles through inhibition of de novo FtsZ assembly and (ii) maintaining proper FtsZ assembly dynamics within the medial FtsZ ring, thereby rendering it sensitive to the factors responsible for coordinating cell growth and cell division.     

SlmA Antagonism of FtsZ Assembly Employs a Two-pronged Mechanism like MinCD

Assembly of the Z-ring over unsegregated nucleoids is prevented by a process called nucleoid occlusion (NO), which in Escherichia coli is partially mediated by SlmA. SlmA is a Z ring antagonist that is spatially regulated and activated by binding to specific DNA sequences (SlmA binding sites, SBSs) more abundant in the origin proximal region of the chromosome. However, the mechanism by which SBS bound SlmA (activated form) antagonizes Z ring assembly is controversial. Here, we report the isolation and characterization of two FtsZ mutants, FtsZ-K190V and FtsZ-D86N that confer resistance to activated SlmA. In trying to understand the basis of resistance of these mutants, we confirmed that activated SlmA antagonizes FtsZ polymerization and determined these mutants were resistant, even though they still bind SlmA. Investigation of SlmA binding to FtsZ revealed activated SlmA binds to the conserved C-terminal tail of FtsZ and that the ability of activated SlmA to antagonize FtsZ assembly required the presence of the tail. Together, these results lead to a model in which SlmA binding to an SBS is activated to bind the tail of FtsZ resulting in further interaction with FtsZ leading to depolymerization of FtsZ polymers. This model is strikingly similar to the model for the inhibitory mechanism of the spatial inhibitor MinCD.     

MipZ, a spatial regulator coordinating chromosome segregation with cell division in Caulobacter

Correct positioning of the division plane is a prerequisite for the generation of daughter cells with a normal chromosome complement. Here, we present a mechanism that coordinates assembly and placement of the FtsZ cytokinetic ring with bipolar localization of the newly duplicated chromosomal origins in Caulobacter. After replication of the polarly located origin region, one copy moves rapidly to the opposite end of the cell in an MreB-dependent manner. A previously uncharacterized essential protein, MipZ, forms a complex with the partitioning protein ParB near the origin of replication and localizes with the duplicated origin regions to the cell poles. MipZ directly interferes with FtsZ polymerization, thereby restricting FtsZ ring formation to midcell, the region of lowest MipZ concentration. The cellular localization of MipZ thus serves the dual function of positioning the FtsZ ring and delaying formation of the cell division apparatus until chromosome segregation has initiated.     

Structure, function and evolution of the mitochondrial division apparatus

Mitochondria are derived from free-living alpha-proteobacteria that were engulfed by eukaryotic host cells through the process of endosymbiosis, and therefore have their own DNA which is organized using basic proteins to form organelle nuclei (nucleoids). Mitochondria divide and are split amongst the daughter cells during cell proliferation. Their division can be separated into two main events: division of the mitochondrial nuclei and division of the matrix (the so-called mitochondrial division, or mitochondriokinesis). In this review, we first focus on the cytogenetical relationships between mitochondrial nuclear division and mitochondriokinesis. Mitochondriokinesis occurs after mitochondrial nuclear division, similar to bacterial cytokinesis. We then describe the fine structure and dynamics of the mitochondrial division ring (MD ring) as a basic morphological background for mitochondriokinesis. Electron microscopy studies first identified a small electron-dense MD ring in the cytoplasm at the constriction sites of dividing mitochondria in the slime mold Physarum polycephalum, and then two large MD rings (with outer cytoplasmic and inner matrix sides) in the red alga Cyanidioschyzon merolae. Now MD rings have been found in all eukaryotes. In the third section, we describe the relationships between the MD ring and the FtsZ ring descended from ancestral bacteria. Other than the GTPase, FtsZ, mitochondria have lost most of the proteins required for bacterial cytokinesis as a consequence of endosymbiosis. The FtsZ protein forms an electron transparent ring (FtsZ or Z ring) in the matrix inside the inner MD ring. For the fourth section, we describe the dynamic association between the outer MD ring with a ring composed of the eukaryote-specific GTPase dynamin. Recent studies have revealed that eukaryote-specific GTPase dynamins form an electron transparent ring between the outer membrane and the MD ring. Thus, mitochondriokinesis is thought to be controlled by a mitochondrial division (MD) apparatus including a dynamic trio, namely the FtsZ, MD and dynamin rings, which consist of a chimera of rings from bacteria and eukaryotes in primitive organisms. Since the genes for the MD ring and dynamin rings are not found in the prokaryotic genome, the host genomes may make these rings to actively control mitochondrial division. In the fifth part, we focus on the dynamic changes in the formation and disassembly of the FtsZ, MD and dynamin rings. FtsZ rings are digested during a later period of mitochondrial division and then finally the MD and dynamin ring apparatuses pinched off the daughter mitochondria, supporting the idea that the host genomes are responsible for the ultimate control of mitochondrial division. We discuss the evolution, from the original vesicle division (VD) apparatuses to VD apparatuses including classical dynamin rings and MD apparatuses. It is likely that the MD apparatuses involving the dynamic trio evolved into the plastid division (PD) apparatus in Bikonta, while in Opisthokonta, the MD apparatus was simplified during evolution and may have branched into the mitochondrial fusion apparatus. Finally, we describe the possibility of intact isolation of large MD/PD apparatuses, the identification of all their proteins and their related genes using C. merolae genome information and TOF-MS analyses. These results will assist in elucidating the universal mechanism and evolution of MD, PD and VD apparatuses.     

A homologue of the bacterial cell division site-determining factor MinD mediates placement of the chloroplast division apparatus

BACKGROUND: Chloroplast division in plant cells occurs by binary fission, yielding two daughter plastids of equal size. Previously, we reported that two Arabidopsis homologues of FtsZ, a bacterial protein that forms a cytokinetic ring during cell division, are essential for plastid division in plants, and may be involved in the formation of plastid-dividing rings on both the stromal and cytosolic surfaces of the chloroplast envelope membranes. In bacteria, positioning of the FtsZ ring at the center of the cell is mediated in part by the protein MinD. Here, we identified AtMinD1, an Arabidopsis homologue of MinD, and investigated whether positioning of the plastid-division apparatus at the plastid midpoint might involve a mechanism similar to that in bacteria. RESULTS: Sequence analysis and in vitro chloroplast import experiments indicated that AtMinD1 contains a transit peptide that targets it to the chloroplast. Transgenic Arabidopsis plants with reduced AtMinD1 expression exhibited variability in chloroplast size and number and asymmetrically constricted chloroplasts, strongly suggesting that the plastid-division machinery is misplaced. Overexpression of AtMinD1 inhibited chloroplast division. These phenotypes resemble those of bacterial mutants with altered minD expression. CONCLUSIONS: Placement of the plastid-division machinery at the organelle midpoint requires a plastid-targeted form of MinD. The results are consistent with a model whereby assembly of the division apparatus is initiated inside the chloroplast by the plastidic form of FtsZ, and suggest that positioning of the cytosolic components of the apparatus is specified by the position of the plastidic components.     

Effects of mutations in Arabidopsis FtsZ1 on plastid division, FtsZ ring formation and positioning, and FtsZ filament morphology in vivo

In plants, chloroplast division FtsZ proteins have diverged into two families, FtsZ1 and FtsZ2. FtsZ1 is more divergent from its bacterial counterparts and lacks a C-terminal motif conserved in most other FtsZs. To begin investigating FtsZ1 structure-function relationships, we first identified a T-DNA insertion mutation in the single FtsZ1 gene in Arabidopsis thaliana, AtFtsZ1-1. Homozygotes null for FtsZ1, though impaired in chloroplast division, could be isolated and set seed normally, indicating that FtsZ1 is not essential for viability. We then mapped five additional atftsZ1-1 alleles onto an FtsZ1 structural model and characterized chloroplast morphologies, FtsZ protein levels and FtsZ filament morphologies in young and mature leaves of the corresponding mutants. atftsZ1-1(G267R), atftsZ1-1(R298Q) and atftsZ1-1(Delta404-433) exhibit reduced FtsZ1 accumulation but wild-type FtsZ2 levels. The semi-dominant atftsZ1-1(G267R) mutation caused the most severe phenotype, altering a conserved residue in the predicted T7 loop. atftsZ1-1(G267R) protein accumulates normally in young leaves but is not detected in rings or filaments. atftsZ1-1(R298Q) has midplastid FtsZ1-containing rings in young leaves, indicating that R298 is not critical for ring formation or positioning despite its conservation. atftsZ1-1(D159N) and atftsZ1-1(G366A) both have overly long, sometimes spiral-like FtsZ filaments, suggesting that FtsZ dynamics are altered in these mutants. However, atftsZ1-1(D159N) exhibits loss of proper midplastid FtsZ positioning while atftsZ1-1(G366A) does not. Finally, truncation of the FtsZ1 C-terminus in atftsZ1-1(Delta404-433) impairs chloroplast division somewhat but does not prevent midplastid Z ring formation. These alleles will facilitate understanding of how the in vitro biochemical properties of FtsZ1 are related to its in vivo function.     

FtsZ ring formation at the chloroplast division site in plants

Among the events that accompanied the evolution of chloroplasts from their endosymbiotic ancestors was the host cell recruitment of the prokaryotic cell division protein FtsZ to function in chloroplast division. FtsZ, a structural homologue of tubulin, mediates cell division in bacteria by assembling into a ring at the midcell division site. In higher plants, two nuclear-encoded forms of FtsZ, FtsZ1 and FtsZ2, play essential and functionally distinct roles in chloroplast division, but whether this involves ring formation at the division site has not been determined previously. Using immunofluorescence microscopy and expression of green fluorescent protein fusion proteins in Arabidopsis thaliana, we demonstrate here that FtsZ1 and FtsZ2 localize to coaligned rings at the chloroplast midpoint. Antibodies specific for recognition of FtsZ1 or FtsZ2 proteins in Arabidopsis also recognize related polypeptides and detect midplastid rings in pea and tobacco, suggesting that midplastid ring formation by FtsZ1 and FtsZ2 is universal among flowering plants. Perturbation in the level of either protein in transgenic plants is accompanied by plastid division defects and assembly of FtsZ1 and FtsZ2 into filaments and filament networks not observed in wild-type, suggesting that previously described FtsZ-containing cytoskeletal-like networks in chloroplasts may be artifacts of FtsZ overexpression.     

Positioning of chemosensory proteins and FtsZ through the Rhodobacter sphaeroides cell cycle

Bacterial chemotaxis depends on signalling through large protein complexes. Each cell must inherit a complex on division, suggesting some co‐ordination with cell division. In Escherichia coli the membrane‐spanning chemosensory complexes are polar and new static complexes form at pre‐cytokinetic sites, ensuring positioning at the new pole after division and suggesting a role for the bacterial cytoskeleton. Rhodobacter sphaeroides has both membrane‐associated and cytoplasmic, chromosome‐associated chemosensory complexes. We followed the relative positions of the two chemosensory complexes, FtsZ and MreB in aerobic and in photoheterotrophic R. sphaeroides cells using fluorescence microscopy. FtsZ forms polar spots after cytokinesis, which redistribute to the midcell forming nodes from which FtsZ extends circumferentially to form the Z‐ring. Membrane‐associated chemosensory proteins form a number of dynamic unit‐clusters with mature clusters containing about 1000 CheW₃ proteins. Individual clusters diffuse randomly within the membrane, accumulating at new poles after division but not colocalizing with either FtsZ or MreB. The cytoplasmic complex colocalizes with FtsZ at midcells in new‐born cells. Before cytokinesis one complex moves to a daughter cell, followed by the second moving to the other cell. These data indicate that two homologous complexes use different mechanisms to ensure partitioning, and neither complex utilizes FtsZ or MreB for positioning.