Escherichia coli mraR gene involved in cell growth and division

Cell wall synthesis and remodelling dynamics determine division site architecture and cell shape in Escherichia coli

The bacterial division apparatus catalyses the synthesis and remodelling of septal peptidoglycan (sPG) to build the cell wall layer that fortifies the daughter cell poles. Understanding of this essential process has been limited by the lack of native three-dimensional views of developing septa. Here, we apply state-of-the-art cryogenic electron tomography (cryo-ET) and fluorescence microscopy to visualize the division site architecture and sPG biogenesis dynamics of the Gram-negative bacterium Escherichia coli. We identify a wedge-like sPG structure that fortifies the ingrowing septum. Experiments with strains defective in sPG biogenesis revealed that the septal architecture and mode of division can be modified to more closely resemble that of other Gram-negative (Caulobacter crescentus) or Gram-positive (Staphylococcus aureus) bacteria, suggesting that a conserved mechanism underlies the formation of different septal morphologies. Finally, analysis of mutants impaired in amidase activation (ΔenvC ΔnlpD) showed that cell wall remodelling affects the placement and stability of the cytokinetic ring. Taken together, our results support a model in which competition between the cell elongation and division machineries determines the shape of cell constrictions and the poles they form. They also highlight how the activity of the division system can be modulated to help generate the diverse array of shapes observed in the bacterial domain.

Roles for both FtsA and the FtsBLQ subcomplex in FtsN-stimulated cell constriction in Escherichia coli

Escherichia coli FtsN is a bitopic membrane protein that is essential for triggering active cell constriction. A small periplasmic subdomain ((E) FtsN) is required and sufficient for function, but its mechanism of action is unclear. We isolated extragenic (E) FtsN*-suppressing mutations that restore division in cells producing otherwise non-functional variants of FtsN. These mapped to the IC domain of FtsA in the cytoplasm and to small subdomains of the FtsB and FtsL proteins in the periplasm. All FtsB and FtsL variants allowed survival without (E) FtsN, but many then imposed a new requirement for interaction between the cytoplasmic domain of FtsN ((N) FtsN) and FtsA. Alternatively, variants of FtsA, FtsB or FtsL acted synergistically to allow cell division in the complete absence of FtsN. Strikingly, moreover, substitution of a single residue in FtsB (E56) proved sufficient to rescue ΔftsN cells as well. In FtsN(+) cells, (E) FtsN*-suppressing mutations promoted cell fission at an abnormally small cell size, and caused cell shape and integrity defects under certain conditions. This and additional evidence support a model in which FtsN acts on either side of the membrane to induce a conformational switch in both FtsA and the FtsBLQ subcomplex to de-repress septal peptidoglycan synthesis and membrane invagination.

A role for the FtsQLB complex in cytokinetic ring activation revealed by an ftsL allele that accelerates division

The cytokinetic apparatus of bacteria is initially formed by the polymerization of the tubulin-like FtsZ protein into a ring structure at midcell. This so-called Z-ring facilitates the recruitment of many additional proteins to the division site to form the mature divisome machine. Although the assembly pathway leading to divisome formation has been well characterized, the mechanisms that trigger cell constriction remain unclear. In this report, we study a 'forgotten' allele of ftsL from Escherichia coli, which encodes a conserved division gene of unknown function. We discovered that this allele promotes the premature initiation of cell division. Further analysis also revealed that the mutant bypasses the requirement for the essential division proteins ZipA, FtsK and FtsN, and partially bypasses the need for FtsA. These findings suggest that rather than serving simply as a protein scaffold within the divisome, FtsL may play a more active role in the activation of the machine. Our results support a model in which FtsL, along with its partners FtsB and FtsQ, function as part of a sensing mechanism that promotes the onset of cell wall remodeling processes needed for the initiation of cell constriction once assembly of the divisome complex is deemed complete.

Identification of a novel function for the FtsL cell division protein from Escherichia coli K12

Analysis of the essential cell division protein FtsL demonstrates the partial conservation of a cysteine-pair within the trans-membrane region which itself is flanked by histidine-pairs in the cytosol and periplasm. Similar arrangements of such amino acids are seen in proteins known to transport/bind metal ions in biological systems. Heterologous expression of ftsL in Escherichia coli K12 confers a Zn(II)-sensitive phenotype and alteration of the candidate metal-ion binding residues cysteine or histidine substantially alters this phenotype. Whilst the cysteine/histidine replacement derivatives of ftsL were able to complement an otherwise ftsL-null strain, the derivative carrying ftsL lacking the cysteine pair was sensitive to raised metal-ion concentrations in the media. We show that ftsL can confer a metal-ion sensitive phenotype and that trans-membrane cysteine residues play a role in FtsL function in elevated metal-ion concentrations.

Streptomyces coelicolor genes ftsL and divIC play a role in cell division but are dispensable for colony formation

We have characterized homologues of the bacterial cell division genes ftsL and divIC in the gram-positive mycelial bacterium Streptomyces coelicolor A3(2). We show by deletion-insertion mutations that ftsL and divIC are dispensable for growth and viability in S. coelicolor. When mutant strains were grown on a conventional rich medium (R2YE, containing high sucrose), inactivation of either ftsL or divIC resulted in the formation of aerial hyphae with partially constricted division sites but no clear separation of prespore compartments. Surprisingly, this phenotype was largely suppressed when strains were grown on minimal medium or sucrose-free R2YE, where division sites in many aerial hyphae had finished constricting and chains of spores were evident. Thus, osmolarity appears to affect the severity of the division defect. Furthermore, double mutant strains deleted for both ftsL and divIC are viable and have medium-dependent phenotypes similar to that of the single mutant strains, suggesting that functions performed by FtsL and DivIC are not absolutely required for septation during growth and sporulation. Alternatively, another division protein may partially compensate for the loss of both FtsL and DivIC on minimal medium or sucrose-free R2YE. Finally, based on transmission electron microscopy observations, we propose that FtsL and DivIC are involved in coordinating symmetrical annular ingrowth of the invaginating septum.

Multiple interactions between the transmembrane division proteins of Bacillus subtilis and the role of FtsL instability in divisome assembly

About 11 essential proteins assemble into a ring structure at the surface of the cell to bring about cytokinesis in bacteria. Several of these proteins have their major domains located outside the membrane, forming an assembly that we call the outer ring (OR). Previous work on division in Bacillus subtilis has shown that four of the OR proteins-FtsL, DivIC, DivIB, and PBP 2B-are interdependent for assembly. This contrasts with the mainly linear pathway for the equivalent proteins in Escherichia coli. Here we show that the interdependent nature of the B. subtilis pathway could be due to effects on FtsL and DivIC stability and that DivIB is an important player in regulating this turnover. Two-hybrid approaches suggest that a multiplicity of protein-protein interactions contribute to the assembly of the OR. DivIC is unusual in interacting strongly only with FtsL. We propose a model for the formation of the OR through the mutual association of the membrane proteins directed by the cytosolic inner-ring proteins.

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.

Principles of macromolecular organization and cell function in bacteria and archaea

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

Intrinsic instability of the essential cell division protein FtsL of Bacillus subtilis and a role for DivIB protein in FtsL turnover

Cell division in most eubacteria is driven by an assembly of about eight conserved division proteins. These proteins form a ring structure that constricts in parallel with the formation of the division septum. Here, we show that one of the division proteins, FtsL, is highly unstable. We also show that the protein is targeted to the ring structure and that targeting occurs in concert with the recruitment of several other membrane-associated division proteins. FtsL stability is further reduced in the absence of DivIB protein (probably homologous to E. coli FtsQ) at high temperature, suggesting that DivIB is involved in the control of FtsL turnover. The reduced stability of FtsL may explain the temperature dependence of divIB mutants, because their phenotype can be suppressed by overexpression of FtsL. The results provide new insights into the roles of the FtsL and DivIB proteins in bacterial cell division.

mraW, an essential gene at the dcw cluster of Escherichia coli codes for a cytoplasmic protein with methyltransferase activity

Three new open reading frames, mraZ, mraW and mraR (also called ftsL), were revealed by DNA sequencing immediately upstream of gene pbpB in the dcw cluster of Escherichia coli. We have found that mraW and mraZ are active genes, coding for two proteins with relative molecular masses of 34 800 and 17 300, respectively. MraW is a cytoplasmic protein that under overproduction condition is also loosely bound to the membrane. Soluble MraW was purified up to 90% by a single high performance electrophoresis (HPEC) step from an extract of an overproducing strain. The protein exhibits a S-adenosyl-dependent methyltransferase activity on membrane-located substrates.

Linkage map of Escherichia coli K-12, edition 10: the traditional map

This map is an update of the edition 9 map by Berlyn et al. (M. K. B. Berlyn, K. B. Low, and K. E. Rudd, p. 1715-1902, in F. C. Neidhardt et al., ed., Escherichia coli and Salmonella: cellular and molecular biology, 2nd ed., vol. 2, 1996). It uses coordinates established by the completed sequence, expressed as 100 minutes for the entire circular map, and adds new genes discovered and established since 1996 and eliminates those shown to correspond to other known genes. The latter are included as synonyms. An alphabetical list of genes showing map location, synonyms, the protein or RNA product of the gene, phenotypes of mutants, and reference citations is provided. In addition to genes known to correspond to gene sequences, other genes, often older, that are described by phenotype and older mapping techniques and that have not been correlated with sequences are included.

Characterization of the essential cell division gene ftsL(yIID) of Bacillus subtilis and its role in the assembly of the division apparatus

We have identified the Bacillus subtilis homologue of the essential cell division gene, ftsL, of Escherichia coli. Repression of ftsL in a strain engineered to carry a conditional promoter results in cell filamentation, with a near immediate arrest of cell division. The filaments show no sign of invagination, indicating that division is blocked at an early stage. FtsL is also shown to be required for septation during sporulation, and depletion of FtsL blocks the activation but not the synthesis of the prespore-specific sigma factor, sigmaF. Immunofluorescence microscopy shows that depletion of FtsL has little or no effect on FtsZ ring formation, but the assembly of other division proteins, DivIB and DivIC, at the site of division is prevented. Repression of FtsL also results in a rapid loss of DivIC protein, indicating that DivIC stability is dependent on the presence of FtsL, in turn suggesting that FtsL is intrinsically unstable. The instability of one or more components of the division apparatus may be important for the cyclic assembly/disassembly of the division apparatus.

Bacterial cell division

Bacteria usually divide by building a central septum across the middle of the cell. This review focuses on recent results indicating that the tubulin-like FtsZ protein plays a central role in cytokinesis as a major component of a contractile cytoskeleton. Assembly of this cytoskeletal element abutting the membrane is a key point for regulation. The characterization of FtsZ homologues in Mycoplasmas, Archaea, and chloroplasts implies that the constriction mechanism is conserved and that FtsZ can constrict in the absence of peptidoglycan synthesis. In most Eubacteria, the internal cytoskeleton must also regulate synthesis of septal peptidoglycan. The Escherichia coli septum-specific penicillin-binding protein 3 (PBP3) forms a complex with other enzymes involved in murein metabolism, suggesting a centrally located transmembrane complex capable of splicing multiple new strands of peptidoglycan into the cell wall. Important questions remain about the spatial and temporal control of bacterial division.

A promoter for the first nine genes of the Escherichia coli mra cluster of cell division and cell envelope biosynthesis genes, including ftsI and ftsW

We constructed a null allele of the ftsI gene encoding penicillin-binding protein 3 of Escherichia coli. It caused blockage of septation and loss of viability when expression of an extrachromosomal copy of ftsI was repressed, providing a final proof that ftsI is an essential cell division gene. In order to complement this null allele, the ftsI gene cloned on a single-copy mini-F plasmid required a region 1.9 kb upstream, which was found to contain a promoter sequence that could direct expression of a promoterless lacZ gene on a mini-F plasmid. This promoter sequence lies at the beginning of the mra cluster in the 2 min region of the E. coli chromosome, a cluster of 16 genes which, except for the first 2, are known to be involved in cell division and cell envelope biosynthesis. Disruption of this promoter, named the mra promoter, on the chromosome by inserting the lac promoter led to cell lysis in the absence of a lac inducer. The defect was complemented by a plasmid carrying a chromosomal fragment ranging from the mra promoter to ftsW, the fifth gene downstream of ftsI, but not by a plasmid lacking ftsW. Although several potential promoter sequences in this region of the mra cluster have been reported, we conclude that the promoter identified in this study is required for the first nine genes of the cluster to be fully expressed.

A complex four-gene operon containing essential cell division gene pbpB in Bacillus subtilis

We have cloned and sequenced the promoter-proximal region of the Bacillus subtilis operon containing the pbpB gene, encoding essential penicillin-binding protein PBP2B. The first two genes in the operon, designated yllB and yllC, are significantly similar to genes of unknown function similarly positioned upstream of pbpB in Escherichia coli. Both B. subtilis genes are shown to be nonessential. The third B. subtilis gene, yllD, is essential, as is the correspondingly positioned ftsL gene of E. coli. The predicted product of yllD is similar to FtsL in size and distribution of charged residues but is not significantly related in primary amino acid sequence. The major promoter for the cluster lies upstream of the first gene, yllB, but at least one minor promoter lies within the yllC gene. The operon is transcribed throughout growth at a low level.

Molecular characterization of the eps gene cluster of Pseudomonas solanacearum and its transcriptional regulation at a single promoter

Production of EPS I, an unusual exopolysaccharide virulence factor of the phytopathogen Pseudomonas solanacearum, requires the 18 kb eps gene cluster. DNA sequence analysis of the first seven genes of eps (epsAPBCDEF), subcellular localization of their products in maxicells, and phoA fusion analysis showed that: (i) epsA, epsB, epsE, and epsF encode exported or membrane-associated proteins probably involved in polymerization and/or export of EPS I; (ii) epsC and epsD encode soluble enzymes probably involved in synthesis of sugar components of EPS I (N-acetylgalactosaminuronic acid and possibly N-acetyltrideoxygalactose, respectively); and (iii) epsP probably encodes a phosphatase involved in EPS I production in an unknown way. Non-polar insertional mutagenesis showed that most, if not all, of these eps genes are absolutely required for production of EPS I. Using random eps::lacZ fusions and primer extension we located a transcription start site and promoter upstream of epsA. Analysis of a plasmid with this promoter fused to lacZ showed that a 140 bp regulatory region upstream of the eps transcription start site was sufficient for normal regulation of eps transcription by the multicomponent virulence gene regulatory network of P. solanacearum. Deletion of this eps promoter from a plasmid-borne epsAPBCDE::lacZ fusion reduced its expression 10-fold, indicating that this promoter alone is responsible for regulated transcription of an eps operon composed of at least epsAPBCDE. Analysis of genomic and plasmid-borne eps::lacZ fusions suggested that most remaining eps genes are part of this same operon or, and this is less likely, comprise a second co-ordinately regulated eps operon.

A complex network regulates expression of eps and other virulence genes of Pseudomonas solanacearum

We have discovered an unusual and complex regulatory network used by the phytopathogen Pseudomonas solanacearum to control transcription of eps, which encodes for production of its primary virulence factor, the exopolysaccharide EPS I. The major modules of this network were shown to be three separate signal transduction systems: PhcA, a LysR-type transcriptional regulator, an dual two-component regulatory systems, VsrA/VsrD and VsrB/VsrC. Using lacZ fusions and RNA analysis, we found that both PhcA and VsrA/VsrD control transcription of another network component, xpsR, which in turn acts in conjunction with vsrB/vsrC to increase transcription of the eps promoter by > 25-fold. Moreover, gel shift DNA binding assays showed that PhcA specifically binds to the xpsR promoter region. Thus, the unique XpsR protein interconnects the three signal transduction systems, forming a network for convergent control of EPS I in simultaneous response to multiple environmental inputs. In addition, we demonstrate that each individual signaling system of the network also acts independently to divergently regulate other unique sets of virulence factors. The purpose of this complex network may be to allow this phytopathogen to both coordinately or independently regulate diverse virulence factors in order to cope with the dynamic situations and conditions encountered during interactions with plants.

GTP-dependent polymerization of Escherichia coli FtsZ protein to form tubules

The FtsZ protein is a GTPase that is essential for cell division in Escherichia coli. During cytokinesis, FtsZ localizes to a ring at the leading edge of septum synthesis. We report the GTP-dependent polymerization of purified FtsZ measured by sedimentation and light scattering. Electron microscopy of polymerized FtsZ revealed structures including tubules 14-20 nm in diameter with longitudinal arrays of protofilaments. FtsZ depolymerized upon removal of GTP and repolymerized after subsequent GTP addition. Mutant FtsZ84 protein polymerized inefficiently, suggesting that polymerization is important for the cellular role of FtsZ in division. The possibility that tubules of FtsZ protein form a cytoskeleton involved in septum synthesis is consistent with our data.

Cloning and sequencing of the cell division gene pbpB, which encodes penicillin-binding protein 2B in Bacillus subtilis

The pbpB gene, which encodes penicillin-binding protein (PBP) 2B of Bacillus subtilis, has been cloned, sequenced, mapped, and mutagenized. The sequence of PBP 2B places it among the class B high-molecular-weight PBPs. It appears to contain three functional domains: an N-terminal domain homologous to the corresponding domain of other class B PBPs, a penicillin-binding domain, and a lengthy carboxy extension. The PBP has a noncleaved signal sequence at its N terminus that presumably serves as its anchor in the cell membrane. Previous studies led to the hypothesis that PBP 2B is required for both vegetative cell division and sporulation septation. Its sequence, map site, and mutant phenotype support this hypothesis. PBP 2B is homologous to PBP 3, the cell division protein encoded by pbpB of Escherichia coli. Moreover, both pbpB genes are located in the same relative position within a cluster of cell division and cell wall genes on their respective chromosomes. However, immediately adjacent to the B. subtilis pbpB gene is spoVD, which appears to be a sporulation-specific homolog of pbpB. Inactivation of SpoVD blocked synthesis of the cortical peptidoglycan in the spore, whereas carboxy truncation of PBP 2B caused cells to grow as filaments. Thus, it appears that a gene duplication has occurred in B. subtilis and that one PBP has evolved to serve a common role in septation during both vegetative growth and sporulation, whereas the other PBP serves a specialized role in sporulation.

Chromosome partition in Escherichia coli

The past year has seen important genetic and biochemical advances in our understanding of the mechanisms that are involved in chromosome partition into two daughter cells in Escherichia coli. Topoisomerase IV and XerCD recombinase have been shown to be required for the unlinking of replicated chromosomes. MukB, an alpha-helical coiled-coil protein, has been shown to be involved in chromosome partition, and this is the first candidate for a bacterial motor protein. Another protein, FtsZ, has been shown to form a constriction ring in cell division and may also relate to chromosome partition.

Escherichia coli cell division

Recent progress in the molecular analysis of bacterial septation and chromosome partitioning suggests that these processes may involve cytoskeletal elements previously thought to be present only in eukaryotic cells. The continued biochemical and genetic analysis of key proteins, such as the tubulin-like FtsZ, should lead to further unravelling of the regulation and mechanism of bacterial cell division.

FtsZ ring in bacterial cytokinesis

FtsZ is localized to a cytokinetic ring at the cell division site in bacteria. In this review a model is discussed that suggests that FtsZ self assembles into a ring at a nucleation site formed on the cytoplasmic membrane under cell-cycle control. This model suggests that formation of the cytokinetic FtsZ ring initiates and coordinates the circumferential invagination of the cytoplasmic membrane and cell wall, leading to formation of the septum. It is also suggested that this process may be conserved among the peptidoglycan-containing eubacteria. In addition, similarities between FtsZ and tubulin are discussed.

Cloning and characterization of ftsN, an essential cell division gene in Escherichia coli isolated as a multicopy suppressor of ftsA12(Ts)

A new cell division gene, ftsN, was identified in Escherichia coli as a multicopy suppressor of the ftsA12(Ts) mutation. Remarkably, multicopy ftsN suppressed ftsI23(Ts) and to a lesser extent ftsQ1(Ts); however, no suppression of the ftsZ84(Ts) mutation was observed. The suppression of ftsA12(Ts), ftsI23(Ts), and ftsQ1(Ts) suggests that FtsN may interact with these gene products during cell division. The ftsN gene was located at 88.5 min on the E. coli genetic map just downstream of the cytR gene. ftsN was essential for cell division, since expression of a conditional null allele led to filamentation and cell death. DNA sequence analysis of the ftsN gene revealed an open reading frame of 319 codons which would encode a protein of 35,725 Da. The predicted gene product had a hydrophobic sequence near its amino terminus similar to the noncleavable signal sequences found in several other Fts proteins. The presumed extracellular domain was unusual in that it was rich in glutamine residues. A 36-kDa protein that was localized to the membrane fraction was detected in minicells containing plasmids with the ftsN gene, confirming that FtsN was a membrane protein.

Chromosome segregation and cytokinesis in bacteria

Substantial progress has recently been made in the understanding of chromosome partitioning and cytokinesis in bacteria. The biochemical properties of some key protein components involved in these processes are beginning to emerge. New evidence supports the recently developed notion that, in prokaryotic cells, basic cell biological processes rely on the activity of previously unidentified cytoskeletal-like elements.

FtsL, an essential cytoplasmic membrane protein involved in cell division in Escherichia coli

We have identified a gene involved in bacterial cell division, located immediately upstream of the ftsI gene in the min 2 region of the Escherichia coli chromosome. This gene, which we named ftsL, was detected through characterization of TnphoA insertions in a plasmid containing this chromosomal region. TnphoA topological analysis and fractionation of alkaline phosphatase fusion proteins indicated that the ftsL gene product is a 13.6-kDa cytoplasmic membrane protein with a cytoplasmic amino terminus, a single membrane-spanning segment, and a periplasmic carboxy terminus. The ftsL gene is essential for cell growth and division. A null mutation in ftsL resulted in inhibition of cell division, formation of long, nonseptate filaments, ultimate cessation of growth, and lysis. Under certain growth conditions, depletion of FtsL or expression of the largest ftsL-phoA fusion produced a variety of cell morphologies, including Y-shaped bacteria, indicating a possible general weakening of the cell wall. The FtsL protein is estimated to be present at about 20 to 40 copies per cell. The periplasmic domain of the protein displays a sequence with features characteristic of leucine zippers, which are involved in protein dimerization.

FtsL, an Essential Cytoplasmic Membrane Protein Involved in Cell Division in Escherichia coli

We have identified a gene involved in bacterial cell division, located immediately upstream of the ftsI gene in the min 2 region of the Escherichia coli chromosome. This gene, which we named ftsL , was detected through characterization of Tn phoA insertions in a plasmid containing this chromosomal region. Tn phoA topological analysis and fractionation of alkaline phosphatase fusion proteins indicated that the ftsL gene product is a 13.6-kDa cytoplasmic membrane protein with a cytoplasmic amino terminus, a single membrane-spanning segment, and a periplasmic carboxy terminus. The ftsL gene is essential for cell growth and division. A null mutation in ftsL resulted in inhibition of cell division, formation of long, nonseptate filaments, ultimate cessation of growth, and lysis. Under certain growth conditions, depletion of FtsL or expression of the largest ftsL-phoA fusion produced a variety of cell morphologies, including Y-shaped bacteria, indicating a possible general weakening of the cell wall. The FtsL protein is estimated to be present at about 20 to 40 copies per cell. The periplasmic domain of the protein displays a sequence with features characteristic of leucine zippers, which are involved in protein dimerization.