Cell division inhibitors SulA and MinCD prevent formation of the FtsZ ring

Mecillinam resistance in Escherichia coli is conferred by loss of a second activity of the AroK protein

Mecillinam, a beta-lactam antibiotic specific to penicillin-binding protein 2 (PBP 2) in Escherichia coli, blocks cell wall elongation and, indirectly, cell division, but its lethality can be overcome by increased levels of ppGpp, the nucleotide effector of the stringent response. We have subjected an E. coli K-12 strain to random insertional mutagenesis with a mini-Tn10 element. One insertion, which was found to confer resistance to mecillinam in relA+ and relA strains, was mapped at 75.5 min on the E. coli map and was located between the promoters and the coding sequence of the aroK gene, which codes for shikimate kinase 1, one of two E. coli shikimate kinases, both of which are involved in aromatic amino acid biosynthesis. The mecillinam resistance conferred by the insertion was abolished in a delta relA delta spoT strain completely lacking ppGpp, and it thus depends on the presence of ppGpp. Furthermore, the insertion increased the ppGpp pool approximately twofold in a relA+ strain. However, this increase was not observed in relA strains, although the insertion still conferred mecillinam resistance in these backgrounds, showing that mecillinam resistance is not due to an increased ppGpp pool. The resistance was also abolished in an ftsZ84(Ts) strain under semipermissive conditions, and the aroK::mini-Tn10 allele partially suppressed ftsZ84(Ts); however, it did not increase the concentration of the FtsZ cell division protein. The insertion greatly decreased or abolished the shikimate kinase activity of AroK in vivo and in vitro. The two shikimate kinases of E. coli are not equivalent; the loss of AroK confers mecillinam resistance, whereas the loss of Arol, does not. Furthermore, the ability of the aroK mutation to confer mecillinam resistance is shown to be independent of polar effects on operon expression and of effects on the availability of aromatic amino acids or shikimic acid. Instead, we conclude that the AroK protein has a second activity, possibly related to cell division regulation, which confers mecillinam sensitivity. We were able to separate the AroK activities mutationally with an aroK mutant allele lacking shikimate kinase activity but still able to confer mecillinam sensitivity.

Multicopy suppressors of the cold-sensitive phenotype of the pcsA68 (dinD68) mutation in Escherichia coli

The Escherichia coli strain cs2-68 is a cold-sensitive (c) mutant that forms a long filamentous cell at 20 degrees C with a large nucleoid mass in its central region. We have recently shown that the pcsA68 mutation causing the cs phenotype is a single-base substitution within the dinD gene, a DNA damage-inducible gene which maps at 82 min. Since null mutants of the pcsA (dinD) gene are viable, with no discernible defect in cell growth, the cs phenotype is attributed to a toxic effect by the mutant protein. In an attempt to identify a target(s) for the toxic pcsA68 mutant protein, we screened for chromosomal fragments on multicopy plasmids that could suppress the cs phenotype. Three different BamHI fragments were found to suppress cold sensitivity, and the lexA, dinG, and dinI genes were identified to be responsible for the suppression in each fragment. DinG shares multiple motifs with many DNA helicases. The complete sequence of dinI revealed that DinI is a small protein of 81 amino acids. It is similar in size and sequence to ImpC of the Salmonella typhimurium plasmid TP110 and to a protein (ORFfs) of the retronphage phi R67, both of which are also under the control of LexA.

FtsZ ring formation in fts mutants

The formation of FtsZ rings (Z rings) in various fts mutants was examined by immunoelectron microscopy and immunofluorescence. In two temperature-sensitive ftsZ mutants which form filaments with smooth morphology, the Z ring was unable to form. In ftsA, ftsI, and ftsQ mutants, which form filaments with an indented morphology, Z rings formed but their contraction was blocked. These results indicate that fully functional ftsA, ftsQ, and ftsI genes are not required for Z-ring formation and are unlikely to have a role in localization of the Z ring. The results also suggest that one function of the Z ring is to localize the activity of other fts gene products.

Mutation in the structural gene for release factor 1 (RF-1) of Salmonella typhimurium inhibits cell division

A temperature-sensitive mutant of Salmonella typhimurium LT2 was isolated. At the nonpermissive temperature cell division stopped and multinucleated filaments were formed. DNA, RNA, or protein synthesis was not affected until after about two generations. Different physiological conditions, such as anaerobiosis and different growth media, suppress the division deficiency at high temperatures. Certain mutations causing a reduced polypeptide chain elongation rate also suppress the division deficiency. The mutation is recessive and shown to be in the structural gene for release factor I (prfA). DNA sequencing of both the wild-type (prfA+) and mutant (prfA101) allele revealed a GC-to-AT transition in codon 168. Like other known prfA mutants, prfA101 can suppress amber mutations. The division defect in the prfA101 mutant strain could not be suppressed by overexpression of the ftsQAZ operon. Moreover, at the nonpermissive temperature the mutant shows a normal heat shock and SOS response and has a normal ppGpp level. We conclude that the prfA101-mediated defect in cell division is not directed through any of these metabolic pathways, which are all known to affect cell division. We speculate that the altered release factor I induces aberrant synthesis of an unidentified protein(s) involved in the elaborate process of septation.

An archaebacterial homologue of the essential eubacterial cell division protein FtsZ

Life falls into three fundamental domains--Archaea, Bacteria, and Eucarya (formerly archaebacteria, eubacteria, and eukaryotes,. respectively). Though Archaea lack nuclei and share many morphological features with Bacteria, molecular analyses, principally of the transcription and translation machineries, have suggested that Archaea are more related to Eucarya than to Bacteria. Currently, little is known about the archaeal cell division apparatus. In Bacteria, a crucial component of the cell division machinery is FtsZ, a GTPase that localizes to a ring at the site of septation. Interestingly, FtsZ is distantly related in sequence to eukaryotic tubulins, which also interact with GTP and are components of the eukaryotic cell cytoskeleton. By screening for the ability to bind radiolabeled nucleotides, we have identified a protein of the hyperthermophilic archaeon Pyrococcus woesei that interacts tightly and specifically with GTP. Furthermore, through screening an expression library of P. woesei genomic DNA, we have cloned the gene encoding this protein. Sequence comparisons reveal that the P. woesei GTP-binding protein is strikingly related in sequence to eubacterial FtsZ and is marginally more similar to eukaryotic tubulins than are bacterial FtsZ proteins. Phylogenetic analyses reinforce the notion that there is an evolutionary linkage between FtsZ and tubulins. These findings suggest that the archaeal cell division apparatus may be fundamentally similar to that of Bacteria and lead us to consider the evolutionary relationships between Archaea, Bacteria, and Eucarya.

Structure, function and controls in microbial division

Several crucial genes required for bacterial division lie close together in a region called the dcw cluster. Within the cluster, gene expression is subject to complex transcriptional regulation, which serves to adjust the cell cycle in response to growth rate. The pivotally important FtsZ protein, which is needed to initiate division, is now known to interact with many other components of the division machinery in Escherichia coli. Some biochemical properties of FtsZ, and of another division protein called FtsA, suggest that they are similar to the eukaryotic proteins tubulin and actin respectively. Cell division needs to be closely co-ordinated with chromosome partitioning. The mechanism of partitioning is poorly understood, though several genes involved in this process, including several muk genes, have been identified. The min genes may participate in both septum positioning and chromosome partitioning. Coupled transcription and translation of membrane-associated proteins might also be important for partitioning. In the event of a failure in the normal partitioning process, Bacillus subtilis, at least, has a mechanism for removing a bisected nucleoid from the division septum.

Transcription factor Spo0A switches the localization of the cell division protein FtsZ from a medial to a bipolar pattern in Bacillus subtilis

Entry into sporulation by the Gram-positive bacterium Bacillus subtilis is governed by two transcription factors, Spo0A and sigma H, and involves a switch in the site of division from a medial to a polar location. We report that at the onset of sporulation, assembly of the cell division protein FtsZ shifts from midcell to potential division sites near both poles. The switch to a bipolar pattern of FtsZ localization is dependent on Spo0A. Additionally, synthesis of an activated form of Spo0A during growth artificially activates the switch in FtsZ localization and results in the formation of polar septa. The sigma H factor, on the other hand, is dispensable for the switch in the position of the FtsZ assembly site, although it is required for formation of the polar septum. Our results suggest that during the transition from growth to sporulation, Spo0A induces the expression of genes that suppress FtsZ assembly at the midcell site and activate sites at both poles, whereas sigma H induces genes required for a subsequent step in cytokinesis.

Cloning and sequence of ftsZ and flanking regions from the cyanobacterium Anabaena PCC 7120

Using degenerate oligodeoxyribonucleotide primers based on conserved regions of the cell-division protein FtsZ, a 220-bp fragment of DNA was amplified by the polymerase chain reaction from Anabaena PCC 7120 (Ana). This fragment, which showed significant homology with Escherichia coli ftsZ, was used as a probe to isolate a 15-kb genomic clone containing ftsZ from an Ana DNA library. Sequence analysis revealed an open reading frame (ORF) encoding a protein of 379 amino acids, with 49% identity with E. coli FtsZ. Upstream of Ana ftsZ is a small, unidentified ORF, transcribed in the same direction. An ORF lying downstream of the ftsZ coding region and transcribed in the opposite orientation, shows homology with bacterial glutathione synthetase-encoding genes. Single copies of ftsZ have been identified in Ana and two other cyanobacteria. Multiple transcripts hybridising to ftsZ were detected by Northern hybridisation.

Overview of controls in the Escherichia coli cell cycle

The harmonious growth and cell-to-cell uniformity of steady-state bacterial populations indicate the existence of a well-regulated cell cycle, responding to a set of internal signals. In Escherichia coli, the key events of this cycle are the initiation of DNA replication, nucleoid segregation and the initiation of cell division. The replication initiator is the DnaA protein. In nucleoid segregation, the MukB protein, required for proper partitioning, may be a member of the myosin-kinesin superfamily of mechanoenzymes. In cell division, the FtsZ protein has a tubulin motif, is a GTPase and polymerizes in a ring around midcell during septation; the FtsA protein has an actin-like structure. The nature of the internal signals triggering these events is not known but candidates include cell mass, the superhelical density of the chromosome and the concentration of two regulatory nucleotides, cyclic AMP and ppGpp. The involvement of cytoskeletal-like proteins in key cycle events encourages the notion of a fundamental biological unity in cell cycle regulation in all organisms.

Overproduction and purification of SulA fusion protein in Escherichia coli and its degradation by Lon protease in vitro

To overproduce extremely unstable SulA protein, which is the cell-division inhibitor of Escherichia coli, we fused the sulA gene to the maltose-binding protein (MBP) fusion vectors with or without the signal sequence (plasmids pMAL-p-SulA and pMAL-c-SulA respectively). The amount of the full-length fusion protein expressed from the plasmid pMAL-p-SulA (pre-MBP-SulA) in E. coli was much larger than that expressed from the plasmid pMAL-c-SulA (MBP-SulA). A major amount of the pre-MBP-SulA fusion protein was expressed in a soluble form and affinity-purified by amylose resin. Since site-specific cleavage of the fusion protein with factor Xa resulted in the precipitation of SulA protein, the pre-MBP-SulA fusion protein was used to study the degradation of SulA protein by E. coli Lon protease in vitro. It was found that only the SulA portion of the fusion protein was degraded by Lon protease in an ATP-dependent manner. This result provides direct evidence that Lon protease plays an important role in the rapid degradation of SulA protein in cells.

Coordinate cell cycle control of a Caulobacter DNA methyltransferase and the flagellar genetic hierarchy

The expression of the Caulobacter ccrM gene and the activity of its product, the M.Ccr II DNA methyltransferase, are limited to a discrete portion of the cell cycle (G. Zweiger, G. Marczynski, and L. Shapiro, J. Mol. Biol. 235:472-485, 1994). Temporal control of DNA methylation has been shown to be critical for normal development in the dimorphic Caulobacter life cycle. To understand the mechanism by which ccrM expression is regulated during the cell cycle, we have identified and characterized the ccrM promoter region. We have found that it belongs to an unusual promoter family used by several Caulobacter class II flagellar genes. The expression of these class II genes initiates assembly of the flagellum just prior to activation of the ccrM promoter in the predivisional cell. Mutational analysis of two M.Ccr II methylation sites located 3' to the ccrM promoter suggests that methylation might influence the temporally controlled inactivation of ccrM transcription. An additional parallel between the ccrM and class II flagellar promoters is that their transcription responds to a cell cycle DNA replication checkpoint. We propose that a common regulatory system coordinates the expression of functionally diverse genes during the Caulobacter cell cycle.

Inhibition and restart of initiation of chromosome replication: effects on exponentially growing Escherichia coli cells

Escherichia coli strains in which initiation of chromosome replication could be specifically blocked while other cellular processes continued uninhibited were constructed. Inhibition of replication resulted in a reduced growth rate and in inhibition of cell division after a time period roughly corresponding to the sum of the lengths of the C and D periods. The division inhibition was not mediated by the SOS regulon. The cells became elongated, and a majority contained a centrally located nucleoid with a fully replicated chromosome. The replication block was reversible, and restart of chromosome replication allowed cell division and rapid growth to resume after a time delay. After the resumption, the septum positions were nonrandomly distributed along the length axis of the cells, and a majority of the divisions resulted in at least one newborn cell of normal size and DNA content. With a transient temperature shift, a single synchronous round of chromosome replication and cell division could be induced in the population, making the constructed system useful for studies of cell cycle-specific events. The coordination between chromosome replication, nucleoid segregation, and cell division in E. coli is discussed.

New horizons for cytokinesis

The mechanism of cytokines is an old problem in cell biology that has received fresh attention recently with a large variety of powerful approaches and experimental systems. Significant advances have been made on the structure of the cortical cytoskeleton, the identification of proteins and genes involved, and the regulatory mechanism. Many surprises have surfaced within the past two years, leading us toward a major revision in our understanding of this important process.

Identification of FtsW and characterization of a new ftsW division mutant of Escherichia coli

The product of the ftsW gene has been identified as a polypeptide that, like the related RodA protein, shows anomalous mobility on sodium dodecyl sulfate-polyacrylamide gel electrophoresis. FtsW is produced at low levels that can be increased by altering the translation initiation region of the mRNA. Overproduction of FtsW strongly inhibits cell growth. A new mutant allele, ftsW201, causes a temperature-dependent block in the initiation stage of cell division which is similar to the division block in ftsZ mutants. The block in initiation of division in the ftsW201 allele is shown to be independent of FtsZ or the FtsZ inhibitor, SulA. In addition, the ftsW201 mutant is hypersensitive to overproduction of the division initiation protein FtsZ at the permissive temperature. Our results suggest a role for FtsW in an early stage of division which may involve an interaction with FtsZ.

The min locus, which confers topological specificity to cell division, is not involved in its coupling with nucleoid separation

In Escherichia coli, nucleoid separation and cell constriction remain tightly linked when division is retarded by altering the level of synthesis of the protein FtsZ. In this study, we have examined the role of the min locus, which is responsible for the inactivation of polar division sites, in the partition-septation coupling mechanism. We conclude that the coupling persists in a delta min strain and that its timing relative to replication remains dependent on the level of FtsZ synthesis. We suggest that the retarded nucleoid segregation observed in min mutants is the result of this coupling in cells with a perturbed pattern of nonpolar divisions.

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.

Thermoinducible filamentation in Escherichia coli due to an altered RNA polymerase beta subunit is suppressed by high levels of ppGpp

The Escherichia coli strain known as GC2553, FB8, UTH1038, or K12S (Luria), considered an F- lambda- wild-type strain, is shown here to carry a cryptic mutation, ftsR1, causing nonlethal filamentation during exponential growth in Luria-Bertani (LB) broth at 42 degrees C and the inability to grow in salt-free LB broth at 42 degrees C. The ftsR1 mutation is completely suppressed in genetic backgrounds which increase RelA-dependent synthesis of the nucleotide ppGpp, i.e., argS201 (Mecr) and alaS21 (Mecr) mutations, affecting aminoacyl-tRNA synthetases, or the presence of a plac-relA' plasmid. These backgrounds also confer resistance in LB broth to the beta-lactam mecillinam, an antibiotic which specifically inhibits penicillin-binding protein 2 and, in wild-type cells, causes an indirect block in cell division. Furthermore, the ftsR1 mutant (but not an isogenic ftsR+ strain) is sensitive to mecillinam in minimal glucose medium at 37 degrees C. Since the division block caused by mecillinam can be overcome by overproduction of the cell division protein FtsZ, we tested the effect of plasmid pZAQ (carrying the ftsZ, ftsA, and ftsQ genes) on the ftsR1 mutant; it suppressed the filamentation in LB broth and the mecillinam sensitivity on minimal glucose medium at 37 degrees C but not the growth defect in salt-free LB broth at 42 degrees C. Genetic analysis indicated that the full phenotype of the ftsR1 mutant is due to a single mutation in the rpoB gene (90 min), coding for the beta subunit of RNA polymerase; we call this allele rpoB369(Fts). We propose that the rpoB369(Fts) mutation alters the specificity of the polymerase and that the mutant enzyme can recover normal activity in the presence of high salt concentrations or via interaction with the nucleotide ppGpp.

Mutations in ftsZ that confer resistance to SulA affect the interaction of FtsZ with GTP

Mutations in the essential cell division gene ftsZ confer resistance to SulA, a cell division inhibitor that is induced as part of the SOS response. In this study we have purified and characterized the gene products of six of these mutant ftsZ alleles, ftsZ1, ftsZ2, ftsZ3, ftsZ9, ftsZ100, and ftsZ114, and compared their properties to those of the wild-type gene product. The binding of GTP was differentially affected by these mutations. FtsZ3 exhibited no detectable GTP binding, and FtsZ9 and FtsZ100 exhibited markedly reduced GTP binding. In contrast, FtsZ1 and FtsZ2 bound GTP almost as well as the wild type, and FtsZ114 displayed increased GTP binding. Furthermore, we observed that all mutant FtsZ proteins exhibited markedly reduced intrinsic GTPase activity. It is likely that mutations in ftsZ that confer sulA resistance alter the conformation of the protein such that it assumes the active form.

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.

Penicillin-binding protein 2 inactivation in Escherichia coli results in cell division inhibition, which is relieved by FtsZ overexpression

Aminoacyl-tRNA synthetase mutants of Escherichia coli are resistant to amdinocillin (mecillinam), a beta-lactam antibiotic which specifically binds penicillin-binding protein 2 (PBP2) and prevents cell wall elongation with concomitant cell death. The leuS(Ts) strain, in which leucyl-tRNA synthetase is temperature sensitive, was resistant to amdinocillin at 37 degrees C because of an increased guanosine 5'-diphosphate 3'-diphosphate (ppGpp) pool resulting from partial induction of the stringent response, but it was sensitive to amdinocillin at 25 degrees C. We constructed a leuS(Ts) delta (rodA-pbpA)::Kmr strain, in which the PBP2 structural gene is deleted. This strain grew as spherical cells at 37 degrees C but was not viable at 25 degrees C. After a shift from 37 to 25 degrees C, the ppGpp pool decreased and cell division was inhibited; the cells slowly carried out a single division, increased considerably in volume, and gradually lost viability. The cell division inhibition was reversible when the ppGpp pool increased at high temperature, but reversion required de novo protein synthesis, possibly of septation proteins. The multicopy plasmid pZAQ, overproducing the septation proteins FtsZ, FtsA, and FtsQ, conferred amdinocillin resistance on a wild-type strain and suppressed the cell division inhibition in the leuS(Ts) delta (rodA-pbpA)::Kmr strain at 25 degrees C. The plasmid pAQ, in which the ftsZ gene is inactivated, did not confer amdinocillin resistance. These results lead us to hypothesize that the nucleotide ppGpp activates ftsZ expression and thus couples cell division to protein synthesis.

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.