Interaction between the min locus and ftsZ

Composition and Biophysical Properties of the Sorting Platform Pods in the Shigella Type III Secretion System

Shigella flexneri, causative agent of bacillary dysentery (shigellosis), uses a type III secretion system (T3SS) as its primary virulence factor. The T3SS injectisome delivers effector proteins into host cells to promote entry and create an important intracellular niche. The injectisome's cytoplasmic sorting platform (SP) is a critical assembly that contributes to substrate selection and energizing secretion. The SP consists of oligomeric Spa33 "pods" that associate with the basal body via MxiK and connect to the Spa47 ATPase via MxiN. The pods contain heterotrimers of Spa33 with one full-length copy associated with two copies of a C-terminal domain (Spa33^C). The structure of Spa33^C is known, but the precise makeup and structure of the pods in situ remains elusive. We show here that recombinant wild-type Spa33 can be prepared as a heterotrimer that forms distinct stable complexes with MxiK and MxiN. In two-hybrid analyses, association of the Spa33 complex with these proteins occurs via the full-length Spa33 component. Furthermore, these complexes each have distinct biophysical properties. Based on these properties, new high-resolution cryo-electron tomography data and architectural similarities between the Spa33 and flagellar FliM-FliN complexes, we provide a preliminary model of the Spa33 heterotrimers within the SP pods. From these findings and evolving models of SP interfaces and dynamics in the Yersinia and Salmonella T3SS, we suggest a model for SP function in which two distinct complexes come together within the context of the SP to contribute to form the complete pod structures during the recruitment of T3SS secretion substrates.

Comparative transcriptomic analysis of global gene expression mediated by (p) ppGpp reveals common regulatory networks in Pseudomonas syringae

BACKGROUND

Pseudomonas syringae is an important plant pathogen, which could adapt many different environmental conditions. Under the nutrient-limited and other stress conditions, P. syringae produces nucleotide signal molecules, i.e., guanosine tetra/pentaphosphate ((p)ppGpp), to globally regulate gene expression. Previous studies showed that (p) ppGpp played an important role in regulating virulence factors in P. syringae pv. tomato DC3000 (PstDC3000) and P. syringae pv. syringae B728a (PssB728a). Here we present a comparative transcriptomic analysis to uncover the overall effects of (p)ppGpp-mediated stringent response in P. syringae.

RESULTS

In this study, we investigated global gene expression profiles of PstDC3000 and PssB728a and their corresponding (p)ppGpp⁰ mutants in hrp-inducing minimal medium (HMM) using RNA-seq. A total of 1886 and 1562 differentially expressed genes (DEGs) were uncovered between the (p)ppGpp⁰ mutants and the wild-type in PstDC3000 and PssB728a, respectively. Comparative transcriptomics identified 1613 common DEGs, as well as 444 and 293 unique DEGs in PstDC3000 and PssB728a, respectively. Functional cluster analysis revealed that (p) ppGpp positively regulated a variety of virulence-associated genes, including type III secretion system (T3SS), type VI secretion system (T6SS), cell motility, cell division, and alginate biosynthesis, while negatively regulated multiple basic physiological processes, including DNA replication, RNA processes, nucleotide biosynthesis, fatty acid metabolism, ribosome protein biosynthesis, and amino acid metabolism in both PstDC3000 and PssB728a. Furthermore, (p) ppGpp had divergent effects on other processes in PstDC3000 and PssB728a, including phytotoxin, nitrogen regulation and general secretion pathway (GSP).

CONCLUSION

In this study, comparative transcriptomic analysis reveals common regulatory networks in both PstDC3000 and PssB728a mediated by (p) ppGpp in HMM. In both P. syringae systems, (p) ppGpp re-allocate cellular resources by suppressing multiple basic physiological activities and enhancing virulence gene expression, suggesting a balance between growth, survival and virulence. Our research is important in that due to similar global gene expression mediated by (p) ppGpp in both PstDC3000 and PssB728a, it is reasonable to propose that (p) ppGpp could be used as a target to develop novel control measures to fight against important plant bacterial diseases.

The cytoplasmic domain of MxiG interacts with MxiK and directs assembly of the sorting platform in the Shigella type III secretion system

Many Gram-negative bacteria use type III secretion systems (T3SSs) to inject virulence effector proteins into eukaryotic cells. The T3SS apparatus (T3SA) is structurally conserved among diverse bacterial pathogens and consists of a cytoplasmic sorting platform, an envelope-spanning basal body, and an extracellular needle with tip complex. The sorting platform is essential for effector recognition and powering secretion. Studies using bacterial "minicells" have revealed an unprecedented level of structural detail of the sorting platform; however, many of the structure-function relationships within this complex remain enigmatic. Here, we report on improved cryo-electron tomographic approaches to enhance the resolution of the Shigella T3SA sorting platform (at ≤2 nm resolution) done in concert with biochemical and genetic methods to define the sorting platform interactome and interactions with the T3SA inner membrane ring (IR). We observed that the sorting platform consists of "pods" with 6-fold symmetry that interact with the Spa47 ATPase via radial extensions comprising MxiN. Most importantly, MxiK maintained an interaction with the IR via specific interactions with the cytoplasmic domain of the IR protein MxiG (MxiG^C), which is a noncanonical forkhead-associated domain, and MxiK has an elongated structure that interacts with the IR via MxiG^C T4 lysozyme-mediated insertional mutagenesis of MxiK revealed its orientation within the sorting platform and enabled disruption of interactions with its binding partners, which abolished sorting platform assembly. Finally, a comparison with the homologous interactions in the Salmonella T3SS sorting platform revealed clear differences in their IR-sorting platform interfaces that have possible mechanistic implications.

Coordination of Chromosome Segregation and Cell Division in Staphylococcus aureus

Productive bacterial cell division and survival of progeny requires tight coordination between chromosome segregation and cell division to ensure equal partitioning of DNA. Unlike rod-shaped bacteria that undergo division in one plane, the coccoid human pathogen Staphylococcus aureus divides in three successive orthogonal planes, which requires a different spatial control compared to rod-shaped cells. To gain a better understanding of how this coordination between chromosome segregation and cell division is regulated in S. aureus, we investigated proteins that associate with FtsZ and the divisome. We found that DnaK, a well-known chaperone, interacts with FtsZ, EzrA and DivIVA, and is required for DivIVA stability. Unlike in several rod shaped organisms, DivIVA in S. aureus associates with several components of the divisome, as well as the chromosome segregation protein, SMC. This data, combined with phenotypic analysis of mutants, suggests a novel role for S. aureus DivIVA in ensuring cell division and chromosome segregation are coordinated.

Backbone and side chain NMR assignments for the N-terminal domain of the cell division regulator MinC from Bacillus subtilis

Bacterial cell division proteins must assemble at the middle of the cell to ensure the viability of both daughter cells. The first step in the assembly of the cell division apparatus is the polymerization of the tubulin-like protein FtsZ into a ring-shaped scaffold, the Z-ring. The Min system contributes to the spatial precision of division by inhibiting FtsZ polymerization at the cell poles. The component of this system that interacts with FtsZ is MinC, a 25 kDa protein that has two domains. The N-terminal domain of MinC is the main responsible for FtsZ inhibition, being sufficient to block Z-ring assembly when overexpressed in vivo, and to inhibit FtsZ polymerization in vitro. Despite intensive studies, little is known about the MinC binding site for FtsZ. We have assigned the backbone and side chain resonances of the MinC N-terminal domain of Bacillus subtilis through NMR spectroscopy. These assignments provide the basis to characterize the interaction between the N-terminal domain of MinC and FtsZ by NMR methods.

Genome sequence of the model sulfate reducer Desulfovibrio gigas: a comparative analysis within the Desulfovibrio genus

Desulfovibrio gigas is a model organism of sulfate-reducing bacteria of which energy metabolism and stress response have been extensively studied. The complete genomic context of this organism was however, not yet available. The sequencing of the D. gigas genome provides insights into the integrated network of energy conserving complexes and structures present in this bacterium. Comparison with genomes of other Desulfovibrio spp. reveals the presence of two different CRISPR/Cas systems in D. gigas. Phylogenetic analysis using conserved protein sequences (encoded by rpoB and gyrB) indicates two main groups of Desulfovibrio spp, being D. gigas more closely related to D. vulgaris and D. desulfuricans strains. Gene duplications were found such as those encoding fumarate reductase, formate dehydrogenase, and superoxide dismutase. Complexes not yet described within Desulfovibrio genus were identified: Mnh complex, a v-type ATP-synthase as well as genes encoding the MinCDE system that could be responsible for the larger size of D. gigas when compared to other members of the genus. A low number of hydrogenases and the absence of the codh/acs and pfl genes, both present in D. vulgaris strains, indicate that intermediate cycling mechanisms may contribute substantially less to the energy gain in D. gigas compared to other Desulfovibrio spp. This might be compensated by the presence of other unique genomic arrangements of complexes such as the Rnf and the Hdr/Flox, or by the presence of NAD(P)H related complexes, like the Nuo, NfnAB or Mnh.

The Kil peptide of bacteriophage λ blocks Escherichia coli cytokinesis via ZipA-dependent inhibition of FtsZ assembly

Assembly of the essential, tubulin-like FtsZ protein into a ring-shaped structure at the nascent division site determines the timing and position of cytokinesis in most bacteria and serves as a scaffold for recruitment of the cell division machinery. Here we report that expression of bacteriophage λ kil, either from a resident phage or from a plasmid, induces filamentation of Escherichia coli cells by rapid inhibition of FtsZ ring formation. Mutant alleles of ftsZ resistant to the Kil protein map to the FtsZ polymer subunit interface, stabilize FtsZ ring assembly, and confer increased resistance to endogenous FtsZ inhibitors, consistent with Kil inhibiting FtsZ assembly. Cells with the normally essential cell division gene zipA deleted (in a modified background) display normal FtsZ rings after kil expression, suggesting that ZipA is required for Kil-mediated inhibition of FtsZ rings in vivo. In support of this model, point mutations in the C-terminal FtsZ-interaction domain of ZipA abrogate Kil activity without discernibly altering FtsZ-ZipA interactions. An affinity-tagged-Kil derivative interacts with both FtsZ and ZipA, and inhibits sedimentation of FtsZ filament bundles in vitro. Together, these data inspire a model in which Kil interacts with FtsZ and ZipA in the cell to prevent FtsZ assembly into a coherent, division-competent ring structure. Phage growth assays show that kil+ phage lyse ∼30% later than kil mutant phage, suggesting that Kil delays lysis, perhaps via its interaction with FtsZ and ZipA.

Imaging DivIVA dynamics using photo-convertible and activatable fluorophores in Bacillus subtilis

Most rod-shape model organisms such as Escherichia coli or Bacillus subtilis utilize two inhibitory systems for correct positioning of the cell division apparatus. While the nucleoid occlusion system acts in vicinity of the nucleoid, the Min system was thought to protect the cell poles from futile division leading to DNA-free miniature cells. The Min system is composed of an inhibitory protein, MinC, which acts at the level of the FtsZ ring formation. MinC is recruited to the membrane by MinD, a member of the MinD/ParA family of Walker-ATPases. Topological positioning of the MinCD complex depends on MinE in E. coli and MinJ/DivIVA in B. subtilis. While MinE drives an oscillation of MinCD in the E. coli cell with a time-dependent minimal concentration at midcell, the B. subtilis system was thought to be stably tethered to the cell poles by MinJ/DivIVA. Recent developments revealed that the Min system in B. subtilis mainly acts at the site of division, where it seems to prevent reinitiation of the division machinery. Thus, MinCD describe a dynamic behavior in B. subtilis. This is somewhat inconsistent with a stable localization of DivIVA at the cell poles. High resolution imaging of ongoing divisions show that DivIVA also enriches at the site of division. Here we analyze whether polar localized DivIVA is partially mobile and can contribute to septal DivIVA and vice versa. For this purpose we use fusions with green to red photoconvertible fluorophores, Dendra2 and photoactivatable PA-GFP. These techniques have proven very powerful to discriminate protein relocalization in vivo. Our results show that B. subtilis DivIVA is indeed dynamic and moves from the poles to the new septum.

Cell size control in bacteria

Like eukaryotes, bacteria must coordinate division with growth to ensure cells are the appropriate size for a given environmental condition or developmental fate. As single-celled organisms, nutrient availability is one of the strongest influences on bacterial cell size. Classic physiological experiments conducted over four decades ago first demonstrated that cell size is directly correlated with nutrient source and growth rate in the Gram-negative bacterium Salmonella typhimurium. This observation subsequently served as the basis for studies revealing a role for cell size in cell cycle progression in a closely related organism, Escherichia coli. More recently, the development of powerful genetic, molecular, and imaging tools has allowed us to identify and characterize the nutrient-dependent pathway responsible for coordinating cell division and cell size with growth rate in the Gram-positive model organism Bacillus subtilis. Here, we discuss the role of cell size in bacterial growth and development and propose a broadly applicable model for cell size control in this important and highly divergent domain of life.

Characterization of ftsZ mutations that render Bacillus subtilis resistant to MinC

Background

Cell division in Bacillus subtilis occurs precisely at midcell. Positional control of cell division is exerted by two mechanisms: nucleoid occlusion, through Noc, which prevents division through nucleoids, and the Min system, where the combined action of the MinC, D and J proteins prevents formation of the FtsZ ring at cell poles or recently completed division sites.

Methodology/Principal Findings

We used a genetic screen to identify mutations in ftsZ that confer resistance to the lethal overexpression of the MinC/MinD division inhibitor. The FtsZ mutants were purified and found to polymerize to a similar or lesser extent as wild type FtsZ, and all mutants displayed reduced GTP hydrolysis activity indicative of a reduced polymerization turnover. We found that even though the mutations conferred in vivo resistance to MinC/D, the purified FtsZ mutants did not display strong resistance to MinC in vitro.

Conclusions/Significance

Our results show that in B. subtilis, overproduction of MinC can be countered by mutations that alter FtsZ polymerization dynamics. Even though it would be very likely that the FtsZ mutants found depend on other Z-ring stabilizing proteins such as ZapA, FtsA or SepF, we found this not to be the case. This indicates that the cell division process in B. subtilis is extremely robust.

A novel component of the division-site selection system of Bacillus subtilis and a new mode of action for the division inhibitor MinCD

Cell division in bacteria is governed by a complex cytokinetic machinery in which the key player is a tubulin homologue, FtsZ. Most rod-shaped bacteria divide precisely at mid-cell between segregated sister chromosomes. Selection of the correct site for cell division is thought to be determined by two negative regulatory systems: the nucleoid occlusion system, which prevents division in the vicinity of the chromosomes, and the Min system, which prevents inappropriate division at the cell poles. In Bacillus subtilis recruitment of the division inhibitor MinCD to cell poles depends on DivIVA, and these proteins were thought to be sufficient for Min function. We have now identified a novel component of the division-site selection system, MinJ, which bridges DivIVA and MinD. minJ mutants are impaired in division because MinCD activity is no longer restricted to cell poles. Although MinCD was thought to act specifically on FtsZ assembly, analysis of minJ and divIVA mutants showed that their block in division occurs downstream of FtsZ. The results support a model in which the main function of the Min system lies in allowing only a single round of division per cell cycle, and that MinCD acts at multiple levels to prevent inappropriate division.

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

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

Plastid division: evolution, mechanism and complexity

BACKGROUND

The continuity of chloroplasts is maintained by division of pre-existing chloroplasts. Chloroplasts originated as bacterial endosymbionts; however, the majority of bacterial division factors are absent from chloroplasts and the eukaryotic host has added several new components. For example, the ftsZ gene has been duplicated and modified, and the Min system has retained MinE and MinD but lost MinC, acquiring at least one new component ARC3. Further, the mechanism has evolved to include two members of the dynamin protein family, ARC5 and FZL, and plastid-dividing (PD) rings were most probably added by the eukaryotic host.

SCOPE

Deciphering how the division of plastids is coordinated and controlled by nuclear-encoded factors is key to our understanding of this important biological process. Through a number of molecular-genetic and biochemical approaches, it is evident that FtsZ initiates plastid division where the coordinated action of MinD and MinE ensures correct FtsZ (Z)-ring placement. Although the classical FtsZ antagonist MinC does not exist in plants, ARC3 may fulfil this role. Together with other prokaryotic-derived proteins such as ARC6 and GC1 and key eukaryotic-derived proteins such as ARC5 and FZL, these proteins make up a sophisticated division machinery. The regulation of plastid division in a cellular context is largely unknown; however, recent microarray data shed light on this. Here the current understanding of the mechanism of chloroplast division in higher plants is reviewed with an emphasis on how recent findings are beginning to shape our understanding of the function and evolution of the components.

CONCLUSIONS

Extrapolation from the mechanism of bacterial cell division provides valuable clues as to how the chloroplast division process is achieved in plant cells. However, it is becoming increasingly clear that the highly regulated mechanism of plastid division within the host cell has led to the evolution of features unique to the plastid division process.

Bacterial cell division: the mechanism and its precison

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

The effects of ftsZ mutation on the production of recombinant protein in Bacillus subtilis

In this paper, the possibility of using a mutation of ftsZ as a pseudo-spore mutant is investigated. ftsZ, which is essential for cell division and sporulation of Bacillus subtilis, was placed under the spac promoter, which is inducible with isopropyl thiogalactose (IPTG). Cell growth of the ftsZ mutant and its beta-galactosidase activity under the aprE promoter were compared with the wild type. In the presence of 1 mM IPTG, cell growth of the ftsZ mutant was almost the same as that of the wild type and its sporulation frequency was slightly lower than that of the wild type. However, under uninduced conditions, cell growth of ftsZ mutant was severely impaired. When induced with 0.2 mM IPTG, the ftsZ mutant showed about 13 times higher beta-galactosidase activity than the wild type. When the ftsZ mutant was used for secretory production of subtilisin, only three times higher extracellular subtilisin activity was measured, compared with the wild type. By real-time PCR investigation, it was revealed that the ftsZ mutant intracellular mRNA level for subtilisin was more than 16 times higher, compared with the wild type. However, it appears that the secretion pathway is somewhat damaged in the ftsZ mutant. These results suggest that the cell division mutant can also be used like a sporulation mutant to produce recombinant proteins, with a precise control of cell growth and induction.

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.

Probing the catalytic activity of a cell division-specific transpeptidase in vivo with beta-lactams

Penicillin-binding protein 3 (PBP3; also called FtsI) is a transpeptidase that catalyzes cross-linking of the peptidoglycan cell wall in the division septum of Escherichia coli. To determine whether the catalytic activity of PBP3 is activated during division, we assayed acylation of PBP3 with three beta-lactams (cephalexin, aztreonam, and piperacillin) in growing cells. Acylation of PBP3 with cephalexin, but not aztreonam or piperacillin, appeared to be stimulated by cell division. Specifically, cephalexin acylated PBP3 about 50% faster in a population of dividing cells than in a population of filamentous cells in which division was inhibited by inactivation or depletion of FtsZ, FtsA, FtsQ, FtsW, or FtsN. However, in a simpler in vitro system using isolated membranes, acylation with cephalexin was not impaired by depletion of FtsW or FtsN. A conflicting previous report that the ftsA3(Ts) allele interferes with acylation of PBP3 was found to be due to the presence of a thermolabile PBP3 in the strain used in that study. The new findings presented here are discussed in light of the hypothesis that the catalytic activity of PBP3 is stimulated by interaction(s) with other division proteins. We suggest that there might be allosteric activation of substrate binding.

Bacterial division: the fellowship of the ring

The Z ring, composed of the tubulin homolog FtsZ, is essential for bacterial cell division. Recently a new protein, ZapA, has been discovered that localizes to the Z ring and stabilizes it, probably by promoting the bundling of FtsZ protofilaments.

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.

Escherichia coli division inhibitor MinCD blocks septation by preventing Z-ring formation

The min system spatially regulates division through the topological regulation of MinCD, an inhibitor of cell division. MinCD was previously shown to inhibit division by preventing assembly of the Z ring (E. Bi and J. Lutkenhaus, J. Bacteriol. 175:1118-1125, 1993); however, this was questioned in a recent report (S. S. Justice, J. Garcia-Lara, and L. I. Rothfield, Mol. Microbiol. 37:410-423, 2000) which indicated that MinCD acted after Z-ring formation and prevented the recruitment of FtsA to the Z ring. This discrepancy was due in part to alternative fixation conditions. We have therefore reinvestigated the action of MinCD and avoided fixation by using green fluorescent protein (GFP) fusions to division proteins. MinCD prevented the localization of both FtsZ-GFP and ZipA-GFP, consistent with it preventing Z-ring assembly. Consistent with a direct interaction between FtsZ and the MinCD inhibitor, we find that increased FtsZ, but not FtsA, suppresses MinCD-induced lethality. Furthermore, strains carrying various alleles of ftsZ, selected on the basis of resistance to the inhibitor SulA, displayed variable resistance to MinCD. These results are consistent with FtsZ as the target of MinCD and confirm that this inhibitor prevents Z-ring assembly.

Polymer stability plays an important role in the positional regulation of FtsZ

We conducted a series of experiments examining the effect of polymer stability on FtsZ localization dynamics in Bacillus subtilis. A loss-of-function mutation in ezrA, a putative polymer-destabilizing factor, suppresses the defects in FtsZ polymer stability associated with minCD overexpression. In addition, a mutation that is predicted to stabilize the FtsZ polymer leads to the formation of polar FtsZ rings. These data support the hypothesis that carefully balanced polymer stability is important for the assembly and localization of FtsZ during the bacterial cell cycle.

Bacterial cell division: regulating Z-ring formation

The earliest stage of cell division in bacteria is the formation of a Z ring, composed of a polymer of the FtsZ protein, at the division site. Z rings appear to be synthesized in a bi-directional manner from a nucleation site (NS) located on the inside of the cytoplasmic membrane. It is the utilization of a NS specifically at the site of septum formation that determines where and when division will occur. However, a Z ring can be made to form at positions other than at the division site. How does a cell regulate utilization of a NS at the correct location and at the right time? In rod-shaped bacteria such as Escherichia coli and Bacillus subtilis, two factors involved in this regulation are the Min system and nucleoid occlusion. It is suggested that in B. subtilis, the main role of the Min proteins is to inhibit division at the nucleoid-free cell poles. In E. coli it is currently not clear whether the Min system can direct a Z ring to the division site at mid-cell or whether its main role is to ensure that division inhibition occurs away from mid-cell, a role analogous to that in B. subtilis. While the nucleoid negatively influences Z-ring formation in its vicinity in these rod-shaped organisms, the exact relationship between nucleoid occlusion and the ability to form a mid-cell Z ring is unresolved. Recent evidence suggests that in B. subtilis and Caulobacter crescentus, utilization of the NS at the division site is intimately linked to the progress of a round of chromosome replication and this may form the basis of achieving co-ordination between chromosome replication and cell division.

Cell division inhibitors SulA and MinC/MinD block septum formation at different steps in the assembly of the Escherichia coli division machinery

SulA and MinCD are specific inhibitors of cell division in Escherichia coli. In this paper, size exclusion chromatography was used to study the effect of the SulA and MinCD division inhibitors on the oligomerization state of endogenous FtsZ in cytoplasmic extracts, and immunofluorescence microscopy was used to determine the effect of SulA and MinCD on the formation of FtsZ, FtsA and ZipA rings at potential division sites. SulA prevented the formation of high-molecular-weight FtsZ polymers by interfering with FtsZ dimerization and subsequent oligomerization. In contrast, the MinCD division inhibitor did not prevent the oligomerization of FtsZ in the cell extracts or the formation of FtsZ and ZipA ring structures in vivo. However, MinCD did prevent the formation of FtsA rings. Increased expression of ftsA suppressed MinCD-induced division inhibition, but had no effect on SulA-induced division inhibition. These results indicate that MinCD blocks the assembly of the septation machinery at a later step than SulA, at the stage at which FtsA is added to the FtsZ ring.

Bacterial division: Finding the dividing line

Division of a cell - whether eukaryotic or prokaryotic - requires accurate spatial coordination. Recent work on the bacterium Escherichia coli has shown that correct placement of the cell division site at the midcell position occurs by a combination of selection against potential polar sites and selection of the midcell site.

The MinC component of the division site selection system in Escherichia coli interacts with FtsZ to prevent polymerization

Positioning of the Z ring at the midcell site in Escherichia coli is assured by the min system, which masks polar sites through topological regulation of MinC, an inhibitor of division. To study how MinC inhibits division, we have generated a MalE-MinC fusion that retains full biological activity. We find that MalE-MinC interacts with FtsZ and prevents polymerization without inhibiting FtsZ's GTPase activity. MalE-MinC19 has reduced ability to inhibit division, reduced affinity for FtsZ, and reduced ability to inhibit FtsZ polymerization. These results, along with MinC localization, suggest that MinC rapidly oscillates between the poles of the cell to destabilize FtsZ filaments that have formed before they mature into polar Z rings.

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 dimerization in vivo

A hybrid assay, based on the properties of the lambda repressor, was developed to detect FtsZ dimerization in Escherichia coli in vivo. A gene fusion comprising the N-terminal end of the lambda cI repressor gene and the complete E. coli ftsZ gene was constructed. The fused protein resulted in a functional lambda repressor and was able to complement the thermosensitive mutant ftsZ84. Using the same strategy, a series of 10 novel mutants of FtsZ that are unable to dimerize was selected, and a deletion analysis of the protein was carried out. Characterization of these mutants allowed the identification of three separate FtsZ portions: the N-terminal of about 150 amino acids; the C-terminal of about 60 amino acids, which corresponds to the less conserved portion of the protein; and a central region of about 150 residues. Mutants belonging to this region would define the dimerization domain of FtsZ.

Effect of minCD on FtsZ ring position and polar septation in Bacillus subtilis

We examined the pattern of FtsZ localization in a Bacillus subtilis minCD mutant. When grown in minimal medium, the majority (approximately 89%) of the minCD mutant cells with an FtsZ ring had a single, medially positioned FtsZ ring. These results indicate that genes in addition to minCD function to restrict the number and position of FtsZ rings. When grown in rich medium, greater than 50% of the minCD mutant cells had multiple FtsZ rings, indicating significant differences in regulation of FtsZ ring formation based on growth medium.

MinCD proteins control the septation process during sporulation of Bacillus subtilis

Mutation of the divIVB locus in Bacillus subtilis causes misplacement of the septum during cell division and allows the formation of anucleate minicells. The divIVB locus contains five open reading frames (ORFs). The last two ORFs (minCD) are homologous to minC and minD of Escherichia coli but a minE homolog is lacking in B. subtilis. There is some similarity between minicell formation and the asymmetric septation that normally occurs during sporulation in terms of polar septum localization. However, it has been proposed that MinCD has no essential role in sporulation septum formation. We have used electron microscopic studies to show septation events during sporulation in some minD strains. We have observed an unusually thin septum at the midcell position in minD and also in minD spoIIE71 mutant cells. Fluorescence microscopy also localized a SpoIIE-green fluorescent protein fusion protein at the midcell site in minD cells. We propose that the MinCD complex plays an important role in asymmetric septum formation during sporulation of B. subtilis cells.

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.

Analysis of the effect of ppGpp on the ftsQAZ operon in Escherichia coli

Escherichia coli loses its rod shape by inactivation of PBP2 (penicillin-binding protein 2), target of the beta-lactam mecillinam. Under these conditions, cell division is blocked in rich medium. Division in the absence of PBP2 activity is restored (and resistance to mecillinam is conferred) when the three cell division proteins FtsQ, FtsA and FtsZ are overproduced, but not when only one or two of them are overproduced. Division in the absence of PBP2 activity is also restored by a doubling in the ppGpp pool, as in the argS201 mutant. However, the nucleotide ppGpp, a transcriptional regulator of many operons, does not govern any of the five promoters of the ftsQAZoperon, as shown by S1 mapping of ftsQAZ mRNA 5' ends in exponentially growing wild-type cells in the mecillinam-resistant argS201 mutant (intermediate ppGpp level) or during the stringent response elicited by isoleucine starvation (high ppGpp level). Furthermore, the concentration of FtsZ protein is not increased in exponentially growing mecillinam-resistant argS201 cells. These results show that the ftsQAZ operon is not the ppGpp target responsible for mecillinam resistance. We are currently trying to identify those targets that, at intermediate ppGpp levels, allow cells to divide as spheres in the absence of PBP2.

Morphogenesis of Escherichia coli

The shape of Escherichia coli is strikingly simple compared to those of higher eukaryotes. In fact, the end result of E. coli morphogenesis is a cylindrical tube with hemispherical caps. It is argued that physical principles affect biological forms. In this view, genes code for products that contribute to the production of suitable structures for physical factors to act upon. After introduction of a physical model, the discussion is focused on the shape-maintaining (peptidoglycan) layer of E. coli. This is followed by a detailed analysis of the structural relationship of the cellular interior to the cytoplasmic membrane. A basic theme of this review is that the transcriptionally active nucleoid and the cytoplasmic translation machinery form a structural continuity with the growing cellular envelope. An attempt has been made to show how this dynamic relationship during the cell cycle affects cell polarity and how it leads to cell division.

Roles of FtsA and FtsZ in activation of division sites

Increasing FtsZ induces the formation of minicells at cell poles but does not increase the frequency or timing of central divisions. A coordinate increase in both FtsZ and FtsA, however, increases the frequency of both polar and central divisions.

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.

The MinE ring: an FtsZ-independent cell structure required for selection of the correct division site in E. coli

E. coli cell division is mediated by the FtsZ ring and associated factors. Selection of the correct division site requires the combined action of an inhibitor of FtsZ ring formation (MinCD) and of a topological specificity factor that somehow prevents MinCD action at the middle of the cell (MinE). Here we show that a biologically active MinE-Gfp fusion accumulates in an annular structure near the middle of young cells. Formation of the MinE ring required MinD but was independent of MinC and continued in nondividing cells in which FtsZ function was inhibited. The results indicate that the MinE ring represents a novel cell structure, which allows FtsZ ring formation at midcell by suppressing MinCD activity at this site.

Cloning, sequencing and expression of the gene encoding elongation factor P in the amino-acid producer Brevibacterium lactofermentum (Corynebacterium glutamicum ATCC 13869)

The Brevibacterium lactofermentum EF-P gene, encoding the elongation factor protein P, was cloned and sequenced. According to DNA sequence analysis of this gene, the B. lactofermentum EF-P protein consists of 187 amino acids with a calculated molecular weight of 20,584. Southern hybridization of an internal fragment of the EF-P gene from B. lactofermentum with chromosomal DNAs from different microorganisms reveals that it is a unique gene product in B. lactofermentum and Corynebacterium glutamicum. The EF-P gene was expressed in E. coli using the T7 expression system and the calculated molecular weight of the expressed protein was 23,000. Disruption experiments using an internal fragment of the EF-P gene or a disrupted EF-P gene in suicide plasmids always failed, suggesting that the gene is needed for cell viability.

MinCD-independent inhibition of cell division by a protein that fuses MalE to the topological specificity factor MinE

We report that MinE, the topological specificity factor of cell division in Escherichia coli, inhibits septation when fused to the C terminus of the maltose-binding protein MalE. This contrasts with overexpression of MinE alone, which affects growth but has no effect on division. Inhibition by MalE-MinE was minCD independent and depended on MinE segments involved in dimerization and prevention of MinCD division inhibition. The SOS and the heat shock responses were not involved, suggesting that the inhibition comes from a direct interaction of MalE-MinE with the septation apparatus. MalE-MinE lethality was suppressed by overexpression of ftsZ, as well as by overexpression of ftsN, a suppressor of temperature-sensitive mutations in genes ftsQ, ftsA, and ftsI. We also report that high-level synthesis of MalE disturbs nucleoid partitioning.

The Escherichia coli histone-like protein HU affects DNA initiation, chromosome partitioning via MukB, and cell division via MinCDE

Escherichia coli hupA hupB double mutants, lacking both subunits (HU1 and HU2) of the histone-like protein HU, accumulate secondary mutations. In some genetic backgrounds, these include mutations in the minCDE operon, inactivating this system of septation control and resulting in the formation of minicells. In the course of the characterization of hupA hupB mutants, we observed that the simultaneous absence of the HU2 subunit and the MukB protein, implicated in chromosome partitioning, is lethal for the bacteria; the integrity of either HU or MukB thus seems to be essential for bacterial growth. The HU protein has been shown to be involved in DNA replication in vitro; we show here that its inactivation in the hupA hupB double mutant disturbs the synchrony of replication initiation in vivo, as evaluated by flow cytometry. Our results suggest that global nucleoid structure, determined in part by the histone-like protein HU, plays a role in DNA replication initiation, in proper chromosome partitioning directed by the MukFEB proteins, and in correct septum placement directed by the MinCDE proteins.

The divIVA minicell locus of Bacillus subtilis

The Bacillus subtilis divIVA1 mutation causes misplacement of the septum during cell division, resulting in the formation of small, circular, anucleate minicells. This study reports the cloning and sequence analysis of 2.4 kb of the B. subtilis chromosome including the divIVA locus. Three open reading frames were identified: orf, whose function is unknown; divIVA; and isoleucyl tRNA synthetase (ileS). We identified the point mutation in the divIVA1 mutant allele. Inactivation of divIVA produces a minicell phenotype, whereas overproduction of DivIVA results in a filamentation phenotype. Mutants with mutations at both of the minicell loci of B. subtilis, divIVA and divIVB, possess a minicell phenotype identical to that of the DivIVB- mutant. The DivIVA-mutants, but not the DivIVB- mutants, show a decrease in sporulation efficiency and a delay in the kinetics of endospore formation. The data support a model in which divIVA encodes the topological specificity subunit of the minCD system. The model suggests that DivIVA acts as a pilot protein, directing minCD to the polar septation sites. DivIVA also appears to be the interface between a sporulation component and MinCD, freeing up the polar septation sites for use during the asymmetric septation event of the sporulation process.

Bacterial cell division and the Z ring

Bacterial cell division occurs through the formation of an FtsZ ring (Z ring) at the site of division. The ring is composed of the tubulin-like FtsZ protein that has GTPase activity and the ability to polymerize in vitro. The Z ring is thought to function in vivo as a cytoskeletal element that is analogous to the contractile ring in many eukaryotic cells. Evidence suggests that the Z ring is utilized by all prokaryotic organisms for division and may also be used by some eukaryotic organelles. This review summarizes our present knowledge about the formation, function, and evolution of the Z ring in prokaryotic cell division.

Interaction between FtsZ and inhibitors of cell division

The interaction between inhibitors of cell division and FtsZ were assessed by using the yeast two-hybrid system. An interaction was observed between FtsZ and SulA, a component of the SOS response, and the interacting regions were mapped to their conserved domains. This interaction was reduced by mutations in sulA and by most mutations in ftsZ that make cell refractory to sulA. No interaction was detected between FtsZ and MinCD, an inhibitory component of the site selection system. However, interactions were observed among various members of the Min system, and MinE was found to reduce the interaction between MinC and MinD. The implications of these findings for cell division are discussed.

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.

Suppression of the pleiotropic effects of HisH and HisF overproduction identifies four novel loci on the Salmonella typhimurium chromosome: osmH, sfiW, sfiX, and sfiY

Insertion mutations that suppress some or all the pleiotropic effects of HisH and HisF overproduction were obtained by using transposons Tn10dTet and Tn10dCam. All suppressor mutations proved to be recessive, indicating that their effects were caused by loss of function; thus, the suppressors identify genes that are necessary to trigger the pleiotropic response when HisH and HisF are overproduced. Genetic mapping of the suppressor mutations identifies four novel loci on the Salmonella typhimurium genetic map. Mutations in osmH (min 49) behave as general suppressors that abolish all manifestations of the pleiotropic response. Mutations in sfiY (min 83) suppress cell division inhibition and thermosensitivity but not osmosensitivity. Mutations that suppress only cell division inhibition define another locus, sfiX (min 44). A fourth novel locus, sfiW (min 19), is also involved in cell division inhibition. The phenotype of sfiW mutations is in turn pleiotropic: they suppress cell division inhibition, make S. typhimurium unable to grow in minimal media, and cause slow growth and abnormal colony and cell shape. The inability of sfiW mutants to grow in minimal medium cannot be relieved by any known nutritional requirement or by the use of carbon sources other than glucose. The hierarchy of suppressor phenotypes and the existence of epistatic effects among suppressor mutations suggest a pathway-like model for the Hisc pleiotropic response.

Analysis of genes encoding the cell division protein FtsZ and a glutathione synthetase homologue in the cyanobacterium Anabaena sp. PCC 7120

Heterocysts, cells specialized in nitrogen fixation in Anabaena sp. PCC 7120, lose the potential for cell division once fully differentiated. This suggests that cell division activity is differentially regulated in heterocysts and vegetative cells. FtsZ has been shown to play a crucial role in bacterial cell division. Two degenerate oligonucleotide primers were designed to detect, by polymerase chain reaction (PCR), an ftsZ homologue from the heterocystous cyanobacterium Anabaena sp. PCC 7120. A PCR-amplified DNA fragment was cloned and used as a probe to isolate the entire ftsZ gene of Anabaena sp. PCC 7120. The deduced amino acid sequence shares strong similarities with other FtsZ proteins, suggesting remarkable conservation of the FtsZ protein during evolution. An ORF downstream of ftsZ, which would be transcribed in the opposite direction compared to ftsZ, could encode a polypeptide with significant sequence similarity to the glutathione synthetase from Escherichia coli. Inactivation experiments in vivo for both ftsZ and the glutathione synthetase gene did not yield any double recombinants either in the presence or in the absence of combined nitrogen, suggesting that both genes are essential for cell growth under these conditions.