Bacterial cell division

Antituberculosis drugs: ten years of research

Tuberculosis is today amongst the worldwide health threats. As resistant strains of Mycobacterium tuberculosis have slowly emerged, treatment failure is too often a fact, especially in countries lacking the necessary health care organisation to provide the long and costly treatment adapted to patients. Because of lack of treatment or lack of adapted treatment, at least two million people will die of tuberculosis this year. Due to this concern, this infectious disease was the focus of renewed scientific interest in the last decade. Regimens were optimized and much was learnt on the mechanisms of action of the antituberculosis drugs used. Moreover, the quest for original drugs overcoming some of the problems of current regimens also became the focus of research programmes and many new series of M. tuberculosis growth inhibitors were reported. This review presents the drugs currently used in antituberculosis treatments and the most advanced compounds undergoing clinical trials. We then provide a description of their mechanism of action along with other series of inhibitors known to act on related biochemical targets. This is followed by other inhibitors of M. tuberculosis growth, including recently reported compounds devoid of a reported mechanism of action.

Mitochondrial biology and disease in Dictyostelium

The cellular slime mold Dictyostelium discoideum has become an increasingly useful model for the study of mitochondrial biology and disease. Dictyostelium is an amoebazoan, a sister clade to the animal and fungal lineages. The mitochondrial biology of Dictyostelium exhibits some features which are unique, others which are common to all eukaryotes, and still others that are otherwise found only in the plant or the animal lineages. The AT-rich mitochondrial genome of Dictyostelium is larger than its mammalian counterpart and contains 56kb (compared to 17kb in mammals) encoding tRNAs, rRNAs, and 33 polypeptides (compared to 13 in mammals). It produces a single primary transcript that is cotranscriptionally processed into multiple monocistronic, dicistronic, and tricistronic mRNAs, tRNAs, and rRNAs. The mitochondrial fission mechanism employed by Dictyostelium involves both the extramitochondrial dynamin-based system used by plant, animal, and fungal mitochondria and the ancient FtsZ-based intramitochondrial fission process inherited from the bacterial ancestor. The mitochondrial protein-import apparatus is homologous to that of other eukaryote, and mitochondria in Dictyostelium play an important role in the programmed cell death pathways. Mitochondrial disease in Dictyostelium has been created both by targeted gene disruptions and by antisense RNA and RNAi inhibition of expression of essential nucleus-encoded mitochondrial proteins. This has revealed a regular pattern of aberrant mitochondrial disease phenotypes caused not by ATP insufficiency per se, but by chronic activation of the universal eukaryotic energy-sensing protein kinase AMPK. This novel insight into the cytopathological mechanisms of mitochondrial dysfunction suggests new possibilities for therapeutic intervention in mitochondrial and neurodegenerative diseases.

Cytoskeletal cytoplasmic filament ribbon of Treponema: a member of an intermediate-like filament protein family

Development of genetic systems for many bacterial genera, including Treponema, now allow the study of structures that are specific to certain pathogens. The cytoplasmic filament ribbon of treponemes that is involved in the cell division cycle has a unique organization. Cytoplasmic bridging proteins connect the filaments, maintaining the distance between them and providing the overall ribbon-like structure. The filaments are anchored by proteins associated with the inner membrane. Each filament is composed of a unique monomer, the cytoplasmic filament protein A (CfpA), with coiled-coils secondary structures. CfpA is part of a growing family of proteins that we propose to call bacterial intermediate-like filaments (BILF).

Imaging peptidoglycan biosynthesis in Bacillus subtilis with fluorescent antibiotics

The peptidoglycan (PG) layers surrounding bacterial cells play an important role in determining cell shape. The machinery controlling when and where new PG is made is not understood, but is proposed to involve interactions between bacterial actin homologs such as Mbl, which forms helical cables within cells, and extracellular multiprotein complexes that include penicillin-binding proteins. It has been suggested that labeled antibiotics that bind to PG precursors may be useful for imaging PG to help determine the genes that control the biosynthesis of this polymer. Here, we compare the staining patterns observed in Bacillus subtilis using fluorescent derivatives of two PG-binding antibiotics, vancomycin and ramoplanin. The staining patterns for both probes exhibit a strong dependence on probe concentration, suggesting antibiotic-induced perturbations in PG synthesis. Ramoplanin probes may be better imaging agents than vancomycin probes because they yield clear staining patterns at concentrations well below their minimum inhibitory concentrations. Under some conditions, both ramoplanin and vancomycin probes produce helicoid staining patterns along the cylindrical walls of B. subtilis cells. This sidewall staining is observed in the absence of the cytoskeletal protein Mbl. Although Mbl plays an important role in cell shape determination, our data indicate that other proteins control the spatial localization of the biosynthetic complexes responsible for new PG synthesis along the walls of B. subtilis cells.

Effect of the morphogenebolAon the permeability of theEscherichia coliouter membrane

Escherichia coli bolA is a morphogene involved in stress response and cell division. Overexpression of bolA induces biofilm formation and affects the levels of carboxypeptidases PBP5, PBP6 and beta-lactamase AmpC. In this study, we have shown that changes in the expression of bolA result in alterations in the properties of the outer membrane. The sensitivity to detergents and vancomycin was reduced when bolA was overexpressed and fluorescent probes indicated that different levels of bolA had an effect on outer membrane protein accessibility. Moreover, bolA was shown to be involved in the modulation of the OmpF/OmpC balance.

Mycobacterium tuberculosis cells growing in macrophages are filamentous and deficient in FtsZ rings

FtsZ, a bacterial homolog of tubulin, forms a structural element called the FtsZ ring (Z ring) at the predivisional midcell site and sets up a scaffold for the assembly of other cell division proteins. The genetic aspects of FtsZ-catalyzed cell division and its assembly dynamics in Mycobacterium tuberculosis are unknown. Here, with an M. tuberculosis strain containing FtsZ(TB) tagged with green fluorescent protein as the sole source of FtsZ, we examined FtsZ structures under various growth conditions. We found that midcell Z rings are present in approximately 11% of actively growing cells, suggesting that the low frequency of Z rings is reflective of their slow growth rate. Next, we showed that SRI-3072, a reported FtsZ(TB) inhibitor, disrupted Z-ring assembly and inhibited cell division and growth of M. tuberculosis. We also showed that M. tuberculosis cells grown in macrophages are filamentous and that only a small fraction had midcell Z rings. The majority of filamentous cells contained nonring, spiral-like FtsZ structures along their entire length. The levels of FtsZ in bacteria grown in macrophages or in broth were comparable, suggesting that Z-ring formation at midcell sites was compromised during intracellular growth. Our results suggest that the intraphagosomal milieu alters the expression of M. tuberculosis genes affecting Z-ring formation and thereby cell division.

Borrelia burgdorferi ftsZ plays a role in cell division

ftsZ is essential for cell division in many microorganisms. In Escherichia coli and Bacillus subtilis, FtsZ plays a role in ring formation at the leading edge of the cell division septum. An ftsZ homologue is present in the Borrelia burgdorferi genome (ftsZ(Bbu)). Its gene product (FtsZ(Bbu)) is strongly homologous to other bacterial FtsZ proteins, but its function has not been established. Because loss-of-function mutants of ftsZ(Bbu) might be lethal, the tetR/tetO system was adapted for regulated control of this gene in B. burgdorferi. Sixty-two nucleotides of an ftsZ(Bbu) antisense DNA sequence under the control of a tetracycline-responsive modified hybrid borrelial promoter were cloned into pKFSS1. This construct was electroporated into a B. burgdorferi host strain carrying a chromosomally located tetR under the control of the B. burgdorferi flaB promoter. After induction by anhydrotetracycline, expression of antisense ftsZ RNA resulted in generation of filamentous B. burgdorferi that were unable to divide and grew more slowly than uninduced cells. To determine whether FtsZ(Bbu) could interfere with the function of E. coli FtsZ, ftsZ(Bbu) was amplified from chromosomal DNA and placed under the control of the tetracycline-regulated hybrid promoter. After introduction of the construct into E. coli and induction with anhydrotetracycline, overexpression of ftsZ(Bbu) generated a filamentous phenotype. This suggested interference of ftsZ(Bbu) with E. coli FtsZ function and confirmed the role of ftsZ(Bbu) in cell division. This is the first report of the generation of a B. burgdorferi conditional lethal mutant equivalent by tetracycline-controlled expression of antisense RNA.

Genes for the peptidoglycan synthesis pathway are essential for chloroplast division in moss

The general consensus is that a cyanobacterium phagocytosed by a host cell evolved into the plastids of red and green algae, land plants, and glaucophytes. In contrast to the plastids of glaucophytes, which retain a cyanobacterial-type peptidoglycan layer, no wall-like structures have been detected in plastids from other sources. Although the genome of Arabidopsis thaliana contains five genes that are essential for peptidoglycan synthesis, MurE, MurG, two genes for D-Ala-D-Ala ligase (Ddl), and the gene for translocase I (MraY), their functions have not been determined. We report that the moss Physcomitrella patens has nine homologous genes related to peptidoglycan biosynthesis: MurA, B, C, D, E, and F, Ddl, genes for the penicillin-binding protein Pbp, and dd-carboxypeptidase (Dac). Corroborating a computer prediction, analysis of the GFP fusion proteins with the N terminus of PpMurE or of PpPbp suggests that these proteins are located in the chloroplasts. Gene disruption of the PpMurE gene in P. patens resulted in the appearance of macrochloroplasts both in protonema and in leaf cells. Moreover, gene knockout of the P. patens Pbp gene showed inhibition of chloroplast division in this moss; however, no Pbp gene was found in A. thaliana.

Structure, function and evolution of the mitochondrial division apparatus

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

Targeted overexpression of the Escherichia coli MinC protein in higher plants results in abnormal chloroplasts

Higher plant chloroplast division involves some of the same types of proteins that are required in prokaryotic cell division. These include two of the three Min proteins, MinD and MinE, encoded by the min operon in bacteria. Noticeably absent from annotated sequences from higher plants is a MinC homologue. A higher plant functional MinC homologue that would interfere with FtsZ polymerization, has yet to be identified. We sought to determine whether expression of the bacterial MinC in higher plants could affect chloroplast division. The Escherichia coli minC (EcMinC) gene was isolated and inserted behind the Arabidopsis thaliana RbcS transit peptide sequence for chloroplast targeting. This TP-EcMinC gene driven by the CaMV 35S(2) constitutive promoter was then transformed into tobacco (Nicotiana tabacum L.). Abnormally large chloroplasts were observed in the transgenic plants suggesting that overexpression of the E. coli MinC perturbed higher plant chloroplast division.

SepF, a novel FtsZ-interacting protein required for a late step in cell division

Cell division in nearly all bacteria is initiated by polymerization of the conserved tubulin-like protein FtsZ into a ring-like structure at midcell. This Z-ring functions as a scaffold for a group of conserved proteins that execute the synthesis of the division septum (the divisome). Here we describe the identification of a new cell division protein in Bacillus subtilis. This protein is conserved in Gram positive bacteria, and because it has a role in septum development, we termed it SepF. sepF mutants are viable but have a cell division defect, in which septa are formed slowly and with a severely abnormal morphology. Yeast two-hybrid analysis showed that SepF can interact with itself and with FtsZ. Accordingly, fluorescence microscopy showed that SepF accumulates at the site of cell division, and this localization depends on the presence of FtsZ. Combination of mutations in sepF and ezrA, encoding another Z-ring interacting protein, had a synthetic lethal division effect. We conclude that SepF is a new member of the Gram positive divisome, required for proper execution of septum synthesis.

Manipulation of starch granule size distribution in potato tubers by modulation of plastid division

Starch granule size is an important parameter for starch applications in industry. Starch granules are formed in amyloplasts, which are, like chloroplasts, derived from proplastids. Division processes and associated machinery are likely to be similar for all plastids. Essential roles for FtsZ proteins in plastid division in land plants have been revealed. FtsZ forms the so-called Z ring which, together with inner and outer plastid division rings, brings about constriction of the plastid. It has been shown that modulation of the expression level of FtsZ may result in altered chloroplast size and number. To test whether FtsZ is also involved in amyloplast division and whether this, in turn, may affect the starch granule size in crop plants, FtsZ protein levels were either reduced or increased in potato. As shown previously in other plant species, decreased StFtsZ1 protein levels in leaves resulted in a decrease in the number of chloroplasts in guard cells. More interestingly, plants with increased StFtsZ1 protein levels in tubers resulted in less, but larger, starch granules. This suggests that the stoichiometry between StFtsZ1 and other components of the plastid division machinery is important for its function. Starch from these tubers also had altered pasting properties and phosphate content. The importance of our results for the starch industry is discussed.

Tethering the Z ring to the membrane through a conserved membrane targeting sequence in FtsA

The cytokinetic Z ring is required for bacterial cell division. It consists of polymers of FtsZ, the bacterial ancestor of eukaryotic tubulin, linked to the cytoplasmic membrane. Formation of a Z ring in Escherichia coli occurs as long as one of two proteins, ZipA or FtsA, is present. Both of these proteins bind FtsZ suggesting that they might function to tether FtsZ filaments to the membrane. Although ZipA has a transmembrane domain and therefore can function as a membrane anchor, interaction of FtsA with the membrane has not been explored. In this study we demonstrate that FtsA, which is structurally related to eukaryotic actin, has a conserved C-terminal amphipathic helix that is essential for FtsA function. It is required to target FtsA to the membrane and subsequently to the Z ring. As FtsA is much more widely conserved in bacteria than ZipA, it is likely that FtsA serves as the principal membrane anchor for the Z ring.

Interaction network among Escherichia coli membrane proteins involved in cell division as revealed by bacterial two-hybrid analysis

Formation of the Escherichia coli division septum is catalyzed by a number of essential proteins (named Fts) that assemble into a ring-like structure at the future division site. Several of these Fts proteins are intrinsic transmembrane proteins whose functions are largely unknown. Although these proteins appear to be recruited to the division site in a hierarchical order, the molecular interactions underlying the assembly of the cell division machinery remain mostly unspecified. In the present study, we used a bacterial two-hybrid system based on interaction-mediated reconstitution of a cyclic AMP (cAMP) signaling cascade to unravel the molecular basis of septum assembly by analyzing the protein interaction network among E. coli cell division proteins. Our results indicate that the Fts proteins are connected to one another through multiple interactions. A deletion mapping analysis carried out with two of these proteins, FtsQ and FtsI, revealed that different regions of the polypeptides are involved in their associations with their partners. Furthermore, we showed that the association between two Fts hybrid proteins could be modulated by the coexpression of a third Fts partner. Altogether, these data suggest that the cell division machinery assembly is driven by the cooperative association among the different Fts proteins to form a dynamic multiprotein structure at the septum site. In addition, our study shows that the cAMP-based two-hybrid system is particularly appropriate for analyzing molecular interactions between membrane proteins.

Cephalosporins in clinical development

Bacterial resistance to established classes of antibiotics in clinical use is continuing to increase, making the need for new agents that can be used to treat the newly multi-drug resistant organisms steadily more urgent. Cephalosporins have been a successful group of antibiotics since they were first introduced to combat drug-resistant organisms, including staphylococci. The history of cephalosporins has emphasised an improvement of their stability towards beta-lactamases, thus expanding their spectrum of activity against important Gram-negative pathogens. New cephalosporins that have potent activity against multi-resistant Gram-positive bacteria, including methicillin-resistant staphylococci and penicillin-resistant pneumococci have recently entered clinical development. At least two of these, BAL-5788 and S-3578, also have Gram-negative activity, which is comparable to that of the third-and fourth-generation cephalosporins, making them broad-spectrum agents that could be used in hospital infections where methicillin-resistant staphylococci is likely to be present.

Caulobacter crescentus requires RodA and MreB for stalk synthesis and prevention of ectopic pole formation

Caulobacter crescentus cells treated with amdinocillin, an antibiotic which specifically inhibits the cell elongation transpeptidase penicillin binding protein 2 in Escherichia coli, exhibit defects in stalk elongation and morphology, indicating that stalk synthesis may be a specialized form of cell elongation. In order to investigate this possibility further, we examined the roles of two other proteins important for cell elongation, RodA and MreB. We show that, in C. crescentus, the rodA gene is essential and that RodA depletion leads to a loss of control over stalk and cell body diameter and a stalk elongation defect. In addition, we demonstrate that MreB depletion leads to a stalk elongation defect and conclude that stalk elongation is a more constrained form of cell elongation. Our results strongly suggest that MreB by itself does not determine the diameter of the cell body or stalk. Finally, we show that cells recovering from MreB depletion exhibit a strong budding and branching cell body phenotype and possess ectopic poles, as evidenced by the presence of multiple, misplaced, and sometimes highly branched stalks at the ends of these buds and branches. This phenotype is also seen to a lesser extent in cells recovering from RodA depletion and amdinocillin treatment. We conclude that MreB, RodA, and the target(s) of amdinocillin all contribute to the maintenance of cellular polarity in C. crescentus.

New temperature-sensitive alleles of ftsZ in Escherichia coli

We isolated five new temperature-sensitive alleles of the essential cell division gene ftsZ in Escherichia coli, using P1-mediated, localized mutagenesis. The five resulting single amino acid changes (Gly109-->Ser109 for ftsZ6460, Ala129-->Thr129 for ftsZ972, Val157-->Met157 for ftsZ2066, Pro203-->Leu203 for ftsZ9124, and Ala239-->Val239 for ftsZ2863) are distributed throughout the FtsZ core region, and all confer a lethal cell division block at the nonpermissive temperature of 42 degrees C. In each case the division block is associated with loss of Z-ring formation such that fewer than 2% of cells show Z rings at 42 degrees C. The ftsZ9124 and ftsZ6460 mutations are of particular interest since both result in abnormal Z-ring formation at 30 degrees C and therefore cause significant defects in FtsZ polymerization, even at the permissive temperature. Neither purified FtsZ9124 nor purified FtsZ6460 exhibited polymerization when it was assayed by light scattering or electron microscopy, even in the presence of calcium or DEAE-dextran. Hence, both mutations also cause defects in FtsZ polymerization in vitro. Interestingly, FtsZ9124 has detectable GTPase activity, although the activity is significantly reduced compared to that of the wild-type FtsZ protein. We demonstrate here that unlike expression of ftsZ84, multicopy expression of the ftsZ6460, ftsZ972, and ftsZ9124 alleles does not complement the respective lethalities at the nonpermissive temperature. In addition, all five new mutant FtsZ proteins are stable at 42 degrees C. Therefore, the novel isolates carrying single ftsZ(Ts) point mutations, which are the only such strains obtained since isolation of the classical ftsZ84 mutation, offer significant opportunities for further genetic characterization of FtsZ and its role in cell division.

Two types of FtsZ proteins in mitochondria and red-lineage chloroplasts: the duplication of FtsZ is implicated in endosymbiosis

The ancestors of plastids and mitochondria were once free-living bacteria that became organelles as a result of endosymbiosis. According to this theory, a key bacterial division protein, FtsZ, plays a role in plastid division in algae and plants as well as in mitochondrial division in lower eukaryotes. Recent studies have shown that organelle division is a process that combines features derived from the bacterial division system with features contributed by host eukaryotic cells. Two nonredundant versions of FtsZ, FtsZ1 and FtsZ2, have been identified in green-lineage plastids, whereas most bacteria have a single ftsZ gene. To examine whether there is also more than one type of FtsZ in red-lineage chloroplasts (red algal chloroplasts and chloroplasts that originated from the secondary endosymbiosis of red algae) and in mitochondria, we obtained FtsZ sequences from the complete sequence of the primitive red alga Cyanidioschyzon merolae and the draft sequence of the stramenopile (heterokont) Thalassiosira pseudonana. Phylogenetic analyses that included known FtsZ proteins identified two types of chloroplast FtsZ in red algae (FtsZA and FtsZB) and stramenopiles (FtsZA and FtsZC). These analyses also showed that FtsZB emerged after the red and green lineages diverged, while FtsZC arose by the duplication of an ftsZA gene that in turn descended from a red alga engulfed by the ancestor of stramenopiles. A comparison of the predicted proteins showed that like bacterial FtsZ and green-lineage FtsZ2, FtsZA has a short conserved C-termmal sequence (the C-terminal core domain), whereas FtsZB and FtsZC, like the green-lineage FtsZ1, lack this sequence. In addition, the Cyanidioschyzon and Dictyostelium genomes encode two types of mitochondrial FtsZ proteins, one of which lacks the C-terminal variable domain. These results suggest that the acquisition of an additional FtsZ protein with a modified C terminus was common to the primary and secondary endosymbioses that produced plastids and that this also occurred during the establishment of mitochondria, presumably to regulate the multiplication of these organelles.

Structure of Mycobacterium tuberculosis FtsZ reveals unexpected, G protein-like conformational switches

We report three crystal structures of the Mycobacterium tuberculosis cell division protein FtsZ, as the citrate, GDP, and GTPgammaS complexes, determined at 1.89, 2.60, and 2.08A resolution. MtbFtsZ crystallized as a tight, laterally oriented dimer distinct from the longitudinal polymer observed for alphabeta-tubulin. Mutational data on Escherichia coli FtsZ suggest that this dimer interface is important for proper protofilament and "Z-ring" assembly and function. An alpha-to-beta secondary structure conformational switch at the dimer interface is spatially analogous to, and has many of the hallmarks of, the Switch I conformational changes exhibited by G-proteins upon activation. The presence of a gamma-phosphate in the FtsZ active site modulates the conformation of the "tubulin" loop T3 (spatially analogous to the G-protein Switch II); T3 switching upon gamma-phosphate ligation is directly coupled to the alpha-to-beta switch by steric overlap. The dual conformational switches observed here for the first time in an FtsZ link GTP binding and hydrolysis to FtsZ (and tubulin) lateral assembly and Z-ring contraction, and they are suggestive of an underappreciated functional analogy between FtsZ, tubulin and G-proteins.

The straight and curved conformation of FtsZ protofilaments-evidence for rapid exchange of GTP into the curved protofilament

Bacterial cell division protein FtsZ assembles into protofilaments, which can adopt a straight or curved conformation, similar to its eukaryotic homolog, tubulin. The straight protofilaments can assemble into sheets with a lattice similar to the microtubule wall. The curved protofilaments can form rings when adsorbed to a lipid monolayer, but in solution they form helices. 4 helices assemble together to make a tube, the characteristic polymer of the curved protofilament. GTP favors the straight conformation, while GDP favors the curved. We show here that addition of EDTA and GTP to tubes causes a rapid transformation to straight protofilament sheets. Apparently when the magnesium is chelated the GDP in the curved protofilaments dissociates rapidly and is replaced with GTP, and this GTP induces the transition to straight protofilaments.

Cell division gene cluster in Spiroplasma kunkelii: functional characterization of ftsZ and the first report of ftsA in mollicutes

Spiroplasma kunkelii is a helical, wall-less bacterium that causes corn stunt disease. In adaptation to its phloem-inhabiting parasitic lifestyle, the bacterium has undergone a reductive evolutionary process and, as a result, possesses a compact genome with a gene set approaching the minimal complement necessary for multiplication and pathogenesis. We cloned a much-reduced cell division gene cluster from S. kunkelii and functionally characterized the key division gene, ftsZ(sk). The 1236-bp open reading frame of ftsZ(sk) is capable of encoding a protein with a calculated molecular mass of 44.1 kDa. Protein sequence alignment revealed that FtsZ(sk) is remarkably similar to FtsZ proteins from other eubacteria, and possesses the conserved GTP-binding and hydrolyzing motifs. We demonstrated that overexpression of ftsZ(sk) in Escherichia coli causes transgression of the host cell division, resulting in a filamentous phenotype. We also report, for the first time, the presence of a ftsA gene in the cell division cluster of a mollicute species.

Growth and division of Streptococcus pneumoniae: localization of the high molecular weight penicillin-binding proteins during the cell cycle

The bacterial peptidoglycan, the main component of the cell wall, is synthesized by the penicillin-binding proteins (PBPs). We used immunofluorescence microscopy to determine the cellular localization of all the high molecular weight PBPs of the human pathogen Streptococcus pneumoniae, for a wild type and for several PBP-deficient strains. Progression through the cell cycle was investigated by the simultaneous labelling of DNA and the FtsZ protein. Our main findings are: (i) the temporal dissociation of cell wall synthesis, inferred by the localization of PBP2x and PBP1a, from the constriction of the FtsZ-ring; (ii) the localization of PBP2b and PBP2a at duplicated equatorial sites indicating the existence of peripheral peptidoglycan synthesis, which implies a similarity between the mechanism of cell division in bacilli and streptococci; (iii) the abnormal localization of some class A PBPs in PBP-defective mutants which may explain the apparent redundancy of these proteins in S. pneumoniae.

Discovery of a small molecule that inhibits cell division by blocking FtsZ, a novel therapeutic target of antibiotics

The emergence of bacterial resistance to antibiotics is a major health problem and, therefore, it is critical to develop new antibiotics with novel modes of action. FtsZ, a tubulin-like GTPase, plays an essential role in bacterial cell division, and its homologs are present in almost all eubacteria and archaea. During cell division, FtsZ forms polymers in the presence of GTP that recruit other division proteins to make the cell division apparatus. Therefore, inhibition of FtsZ polymerization will prevent cells from dividing, leading to cell death. Using a fluorescent FtsZ polymerization assay, the screening of >100,000 extracts of microbial fermentation broths and plants followed by fractionation led to the identification of viriditoxin, which blocked FtsZ polymerization with an IC50 of 8.2 microg/ml and concomitant GTPase inhibition with an IC50 of 7.0 microg/ml. That the mode of antibacterial action of viriditoxin is via inhibition of FtsZ was confirmed by the observation of its effects on cell morphology, macromolecular synthesis, DNA-damage response, and increased minimum inhibitory concentration as a result of an increase in the expression of the FtsZ protein. Viriditoxin exhibited broad-spectrum antibacterial activity against clinically relevant Gram-positive pathogens, including methicillin-resistant Staphylococcus aureus and vancomycin-resistant Enterococci, without affecting the viability of eukaryotic cells.

Characterization of divIVA and other genes located in the chromosomal region downstream of the dcw cluster in Streptococcus pneumoniae

We analyzed the chromosome region of Streptococcus pneumoniae located downstream of the division and cell wall (dcw) cluster that contains the homolog of the Bacillus subtilis cell division gene divIVA and some genes of unknown function. Inactivation of divIVA in S. pneumoniae resulted in severe growth inhibition and defects in cell shape, nucleoid segregation, and cell division. Inactivation of the ylm genes resulted in some morphological and/or division abnormalities, depending on the inactivated gene. Transcriptional analysis revealed a relationship between these genes and the ftsA and ftsZ cell division genes, also indicating that the connection between the dcw cluster and the divIVA region is more extensive than just chromosomal position and gene organization.

ARC6 is a J-domain plastid division protein and an evolutionary descendant of the cyanobacterial cell division protein Ftn2

Replication of chloroplasts is essential for achieving and maintaining optimal plastid numbers in plant cells. The plastid division machinery contains components of both endosymbiotic and host cell origin, but little is known about the regulation and molecular mechanisms that govern the division process. The Arabidopsis mutant arc6 is defective in plastid division, and its leaf mesophyll cells contain only one or two grossly enlarged chloroplasts. We show here that arc6 chloroplasts also exhibit abnormal localization of the key plastid division proteins FtsZ1 and FtsZ2. Whereas in wild-type plants, the FtsZ proteins assemble into a ring at the plastid division site, chloroplasts in the arc6 mutant contain numerous short, disorganized FtsZ filament fragments. We identified the mutation in arc6 and show that the ARC6 gene encodes a chloroplast-targeted DnaJ-like protein localized to the plastid envelope membrane. An ARC6-green fluorescent protein fusion protein was localized to a ring at the center of the chloroplasts and rescued the chloroplast division defect in the arc6 mutant. The ARC6 gene product is related closely to Ftn2, a prokaryotic cell division protein unique to cyanobacteria. Based on the FtsZ filament morphology observed in the arc6 mutant and in plants that overexpress ARC6, we hypothesize that ARC6 functions in the assembly and/or stabilization of the plastid-dividing FtsZ ring. We also analyzed FtsZ localization patterns in transgenic plants in which plastid division was blocked by altered expression of the division site-determining factor AtMinD. Our results indicate that MinD and ARC6 act in opposite directions: ARC6 promotes and MinD inhibits FtsZ filament formation in the chloroplast.

Effects of antibiotics that inhibit the bacterial peptidoglycan synthesis pathway on moss chloroplast division

Moss chloroplasts should prove useful for studying the cyanobacteria-derived system in chloroplasts. To determine the effects of antibiotics that inhibit bacterial peptidoglycan synthesis, the numbers of chloroplasts in treated Physcomitrella patens cells were counted. Ampicillin and D-cycloserine caused a rapid decrease in the number of chloroplasts per cell. Fosfomycin affected half of the cells, while vancomycin affected a few cells. Conversely, bacitracin had no effect. With the decrease in chloroplast number, macrochloroplasts appeared in antibiotic-treated cells. Removal of the antibiotics resulted in the recovery of chloroplast number, suggesting that the decrease in number was directly dependent on the antibiotic treatment. Microscopic observations showed that the decrease in the number of chloroplasts resulted from cell division without chloroplast division. These results suggest that enzymes derived from the bacterial peptidoglycan synthesis pathway are related to moss chloroplast division.

Crystal structure of the SOS cell division inhibitor SulA and in complex with FtsZ

SulA halts cell division in Escherichia coli by binding to the major component of the division machinery FtsZ. We have solved the crystal structure of SulA alone and in complex with FtsZ from Pseudomonas aeruginosa. SulA is expressed when the SOS response is induced. This is a mechanism to inhibit cell division and repair DNA in the event of DNA damage. FtsZ is a tubulin-like protein that forms polymers, with the active-site GTPase split across two monomers. One monomer provides the GTP-binding site and the other, through its T7 loop nucleotide hydrolysis. Our structures show that SulA is a dimer, and that SulA inhibits cell division neither by binding the nucleotide-binding site nor by inducing conformational changes in FtsZ. Instead, SulA binds the T7 loop surface of FtsZ, opposite the nucleotide-binding site, blocking polymer formation. These findings explain why GTP hydrolysis and polymer turnover are required for SulA inhibition.

Plastid division: its origins and evolution

Photosynthetic eukaryotes have evolved plastid division mechanisms since acquisition of plastids through endosymbiosis. The emerging evolutionary origin of the plastid division mechanism is remarkably complex. The constituents of the division apparatus of plastids may have complex origins. The one constituent is the plastid FtsZ ring taken over from the cyanobacteria-like ancestral endosymbionts. The second is the doublet of concentric plastid dividing rings (or triplet in red algae), possibly acquired by ancestral host eukaryotes following the primary endosymbiotic event. Placement of the division apparatus at the correct division site may involve a system analogous to the bacterial Min system. Plastid nucleoid partitioning may be mediated by binding to envelope or thylakoid membranes. Multiple copies of plastid DNA and symmetrical distribution of the nucleoids in the plastids may permit faithful transmission to daughter plastids via equal binary plastid divisions. Cyanelles retain peptidoglycan wall and cyanelle division occurs through septum formation such as bacterial cell division. Cyanelle division involves the cyanelle ring analogous to the inner stromal plastid-dividing (PD) ring. According to the prevailing hypothesis that primary endosymbiosis occurred only once, cyanelle division may represent an intermediate stage between cyanobacterial division and the well-known plastid division among extant plants. With the secondary plastids, which are surrounded by three or four membranes, the PD ring also participates in division of the inner two "true" plastid envelope membranes, and the third and the outermost membranes divide by unknown mechanisms.

Glucosylglycerol, a compatible solute, sustains cell division under salt stress

The cyanobacterium Synechocystis sp. PCC 6803 accumulates the compatible solute glucosylglycerol (GG) and sucrose under salt stress. Although the molecular mechanisms for GG synthesis including regulation of the GG-phosphate synthase (ggpS) gene, which encodes GgpS, has been intensively investigated, the role of GG in protection against salt stress remains poorly understood. In our study of the role of GG in the tolerance to salt stress, we found that salt stress due to 450 mM NaCl inhibited cell division and significantly increased cell size in DeltaggpS mutant cells, whereas the inhibition of cell division and increase in cell size were observed in wild-type cells at high concentrations of NaCl, such as 800 mM. Electron microscopy revealed that, in DeltaggpS cells, separation of daughter cells was incomplete, and aborted division could be recognized by the presence of a structure that resembled a division ring. The addition of GG to the culture medium protected DeltaggpS cells against salt stress and reversed the adverse effects of NaCl on cell division and cell size. These observations suggest that GG is important for salt tolerance and thus for the proper division of cells under salt stress conditions.

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.

Dynamic recruitment of dynamin for final mitochondrial severance in a primitive red alga

Dynamins are a eukaryote-specific family of GTPases. Some family members are involved in diverse and varied cellular activities. Here, we report that the primitive red alga Cyanidioschyzon merolae retains only one dynamin homolog, CmDnm1, belonging to the mitochondrial division subfamily. Previously, the bacterial cell division protein, FtsZ, was shown to localize at the mitochondrial division site in the alga. We showed that FtsZ and dynamin coexist as mitochondrial division-associated proteins that act during different phases of division. CmDnm1 was recruited from 10-20 cytoplasmic patches (dynamin patches) to the midpoint of the constricted mitochondrion-dividing ring (MD ring), which was observed as an electron-dense structure on the cytoplasmic side. CmDnm1 is probably not required for early constriction; it forms a ring or spiral when the outer mitochondrial membrane is finally severed, whereas the FtsZ and MD rings are formed before constriction. It is thought that the FtsZ, MD, and dynamin rings are involved in scaffolding, constriction, and final separation, respectively. In eukaryotes, mitochondrial severance is probably the most conserved role for the dynamin family.

Inhibition of plastid division by ampicillin in the pteridophyte Selaginella nipponica Fr. et Sav

We investigated the effect of the beta-lactam antibiotic, ampicillin, on plastid division in the pteridophyte Selaginella nipponica. Guard cells of plantlets treated with 1 mM ampicillin only often had one plastid, whereas guard cells of untreated plantlets had two to four plastids. We generated a S. nipponica cell culture system and used it to investigate the effects of ampicillin. Treatment with 1 mM ampicillin had no effect on cell division in culture. We classified cultured cells into four types based on the number of plastids they contained: one (Type I), two (Type II), three or four (Type III) and more than five (Type IV). After 3 d in culture, the percentage of each cell type (I-IV) was 29.5, 46.7, 20.9, and 1.9%, respectively. Subsequently, the percentage of Types III and IV increased gradually, reaching 61.9 and 11.4%, respectively, after 15 d in culture in the absence of ampicillin. When 1 mM ampicillin was added, there was a minimal increase in the number of Type III and IV cells, with high percentages of Type I and II cells (32.4 and 45.7%, respectively) after 15 d. These results suggest that ampicillin inhibits plastid division in S. nipponica.

Evolutionary history, structural features and biochemical diversity of the NlpC/P60 superfamily of enzymes

BACKGROUND

Peptidoglycan is hydrolyzed by a diverse set of enzymes during bacterial growth, development and cell division. The N1pC/P60 proteins define a family of cell-wall peptidases that are widely represented in various bacterial lineages. Currently characterized members are known to hydrolyze D-gamma-glutamyl-meso-diaminopimelate or N-acetylmuramate-L-alanine linkages.

RESULTS

Detailed analysis of the N1pC/P60 peptidases showed that these proteins define a large superfamily encompassing several diverse groups of proteins. In addition to the well characterized P60-like proteins, this superfamily includes the AcmB/LytN and YaeF/YiiX families of bacterial proteins, the amidase domain of bacterial and kinetoplastid glutathionylspermidine synthases (GSPSs), and several proteins from eukaryotes, phages, poxviruses, positive-strand RNA viruses, and certain archaea. The eukaryotic members include lecithin retinol acyltransferase (LRAT), nematode developmental regulator Egl-26, and candidate tumor suppressor H-rev107. These eukaryotic proteins, along with the bacterial YaeF/poxviral G6R family, show a circular permutation of the catalytic domain. We identified three conserved residues, namely a cysteine, a histidine and a polar residue, that are involved in the catalytic activities of this superfamily. Evolutionary analysis of this superfamily shows that it comprises four major families, with diverse domain architectures in each of them.

CONCLUSIONS

Several related, but distinct, catalytic activities, such as murein degradation, acyl transfer and amide hydrolysis, have emerged in the N1pC/P60 superfamily. The three conserved catalytic residues of this superfamily are shown to be equivalent to the catalytic triad of the papain-like thiol peptidases. The predicted structural features indicate that the N1pC/P60 enzymes contain a fold similar to the papain-like peptidases, transglutaminases and arylamine acetyltransferases.

A novel gene that bears a DnaJ motif influences cyanobacterial cell division

Transposon Tn5-692 mutagenizes Synechococcus sp. strain PCC 7942 efficiently. The predicted product of the gene mutated in the Tn5-692-derived cell division mutant FTN2 has an N-terminal DnaJ domain, as have its cyanobacterial and plant orthologs. Anabaena sp. strain PCC 7120, when mutated in genes orthologous to ftn2 and ftn6, forms akinete-like cells.

Fluorescent assay for polymerization of purified bacterial FtsZ cell-division protein

Septum formation in Escherichia coli is a complex cascade of interactions among cell-division proteins. The tubulin-like FtsZ division protein localizes to the division site and serves a cytoskeletal role during septum formation. A novel fluorescent-based 96-well format filter assay has been developed to measure the polymerization of FtsZ. A mixture of monomers and aggregates (38 to approximately 200 KDa in range) of purified wild-type FtsZ and a fluorescently tagged derivative of FtsZ protein in stoichiometric ratio passes through a 0.2-microm filter membrane, while polymerized FtsZ is retained on the filter. Addition of the SulA protein to the assay leads to rapid disassembly of existing FtsZ polymers, demonstrating its natural regulatory effect on FtsZ under the assay conditions. This assay is sensitive (requiring 2 microM FtsZ or less) and facilitates high-throughput screening of factors affecting FtsZ polymerization.

Characterization of the Mycoplasma hominis ftsZ gene and its sequence variability in mycoplasma clinical isolates

We cloned and sequenced Mycoplasma hominis chromosomal fragment containing ftsZ gene. The wild-type expression of the gene was shown at RNA level by reverse transcription followed by PCR amplification. We revealed that M. hominis FtsZ had a comparatively low similarity to proteins of Mycoplasma genitalium and Mycoplasma pneumoniae. After full ftsZ gene sequencing for 14 clinical isolates of M. hominis, single-nucleotide substitutions were found in 21 positions, 6 of them being common for almost all isolates. This ftsZ gene polymorphism may be used for subtyping of M. hominis in clinical samples. Expression of the M. hominis ftsZ gene in different Escherichia coli strains was also demonstrated, and M. hominis FtsZ protein was purified from E. coli cells transformed with recombinant expression plasmid. Complementation between the M. hominis FtsZ and E. coli FtsZ could be shown. The comparison of FtsZ protein structures may also be used for investigation of bacterial phylogenetic relationships.

Rapid assembly dynamics of the Escherichia coli FtsZ-ring demonstrated by fluorescence recovery after photobleaching

FtsZ, the major cytoskeletal component of the bacterial cell-division machine, assembles into a ring (the Z-ring) that contracts at septation. FtsZ is a bacterial homolog of tubulin, with similar tertiary structure, GTP hydrolysis, and in vitro assembly. We used green fluorescent protein-labeled FtsZ and fluorescence recovery after photobleaching to show that the E. coli Z-ring is extremely dynamic, continually remodeling itself with a half-time of 30 s. ZipA, a membrane protein involved in cell division that colocalizes with FtsZ, was equally dynamic. The Z-ring of the mutant ftsZ84, which has 1/10 the guanosine triphosphatase activity of wild-type FtsZ in vitro, showed a 9-fold slower turnover in vivo. This finding implies that assembly dynamics are determined primarily by GTP hydrolysis. Despite the greatly reduced assembly dynamics, the ftsZ84 cells divide with a normal cell-cycle time.

GTP hydrolysis of cell division protein FtsZ: evidence that the active site is formed by the association of monomers

The essential prokaryotic cell division protein FtsZ is a tubulin homologue that forms a ring at the division site. FtsZ forms polymers in a GTP-dependent manner. Recent biochemical evidence has shown that FtsZ forms multimeric structures in vitro and in vivo and functions as a self-activating GTPase. Structural analysis of FtsZ points to an important role for the highly conserved tubulin-like loop 7 (T7-loop) in the self-activation of GTP hydrolysis. The T7-loop was postulated to form the active site together with the nucleotide-binding site on an adjacent FtsZ monomer. To characterize the role of the T7-loop of Escherichia coli FtsZ, we have mutagenized residues M206, N207, D209, D212, and R214. All the mutant proteins, except the R214 mutant, are severely affected in polymerization and GTP hydrolysis. Charged residues D209 and D212 cannot be substituted with a glutamate residue. All mutants interact with wild-type FtsZ in vitro, indicating that the T7-loop mutations do not abolish FtsZ self-association. Strikingly, in mixtures of wild-type and mutant proteins, most mutants are capable of inhibiting wild-type GTP hydrolysis. We conclude that the T7-loop is part of the active site for GTP hydrolysis, formed by the association of two FtsZ monomers.

Assembly of an FtsZ mutant deficient in GTPase activity has implications for FtsZ assembly and the role of the Z ring in cell division

FtsZ, the ancestral homologue of eukaryotic tubulins, assembles into the Z ring, which is required for cytokinesis in prokaryotic cells. Both FtsZ and tubulin have a GTPase activity associated with polymerization. Interestingly, the ftsZ2 mutant is viable, although the FtsZ2 mutant protein has dramatically reduced GTPase activity due to a glycine-for-aspartic acid substitution within the synergy loop. In this study, we have examined the properties of FtsZ2 and found that the reduced GTPase activity is not enhanced by DEAE-dextran-induced assembly, indicating it has a defective catalytic site. In the absence of DEAE-dextran, FtsZ2 fails to assemble unless supplemented with wild-type FtsZ. FtsZ has to be at or above the critical concentration for copolymerization to occur, indicating that FtsZ is nucleating the copolymers. The copolymers formed are relatively stable and appear to be stabilized by a GTP-cap. These results indicate that FtsZ2 cannot nucleate assembly in vitro, although it must in vivo. Furthermore, the stability of FtsZ-FtsZ2 copolymers argues that FtsZ2 polymers would be stable, suggesting that stable FtsZ polymers are able to support cell division.

Crystal structure of PBP2x from a highly penicillin-resistant Streptococcus pneumoniae clinical isolate: a mosaic framework containing 83 mutations

Penicillin-binding proteins (PBPs) are the main targets for beta-lactam antibiotics, such as penicillins and cephalosporins, in a wide range of bacterial species. In some Gram-positive strains, the surge of resistance to treatment with beta-lactams is primarily the result of the proliferation of mosaic PBP-encoding genes, which encode novel proteins by recombination. PBP2x is a primary resistance determinant in Streptococcus pneumoniae, and its modification is an essential step in the development of high level beta-lactam resistance. To understand such a resistance mechanism at an atomic level, we have solved the x-ray crystal structure of PBP2x from a highly penicillin-resistant clinical isolate of S. pneumoniae, Sp328, which harbors 83 mutations in the soluble region. In the proximity of the Sp328 PBP2x* active site, the Thr(338) --> Ala mutation weakens the local hydrogen bonding network, thus abrogating the stabilization of a crucial buried water molecule. In addition, the Ser(389) --> Leu and Asn(514) --> His mutations produce a destabilizing effect that generates an "open" active site. It has been suggested that peptidoglycan substrates for beta-lactam-resistant PBPs contain a large amount of abnormal, branched peptides, whereas sensitive strains tend to catalyze cross-linking of linear forms. Thus, in vivo, an "open" active site could facilitate the recognition of distinct, branched physiological substrates.

The chromosome partitioning protein, ParB, is required for cytokinesis in Caulobacter crescentus

In Caulobacter crescentus, the genes encoding the chromosome partitioning proteins, ParA and ParB, are essential. Depletion of ParB resulted in smooth filamentous cells in which DNA replication continued. Immunofluorescence microscopy revealed that the formation of FtsZ rings at the mid-cell, the earliest molecular event in the initiation of bacterial cell division, was blocked in cells lacking ParB. ParB binds sequences near the C. crescentus origin of replication. Cell cycle experiments show that the formation of bipolarly localized ParB foci, and presumably localization of the origin of replication to the cell poles, preceded the formation of FtsZ rings at the mid-cell by 20 min. These results suggest that one major function of ParA and ParB may be to regulate the initiation of cytokinesis in C. crescentus.

The bacterial ParA-ParB partitioning proteins

A pair of genes designated parA and parB are encoded by many low copy number plasmids and bacterial chromosomes. They work with one or more cis-acting sites termed centromere-like sequences to ensure better than random predivisional partitioning of the DNA molecule that encodes them. The centromere-like sequences nucleate binding of ParB and titrate sufficient protein to create foci, which are easily visible by immuno-fluorescence microscopy. These foci normally follow the plasmid or the chromosomal replication oriC complexes. ParA is a membrane-associated ATPase that is essential for this symmetric movement of the ParB foci. In Bacillus subtilis ParA oscillates from end to end of the cell as does MinD of E. coli, a relative of the ParA family. ParA may facilitate ParB movement along the inner surface of the cytoplasmic membrane to encounter and become tethered to the next replication zone. The ATP-bound form of ParA appears to adopt the conformation needed to drive partition. Hydrolysis to create ParA-ADP or free ParA appears to favour a form that is not located at the pole and binds to DNA rather than the partition complex. Definition of the protein domains needed for interaction with membranes and the conformational changes that occur on interaction with ATP/ADP will provide insights into the partitioning mechanism and possible targets for inhibitors of partitioning.

HscA is involved in the dynamics of FtsZ-ring formation in Escherichia coli K12

BACKGROUND

FtsZ, a homologue of eukaryotic tubulin, localizes throughout the cytoplasm in non-dividing Escherichia coli. However, it assembles in cytokinetic rings at the early stages of septation. Factors controlling the dynamics of FtsZ ring formation are unknown, and the molecular mechanism governing these dynamics is yet to be determined.

RESULTS

At 42 degrees C, JE10715 mutant bacteria formed multinucleated filaments with a highly reduced number of FtsZ-rings at potential division sites. The JE10715 phenotype resulted from a mis-sense mutation in the hscA gene which encodes a heat shock Hsp70 family protein, with a single alanine-to-valine substitution at position 192 within the ATPase domain. Both JE10715 and the hscA knockout strain of JE10715 were completely complemented by a plasmid-born, wild-type hscA gene, but not by a mutant-type hscA715 gene. An hscA conditional knockout of the wild-type strain under non-permissive conditions exhibited longer rod cells with an abnormal localization of FtsZ. The over-expression of dnaK partially complemented the JE10715 mutation. In vitro, the ATPase activity of the mutant protein HscA715 was reduced to 63% of wild-type HscA activity. HscA co-sedimented with FtsZ-polymers in the presence of GTP.

CONCLUSION

HscA is involved in FtsZ-ring formation, through a chaperon-like interaction with FtsZ. Defects in hscA, however, can partially be compensated for by redundant genes, including the wild-type dnaK.

Crystal structure of the bacterial cell division inhibitor MinC

Bacterial cell division requires accurate selection of the middle of the cell, where the bacterial tubulin homologue FtsZ polymerizes into a ring structure. In Escherichia coli, site selection is dependent on MinC, MinD and MINE: MinC acts, with MinD, to inhibit division at sites other than the midcell by directly interacting with FTSZ: Here we report the crystal structure to 2.2 A of MinC from Thermotoga maritima. MinC consists of two domains separated by a short linker. The C-terminal domain is a right-handed beta-helix and is involved in dimer formation. The crystals contain two different MinC dimers, demonstrating flexibility in the linker region. The two-domain architecture and dimerization of MinC can be rationalized with a model of cell division inhibition. MinC does not act like SulA, which affects the GTPase activity of FtsZ, and the model can explain how MinC would select for the FtsZ polymer rather than the monomer.

Escherichia coli FtsZ polymers contain mostly GTP and have a high nucleotide turnover

The cell division protein FtsZ is a GTPase structurally related to tubulin and, like tubulin, it assembles in vitro into filaments, sheets and other structures. To study the roles that GTP binding and hydrolysis play in the dynamics of FtsZ polymerization, the nucleotide contents of FtsZ were measured under different polymerizing conditions using a nitrocellulose filter-binding assay, whereas polymerization of the protein was followed in parallel by light scattering. Unpolymerized FtsZ bound 1 mol of GTP mol(-1) protein monomer. At pH 7.5 and in the presence of Mg(2+) and K(+), there was a strong GTPase activity; most of the bound nucleotide was GTP during the first few minutes but, later, the amount of GTP decreased in parallel with depolymerization, whereas the total nucleotide contents remained invariant. These results show that the long FtsZ polymers formed in solution contain mostly GTP. Incorporation of nucleotides into the protein was very fast either when the label was introduced at the onset of the reaction or subsequently during polymerization. Molecular modelling of an FtsZ dimer showed the presence of a cleft between the two subunits maintaining the nucleotide binding site open to the medium. These results show that the FtsZ polymers are highly dynamic structures that quickly exchange the bound nucleotide, and this exchange can occur in all the subunits.

Regions of FtsZ important for self-interaction in Staphylococcus aureus

Since FtsZ is structurally similar to eukaryotic tubulin, selective regions of S. aureus FtsZ have been swapped with equivalent regions from tubulin. These mutant FtsZ proteins were analyzed for their interaction with both wildtype FtsZ and FtsA using the yeast two-hybrid assay. Specific regions were identified which abrogated FtsZ-FtsZ binding but unaffected the FtsA-FtsZ interaction. This work will further our understanding of the regions of FtsZ critical for its physiological role.