Characterization of the ftsZ gene from Mycoplasma pulmonis, an organism lacking a cell wall

Dynamics of interdomain rotation facilitates FtsZ filament assembly

FtsZ, the tubulin homolog essential for bacterial cell division, assembles as the Z-ring at the division site, and directs peptidoglycan synthesis by treadmilling. It is unclear how FtsZ achieves kinetic polarity that drives treadmilling. To obtain insights into fundamental features of FtsZ assembly dynamics independent of peptidoglycan synthesis, we carried out structural and biochemical characterization of FtsZ from the cell wall-less bacteria, Spiroplasma melliferum (SmFtsZ). Interestingly the structures of SmFtsZ, bound to GDP and GMPPNP respectively, were captured as domain swapped dimers. SmFtsZ was found to be a slower GTPase with a higher critical concentration (CC) compared to Escherichia coli FtsZ (EcFtsZ). In FtsZs, a conformational switch from R-state (close) to T-state (open) favors polymerization. We identified that Phe224, located at the interdomain cleft of SmFtsZ, is crucial for R- to T-state transition. SmFtsZF224M exhibited higher GTPase activity and lower CC, whereas the corresponding EcFtsZM225F resulted in cell division defects in E. coli. Our results demonstrate that relative rotation of the domains is a rate-limiting step of polymerization. Our structural analysis suggests that the rotation is plausibly triggered upon addition of a GTP-bound monomer to the filament through interaction of the preformed N-terminal domain (NTD). Hence, addition of monomers to the NTD-exposed end of filament is slower in comparison to the C-terminal domain (CTD) end, thus explaining kinetic polarity. In summary, the study highlights the importance of interdomain interactions and conformational changes in regulating FtsZ assembly dynamics.

A genetic toolkit and gene switches to limit Mycoplasma growth for biosafety applications

Mycoplasmas have exceptionally streamlined genomes and are strongly adapted to their many hosts, which provide them with essential nutrients. Owing to their relative genomic simplicity, Mycoplasmas have been used to develop chassis for biotechnological applications. However, the dearth of robust and precise toolkits for genomic manipulation and tight regulation has hindered any substantial advance. Herein we describe the construction of a robust genetic toolkit for M. pneumoniae, and its successful deployment to engineer synthetic gene switches that control and limit Mycoplasma growth, for biosafety containment applications. We found these synthetic gene circuits to be stable and robust in the long-term, in the context of a minimal cell. With this work, we lay a foundation to develop viable and robust biosafety systems to exploit a synthetic Mycoplasma chassis for live attenuated vectors for therapeutic applications.

Mycoplasmas are minimal cell model organisms but lack genetic tools. Here the authors provide a robust genetic toolkit for Mycoplasma demonstrating gene circuit engineering applications.

Chlamydial MreB Directs Cell Division and Peptidoglycan Synthesis in Escherichia coli in the Absence of FtsZ Activity

Cell division is the ultimate process for the propagation of bacteria, and FtsZ is an essential protein used by nearly all bacteria for this function. Chlamydiae belong to a small group of bacteria that lack the universal cell division protein FtsZ but still divide by binary fission. Chlamydial MreB is a member of the shape-determining MreB/Mbl family of proteins responsible for rod shape morphology in Escherichia coliChlamydia also encodes a homolog of RodZ, an MreB assembly cytoskeletal protein that links MreB to cell wall synthesis proteins. We hypothesized that MreB directs cell division in Chlamydia and that chlamydial MreB could replace FtsZ function for cell division in E. coli Overexpression of chlamydial mreB-rodZ in E. coli induced prominent morphological changes with production of large swollen or oval bacteria, eventually resulting in bacterial lysis. Low-level expression of chlamydial mreB-rodZ restored viability of a lethal ΔmreB mutation in E. coli, although the bacteria lost their typical rod shape and grew as rounded cells. When FtsZ activity was inhibited by overexpression of SulA in the ΔmreB mutant of E. coli complemented with chlamydial mreB-rodZ, spherical E. coli grew and divided. Localization studies using a fluorescent fusion chlamydial MreB protein indicated that chlamydial RodZ directs chlamydial MreB to the E. coli division septum. These results demonstrate that chlamydial MreB, in partnership with chlamydial RodZ, acts as a cell division protein. Our findings suggest that an mreB-rodZ-based mechanism allows Chlamydia to divide without the universal division protein FtsZ.IMPORTANCE The study of Chlamydia growth and cell division is complicated by its obligate intracellular nature and biphasic lifestyle. Chlamydia also lacks the universal division protein FtsZ. We employed the cell division system of Escherichia coli as a surrogate to identify chlamydial cell division proteins. We demonstrate that chlamydial MreB, together with chlamydial RodZ, forms a cell division and growth complex that can replace FtsZ activity and support cell division in E. coli Chlamydial RodZ plays a major role in directing chlamydial MreB localization to the cell division site. It is likely that the evolution of chlamydial MreB and RodZ to form a functional cell division complex allowed Chlamydia to dispense with its FtsZ-based cell division machinery during genome reduction. Thus, MreB-RodZ represents a possible mechanism for cell division in other bacteria lacking FtsZ.

An Escherichia coli expression model reveals the species-specific function of FtsA from Neisseria gonorrhoeae in cell division

Escherichia coli (Ec) has been used to study the function of cell division proteins from different microorganisms, especially when genetic tools are limited for studying these proteins in their native hosts. The expression of ftsA from Neisseria gonorrhoeae (Ng) disrupted cell division in E. coli resulting in a significant increase in cell length. In some cells, FtsANg localised to the division site and the poles of E. coli cells, but the majority of cells showed no specifical localisation. FtsANg did not complement an E. coli ftsA mutant strain. Bacterial two-hybrid and GST pull-down assays indicated that FtsANg interacted with FtsNEc, but no other cell division proteins from E. coli. This interaction was mediated through the 2A and 2B subdomains of FtsANg. This evidence suggests that the function of FtsANg is species specific.

FtsZ Constriction Force - Curved Protofilaments Bending Membranes

FtsZ assembles in vitro into protofilaments (pfs) that are one subunit thick and ~50 subunits long. In vivo these pfs assemble further into the Z ring, which, along with accessory division proteins, constricts to divide the cell. We have reconstituted Z rings in liposomes in vitro, using pure FtsZ that was modified with a membrane targeting sequence to directly bind the membrane. This FtsZ-mts assembled Z rings and constricted the liposomes without any accessory proteins. We proposed that the force for constriction was generated by a conformational change from straight to curved pfs. Evidence supporting this mechanism came from switching the membrane tether to the opposite side of the pf. These switched-tether pfs assembled "inside-out" Z rings, and squeezed the liposomes from the outside, as expected for the bending model. We propose three steps for the full process of cytokinesis: (a) pf bending generates a constriction force on the inner membrane, but the rigid peptidoglycan wall initially prevents any invagination; (b) downstream proteins associate to the Z ring and remodel the peptidoglycan, permitting it to follow the constricting FtsZ to a diameter of ~250 nm; the final steps of closure of the septum and membrane fusion are achieved by excess membrane synthesis and membrane fluctuations.

Recent advances in the discovery and development of antibacterial agents targeting the cell-division protein FtsZ

With the emergence of multidrug-resistant bacterial strains, there is a dire need for new drug targets for antibacterial drug discovery and development. Filamentous temperature sensitive protein Z (FtsZ), is a GTP-dependent prokaryotic cell division protein, sharing less than 10% sequence identity with the eukaryotic cell division protein, tubulin. FtsZ forms a dynamic Z-ring in the middle of the cell, leading to septation and subsequent cell division. Inhibition of the Z-ring blocks cell division, thus making FtsZ a highly attractive target. Various groups have been working on natural products and synthetic small molecules as inhibitors of FtsZ. This review summarizes the recent advances in the development of FtsZ inhibitors, focusing on those in the last 5years, but also includes significant findings in previous years.

Membrane remodelling in bacteria

In bacteria the ability to remodel membrane underpins basic cell processes such as growth, and more sophisticated adaptations like inter-cell crosstalk, organelle specialisation, and pathogenesis. Here, selected examples of membrane remodelling in bacteria are presented and the diverse mechanisms for inducing membrane fission, fusion, and curvature discussed. Compared to eukaryotes, relatively few curvature-inducing proteins have been characterised so far. Whilst it is likely that many such proteins remain to be discovered, it also reflects the importance of alternative membrane remodelling strategies in bacteria where passive mechanisms for generating curvature are utilised.

FtsZ Protofilament Curvature Is the Opposite of Tubulin Rings

FtsZ protofilaments (pfs) form the bacterial cytokinetic Z ring. Previous work suggested that a conformational change from straight to curved pfs generated the constriction force. In the simplest model, the C-terminal membrane tether is on the outside of the curved pf, facing the membrane. Tubulin, a homologue of FtsZ, also forms pfs with a curved conformation. However, it is well-established that tubulin rings have the C terminus on the inside of the ring. Could FtsZ and tubulin rings have the opposite curvature? In this study, we explored the FtsZ curvature direction by fusing large protein tags to the FtsZ termini. Thin section electron microscopy showed that the C-terminal tag was on the outside, consistent with the bending pf model. This has interesting implications for the evolution of tubulin. Tubulin likely began with the curvature of FtsZ, but evolution managed to reverse direction to produce outward-curving rings, which are useful for pulling chromosomes.

A structural framework for a near-minimal form of life: mass and compositional analysis of the helical mollicute Spiroplasma melliferum BC3

Spiroplasma melliferum is a wall-less bacterium with dynamic helical geometry. This organism is geometrically well defined and internally well ordered, and has an exceedingly small genome. Individual cells are chemotactic, polar, and swim actively. Their dynamic helicity can be traced at the molecular level to a highly ordered linear motor (composed essentially of the proteins fib and MreB) that is positioned on a defined helical line along the internal face of the cell's membrane. Using an array of complementary, informationally overlapping approaches, we have taken advantage of this uniquely simple, near-minimal life-form and its helical geometry to analyze the copy numbers of Spiroplasma's essential parts, as well as to elucidate how these components are spatially organized to subserve the whole living cell. Scanning transmission electron microscopy (STEM) was used to measure the mass-per-length and mass-per-area of whole cells, membrane fractions, intact cytoskeletons and cytoskeletal components. These local data were fit into whole-cell geometric parameters determined by a variety of light microscopy modalities. Hydrodynamic data obtained by analytical ultracentrifugation allowed computation of the hydration state of whole living cells, for which the relative amounts of protein, lipid, carbohydrate, DNA, and RNA were also estimated analytically. Finally, ribosome and RNA content, genome size and gene expression were also estimated (using stereology, spectroscopy and 2D-gel analysis, respectively). Taken together, the results provide a general framework for a minimal inventory and arrangement of the major cellular components needed to support life.

Skin and bones: the bacterial cytoskeleton, cell wall, and cell morphogenesis

The bacterial world is full of varying cell shapes and sizes, and individual species perpetuate a defined morphology generation after generation. We review recent findings and ideas about how bacteria use the cytoskeleton and other strategies to regulate cell growth in time and space to produce different shapes and sizes.

The structure of FtsZ filaments in vivo suggests a force-generating role in cell division

In prokaryotes, FtsZ (the filamentous temperature sensitive protein Z) is a nearly ubiquitous GTPase that localizes in a ring at the leading edge of constricting plasma membranes during cell division. Here we report electron cryotomographic reconstructions of dividing Caulobacter crescentus cells wherein individual arc-like filaments were resolved just underneath the inner membrane at constriction sites. The filaments' position, orientation, time of appearance, and resistance to A22 all suggested that they were FtsZ. Predictable changes in the number, length, and distribution of filaments in cells where the expression levels and stability of FtsZ were altered supported that conclusion. In contrast to the thick, closed-ring-like structure suggested by fluorescence light microscopy, throughout the constriction process the Z-ring was seen here to consist of just a few short (approximately 100 nm) filaments spaced erratically near the division site. Additional densities connecting filaments to the cell wall, occasional straight segments, and abrupt kinks were also seen. An 'iterative pinching' model is proposed wherein FtsZ itself generates the force that constricts the membrane in a GTP-hydrolysis-driven cycle of polymerization, membrane attachment, conformational change, depolymerization, and nucleotide exchange.

Cytoskeleton of mollicutes

Mollicutes are a class of bacteria that lack a peptidoglycan layer but have various cell shapes. They perform chromosome segregation and binary fission in a well-organized manner. Especially, species with polarized cell morphology duplicate their membrane protrusion at a position adjacent to the original one and move the new protrusion laterally to the opposite end pole before cell division. The featured various cell shapes of Mollicutes are supported by cytoskeletal structures composed of proteins. Recent progress in the study of cytoskeletons of walled bacteria and genome sequencing has revealed that the cytoskeletons of Mollicutes are not common with those of other bacteria. Mollicutes have special cytoskeletal proteins and structures that are sometimes not shared even by other mollicute species.

The cytoskeleton of spiroplasma: a complex linear motor

Spiroplasma are wall-less, helical bacteria from the class Mollicutes. The Mollicutes (Mycoplasma, Acholeplasma, Spiroplasma) evolved by regressive evolution to generate one of the simplest and minimal free-living and self-replicating forms of life. The spiroplasmas are the more advanced members in the class and are the closest to their clostridial ancestors. Spiroplasmas were discovered and identified as such only in 1972 and the finding of a unique and well-defined internal cytoskeleton, believed to be uncommon in bacteria, followed in 1973. Structural analysis suggests that the core of the spiroplasmal cytoskeleton is a flat, monolayered ribbon comprised of the 59-kDa fib gene product. The ribbon follows the shortest helical line of the polar cell from end to end. The structural building blocks of the cytoskeletal ribbon are fibrils assembling into a structure with approximately 10-nm axial and lateral repeats. Differential length changes of the fibrils may generate a wide dynamic spectrum of helical and non-helical geometries allowing for directional motility in low Reynolds number environments. The presence of other cytoskeletal elements (FtsZ, FtsA, EF-TU, MreB) has been demonstrated only recently in Spiroplasma cells. The cellular and molecular structure and dynamics of spiroplasmas and their cytoskeletal elements are reviewed.

FtsZ from divergent foreign bacteria can function for cell division in Escherichia coli

FtsZs from Mycoplasma pulmonis (MpuFtsZ) and Bacillus subtilis (BsFtsZ) are only 46% and 53% identical in amino acid sequence to FtsZ from Escherichia coli (EcFtsZ). In the present study we show that MpuFtsZ and BsFtsZ can function for cell division in E. coli provided we make two modifications. First, we replaced their C-terminal tails with that from E. coli, giving the foreign FtsZ the binding site for E. coli FtsA and ZipA. Second, we selected for mutations in the E. coli genome that facilitated division by the foreign FtsZs. These suppressor strains arose at a relatively high frequency of 10(-3) to 10(-5), suggesting that they involve loss-of-function mutations in multigene pathways. These pathways may be negative regulators of FtsZ or structural pathways that facilitate division by slightly defective FtsZ. Related suppressor strains were obtained for EcFtsZ containing certain point mutations or insertions of yellow fluorescent protein. The ability of highly divergent FtsZs to function for division in E. coli is consistent with a two-part mechanism. FtsZ assembles the Z ring, and perhaps generates the constriction force, through self interactions; the downstream division proteins remodel the peptidoglycan wall by interacting with each other and the wall. The C-terminal peptide of FtsZ, which binds FtsA, provides the link between FtsZ assembly and peptidoglycan remodeling.

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.

Characterization of FtsZ homolog from hyperthermophilic archaeon Pyrococcus kodakaraensis KOD1

The gene of bacterial type ftsZ homolog in hyperthermophilic archaeon, Pyrococcus kodakaraensis KOD1 (Pk-ftsZ), was identified. The gene product of the Pk-ftsZ gene is composed of 380 amino acids with a molecular mass of 41,354 Da. In the deduced amino acid sequence of the Pk-ftsZ gene, a glycine-rich sequence (Gly-Gly-Gly-Thr-Gly-Ala-Gly) implicated in GTP binding was well conserved. The Pk-ftsZ gene was overexpressed using Escherichia coli as a host and the recombinant protein was purified. The purified Pk-FtsZ protein exhibited GTPase activity with optimum temperatures higher than 80 degrees C. However, the protein showed little GTPase activity at 40 degrees C, indicating that a high reaction temperature is required for the GTPase activity in accordance with the thermophilic nature of P. kodakaraensis KOD1. The GTP-binding ability of Pk-FtsZ protein could also be detected by UV-induced cross-linking of a protein to [alpha-32P] GTP. The Pk-ftsZ gene was expressed in E. coli cells with a temperature-sensitive ftsZ mutation, E. coli ftsZ84 (ts), but its mutant phenotype of elongated cell form at a nonpermissive temperature (42 degrees C) could not be compensated, possibly because of the thermophilic nature of the Pk-FtsZ. Pk-FtsZ could form protofilaments in a GTP-dependent manner at 90 degrees C. Results of phylogenetic analysis suggest that there might be additional factors required for formation of the Z ring in P. kodakaraensis KOD1.

Overexpression of the ftsZ gene from Corynebacterium glutamicum (Brevibacterium lactofermentum) in Escherichia coli

Our goal in this work was to overexpress the essential cell division FtsZ protein from Corynebacterium glutamicum (Brevibacterium lactofermentum) (FtsZCG) in Escherichia coli to produce anti-FtsZCG polyclonal antibodies. Previous results from our laboratory showed that ftsZCG was not expressed in E. coli in a sufficient amount to purify FtsZCG. However, when ftsZCG (without upstream sequences) was transcriptionally fused to the T7 promoter, different truncated FtsZCG proteins (28-32 kDa) were overexpressed in E. coli, and in all cases, stop codons were created because of DNA deletions or rearrangements. Nevertheless, we were able to overexpress and purify an N-terminally hexa-His-tagged FtsZCG from uninduced E. coli cells carrying a pET-28a(+) derivative, yielding about 5 mg of 98% pure protein per 100-mL culture.

Bacterial cell shape

Bacterial species have long been classified on the basis of their characteristic cell shapes. Despite intensive research, the molecular mechanisms underlying the generation and maintenance of bacterial cell shape remain largely unresolved. The field has recently taken an important step forward with the discovery that eukaryotic cytoskeletal proteins have homologues in bacteria that affect cell shape. Here, we discuss how a bacterium gains and maintains its shape, the challenges still confronting us and emerging strategies for answering difficult questions in this rapidly evolving field.

Bacterial cell division and the septal ring

Cell division in bacteria is mediated by the septal ring, a collection of about a dozen (known) proteins that localize to the division site, where they direct assembly of the division septum. The foundation of the septal ring is a polymer of the tubulin-like protein FtsZ. Recently, experiments using fluorescence recovery after photobleaching have revealed that the Z ring is extremely dynamic. FtsZ subunits exchange in and out of the ring on a time scale of seconds even while the overall morphology of the ring appears static. These findings, together with in vitro studies of purified FtsZ, suggest that the rate-limiting step in turnover of FtsZ polymers is GTP hydrolysis. Another component of the septal ring, FtsK, is involved in coordinating chromosome segregation with cell division. Recent studies have revealed that FtsK is a DNA translocase that facilitates decatenation of sister chromosomes by TopIV and resolution of chromosome dimers by the XerCD recombinase. Finally, two murein hydrolases, AmiC and EnvC, have been shown to localize to the septal ring of Escherichia coli, where they play an important role in separation of daughter cells.

The Microtubule Analog Protein, FtsZ, in the Endosymbiont of Trypanosomatid Protozoa

Blastocrithidia culicis and Crithidia deanei are trypanosomatids that harbor an endosymbiotic bacterium in their cytoplasm. In prokaryotes, numerous proteins are essential for cell division, such as FtsZ, which is encoded by filament-forming temperature-sensitive (fts) genes. FtsZ is the prokaryotic homolog of eukaryotic tubulin and is present in bacteria and archaea, and has also been identified in mitochondria and chloroplasts. FtsZ plays a key role in the initiation of cytokinesis. It self-assembles into the Z ring, which establishes the division plane during septation. In this study, immunoblotting analysis using a FtsZ polyclonal antibody, revealed a 40-kDa band characteristic of FtsZ in endosymbiont fractions and in whole trypanosomatid homogenates, but not in whole cell extracts of aposymbiotic strains. Confocal microscopy and ultrastructural analysis revealed a specific and dispersed labeling over the endosymbiont. Bars and ring-like structures, which are suggestive of the presence of Z-rings, were never observed, even during the division of the symbiont. This peculiar distribution of FtsZ may represent an arrangement of cytoskeleton protein intermediate between prokaryotic and eukaryotic cells. The endosymbiont ftsz gene was completely sequenced after amplification of DNA from symbiont-bearing trypanosomatids or from pure endosymbiont fractions, using PCR and specific primers. The sequences obtained from the endosymbionts from C. deanei and B. culicis were very similar, and were most closely related to bacteria from the genus Pseudomonas.

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.

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.

Analysis of ftsQ mutant alleles in Escherichia coli: complementation, septal localization, and recruitment of downstream cell division proteins

FtsQ, a 276-amino-acid, bitopic membrane protein, is one of the nine proteins known to be essential for cell division in gram-negative bacterium Escherichia coli. To define residues in FtsQ critical for function, we performed random mutagenesis on the ftsQ gene and identified four alleles (ftsQ2, ftsQ6, ftsQ15, and ftsQ65) that fail to complement the ftsQ1(Ts) mutation at the restrictive temperature. Two of the mutant proteins, FtsQ6 and FtsQ15, are functional at lower temperatures but are unable to localize to the division site unless wild-type FtsQ is depleted, suggesting that they compete poorly with the wild-type protein for septal targeting. The other two mutants, FtsQ2 and FtsQ65, are nonfunctional at all temperatures tested and have dominant-negative effects when expressed in an ftsQ1(Ts) strain at the permissive temperature. FtsQ2 and FtsQ65 localize to the division site in the presence or absence of endogenous FtsQ, but they cannot recruit downstream cell division proteins, such as FtsL, to the septum. These results suggest that FtsQ2 and FtsQ65 compete efficiently for septal targeting but fail to promote the further assembly of the cell division machinery. Thus, we have separated the localization ability of FtsQ from its other functions, including recruitment of downstream cell division proteins, and are beginning to define regions of the protein responsible for these distinct capabilities.

Differential regulation of ftsZ transcription during septation of Streptomyces griseus

Streptomyces has been known to form two types of septa. The data in this research demonstrated that Streptomyces griseus forms another type of septum near the base of sporogenic hyphae (basal septum). To understand the regulation of the septation machinery in S. griseus, we investigated the expression of the ftsZ gene. S1 nuclease protection assays revealed that four ftsZ transcripts were differentially expressed during morphological differentiation. The vegetative transcript (emanating from P(veg)) is present at a moderate level during vegetative growth, but is switched off within the first 2 h of sporulation. Two sporulation-specific transcripts predominantly accumulated, and the levels increased by approximately fivefold together shortly before sporulation septa begin to form. Consistently, the sporulation-specific transcripts were expressed much earlier and more abundantly in a group of nonsporulating mutants that form their sporulation septa prematurely. Promoter-probe studies with two different reporter systems confirmed the activities of the putative promoters identified from the 5' end point of the transcripts. The levels and expression timing of promoter activities were consistent with the results of nuclease protection assays. The aseptate phenotype of the P(spo) mutant indicated that the increased transcription from P(spo) is required for sporulation septation, but not for vegetative or basal septum formation.

The bacterial cell-division protein ZipA and its interaction with an FtsZ fragment revealed by X-ray crystallography

In Escherichia coli, FtsZ, a homologue of eukaryotic tubulins, and ZipA, a membrane-anchored protein that binds to FtsZ, are two essential components of the septal ring structure that mediates cell division. Recent data indicate that ZipA is involved in the assembly of the ring by linking FtsZ to the cytoplasmic membrane and that the ZipA-FtsZ interaction is mediated by their C-terminal domains. We present the X-ray crystal structures of the C-terminal FtsZ-binding domain of ZipA and a complex between this domain and a C-terminal fragment of FtsZ. The ZipA domain is a six-stranded beta-sheet packed against three alpha-helices and contains the split beta-alpha-beta motif found in many RNA-binding proteins. The uncovered side of the sheet incorporates a shallow hydrophobic cavity exposed to solvent. In the complex, the 17-residue FtsZ fragment occupies this entire cavity of ZipA and binds as an extended beta-strand followed by alpha-helix. An alanine-scanning mutagenesis analysis of the FtsZ fragment was also performed, which shows that only a small cluster of the buried FtsZ side chains is critical in binding to ZipA.

Identification of an antigen localized to an apparent septum within dividing chlamydiae

The process of chlamydial cell division has not been thoroughly investigated. The lack of detectable peptidoglycan and the absence of an FtsZ homolog within chlamydiae suggest an unusual mechanism for the division process. Our laboratory has identified an antigen (SEP antigen) localized to a ring-like structure at the apparent septum within dividing chlamydial reticulate bodies (RB). Antisera directed against SEP show similar patterns of antigen distribution in Chlamydia trachomatis and Chlamydia psittaci RB. In contrast to localization in RB, SEP in elementary bodies appears diffuse and irregular, suggesting that the distribution of the antigen is developmental-stage specific. Treatment of chlamydiae with inhibitors of peptidoglycan synthesis or culture of chlamydiae in medium lacking tryptophan leads to the formation of nondividing, aberrant RB. Staining of aberrant RB with anti-SEP reveals a marked redistribution of the antigen. Within C. trachomatis-infected cells, ampicillin treatment leads to high levels of SEP accumulation at the periphery of aberrant RB, while in C. psittaci, treatment causes SEP to localize to distinct punctate sites within the bacteria. Aberrancy produced via tryptophan depletion results in a different pattern of SEP distribution. In either case, the reversal of aberrant formation results in the production of normal RB and a redistribution of SEP to the apparent plane of bacterial division. Collectively these studies identify a unique chlamydial-genus-common and developmental-stage-specific antigen that may be associated with RB division.

The division and cell wall gene cluster of Enterococcus hirae S185

A chromosomal 10355-bp segment of Enterococcus hirae S185 contains nine orfs which occur in the same order as the MraW-, FtsL-, PBP3-, MraY-, MurD-, MurG-, FtsQ-, FtsA- and FtsZ-encoding genes of the division and cell wall clusters of Escherichia coli and Bacillus subtilis. The E. hirae DNA segment lacks the genes which in E. coli encode the ligases Ddl, MurC, MurE and MurF and the integral membrane protein FtsW. The encoded E. hirae and E. coli proteins share 25% to 50% identity except FtsL and FtsQ (approximately = 14% identity).

Characterization of Acholeplasma laidlawii ftsZ gene and its gene product

The ftsZ gene was found among representatives of all bacterial groups. FtsZ protein is an essential component of cell division ring. Contraction of this cytoskeleton-like ring is believed to be the universal way of bacterial division. Acholeplasma laidlawii possesses all features of the minimal mycoplasma cell and some traits of cell-wall bacteria and seems to be a promising object for study of basic principles of the bacterial division process. We cloned an A. laidlawii chromosomal fragment containing ftsZ gene and two flanking orf which also were identified. A. laidlawii FtsZ protein has been determined with polyclonal antibodies raised in rabbit. It was demonstrated that ftsZ gene of A. laidlawii could be expressed in E. coli cells. We also revealed that A. laidlawii FtsZ had a low similarity to proteins of Mycoplasma genitalium and M. pneumoniae. The comparison of FtsZ structures may be used for investigation of bacterial phylogenetic relations.

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.

Mollicutes-wall-less bacteria with internal cytoskeletons

The structure and motility of the Mollicutes (Spiroplasma, Mycoplasma, and Acholeplasma) are briefly reviewed. The data are presented from the perspective of prokaryotic and eukaryotic motors, cytoskeletons, and cell motility. The Mollicutes are eubacteria derived from Clostridia by regressive evolution and genome reduction to produce the smallest and simplest free-living and self-replicating cells. Structurally, the Mollicutes are characterized by a complete lack of a cell wall and the presence of an internal cytoskeleton. Spiroplasma, which are helical cells with a flat, ribbon-like cytoskeleton, are amenable to structural and geometrical analysis. Motility and shape changes can be explained and modeled by the cytoskeleton acting as a linear motor.

Molecular biology and pathogenicity of mycoplasmas

The recent sequencing of the entire genomes of Mycoplasma genitalium and M. pneumoniae has attracted considerable attention to the molecular biology of mycoplasmas, the smallest self-replicating organisms. It appears that we are now much closer to the goal of defining, in molecular terms, the entire machinery of a self-replicating cell. Comparative genomics based on comparison of the genomic makeup of mycoplasmal genomes with those of other bacteria, has opened new ways of looking at the evolutionary history of the mycoplasmas. There is now solid genetic support for the hypothesis that mycoplasmas have evolved as a branch of gram-positive bacteria by a process of reductive evolution. During this process, the mycoplasmas lost considerable portions of their ancestors' chromosomes but retained the genes essential for life. Thus, the mycoplasmal genomes carry a high percentage of conserved genes, greatly facilitating gene annotation. The significant genome compaction that occurred in mycoplasmas was made possible by adopting a parasitic mode of life. The supply of nutrients from their hosts apparently enabled mycoplasmas to lose, during evolution, the genes for many assimilative processes. During their evolution and adaptation to a parasitic mode of life, the mycoplasmas have developed various genetic systems providing a highly plastic set of variable surface proteins to evade the host immune system. The uniqueness of the mycoplasmal systems is manifested by the presence of highly mutable modules combined with an ability to expand the antigenic repertoire by generating structural alternatives, all compressed into limited genomic sequences. In the absence of a cell wall and a periplasmic space, the majority of surface variable antigens in mycoplasmas are lipoproteins. Apart from providing specific antimycoplasmal defense, the host immune system is also involved in the development of pathogenic lesions and exacerbation of mycoplasma induced diseases. Mycoplasmas are able to stimulate as well as suppress lymphocytes in a nonspecific, polyclonal manner, both in vitro and in vivo. As well as to affecting various subsets of lymphocytes, mycoplasmas and mycoplasma-derived cell components modulate the activities of monocytes/macrophages and NK cells and trigger the production of a wide variety of up-regulating and down-regulating cytokines and chemokines. Mycoplasma-mediated secretion of proinflammatory cytokines, such as tumor necrosis factor alpha, interleukin-1 (IL-1), and IL-6, by macrophages and of up-regulating cytokines by mitogenically stimulated lymphocytes plays a major role in mycoplasma-induced immune system modulation and inflammatory responses.

The regulation of bacterial cell division: a time and place for it

Temporal and spatial regulation of cell division assures that each daughter cell receives a copy of the chromosome. Within the past year, the application of fluorescence microscopy to the cell biology of bacteria has revealed an increasing number of proteins that are localized within the bacterial cell to carry out DNA segregation and cell division. The localization of these proteins implies the existence of positional information in the cell, but how this information is established is unknown.

Dominant C-terminal deletions of FtsZ that affect its ability to localize in Caulobacter and its interaction with FtsA

The cell division protein FtsZ is composed of three regions based on sequence similarity: a highly conserved N-terminal region of approximately 320 amino acids; a variable spacer region; and a conserved C-terminal region of eight amino acids. We show that FtsZ mutants missing different C-terminal fragments have dominant lethal effects because they block cell division in Caulobacter crescentus by two different mechanisms. Removal of the C-terminal conserved region, the linker, and 40 amino acids from the end of the N-terminal conserved region (FtsZdeltaC281) prevents the localization or the polymerization of FtsZ. Because two-hybrid analysis indicates that FtsZdeltaC281 does not interact with FtsZ, we hypothesize that FtsZdeltaC281 blocks cell division by competing with a factor required for FtsZ localization or that it titrates a factor required for the stability of the FtsZ ring. The removal of 24 amino acids from the C-terminus of FtsZ (FtsZdeltaC485) causes a punctate pattern of FtsZ localization and affects its interaction with FtsA. This suggests that the conserved C-terminal region of FtsZ is required for proper polymerization of FtsZ in Caulobacter and for its interaction with FtsA.

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.

Cell reproduction and morphological changes in Mycoplasma capricolum

The cell reproduction of Mycoplasma capricolum was studied. The velocity of DNA replication fork progression was about 6 kb/min, which is 10 times slower than that of Escherichia coli. The time required for one round of DNA replication accorded with the doubling time. The origin/terminus ratio was 2.0. M. capricolum cell morphology was classified into two types, rod and branched. In the ordinary-growth phase, the rod cells accounted for about 90% of the total population, with branched cells comprising the remaining 10%. The proportion of branched cells increased to 90% following inhibition of DNA replication by nucleoside starvation. An increase in the proportion of branched cells was induced by transfer of a temperature-sensitive mutant deficient in DNA replication to the restrictive temperature. The rod cells had a regular structure, a fixed cell length, and constrictions in the center. The DNA contents of individual rod cells were distributed with a standard deviation of 0.40 of average. The branched cells had irregular structures and a wide distribution of DNA contents. Counting of viable cells revealed that the cells ceased division upon cell type conversion; however, branched cells maintained a reproductive capacity. A model for the reproduction process is proposed.

The role of GTP-binding proteins in mechanochemical movements of microorganisms and their potential to form filamentous structures

Prokaryotic cells contain proteins which form extended chains or multimers that oscillate between monomers and oligomers of varying length. Hydrolysis of nucleoside triphosphates combined with site-specific disposition of substrates and products to monomers and multimers is the driving force of dynamic instability of these molecules. Polymeric structures are connected in some manner to a variety of signaling systems that adhere to the polymeric matrix, including the GTP-binding protein(s), protein kinases and phosphatases, and other proteins or systems that communicate between the cytoplasmic membrane and the cytosol. Flexible organization allowing regulated dynamic movement is one of the key elements in all living cells. In eukaryotic cells actin and tubulin are the two main components of dynamically controlled spatial system. These proteins are noteworthy for their ability to polymerize, reversibly, into filaments or microtubules in association with hydrolysis of ATP or GTP, respectively. As such, they regulate most of the mechanics of cell movement including cell division, cell differentiation, phagocytosis and other dynamic phenomena. Recent evidence revealed that microbial cells create functional domains at specific sites of the cells and can form cytoplasmic tubules and fibers.

Analysis of the interaction of FtsZ with itself, GTP, and FtsA

The interaction of FtsZ with itself, GTP, and FtsA was examined by analyzing the sensitivity of FtsZ to proteolysis and by using the yeast two-hybrid system. The N-terminal conserved domain consisting of 320 amino acids bound GTP, and a central region of FtsZ, encompassing slightly more than half of the protein, was cross-linked to GTP. Site-directed mutagenesis revealed that none of six highly conserved aspartic acid and asparagine residues were required for GTP binding. These results indicate that the specificity determinants for GTP binding are different than those for the GTPase superfamily. The N-terminal conserved domain of FtsZ contained a site for self-interaction that is conserved between FtsZ proteins from distantly related bacterial species. FtsZ320, which was truncated at the end of the conserved domain, was a potent inhibitor of division although it expressed normal GTPase activity and could polymerize. FtsZ was also found to interact directly with FtsA, and this interaction could also be observed between these proteins from distantly related bacterial species.

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.

Colocalization of cell division proteins FtsZ and FtsA to cytoskeletal structures in living Escherichia coli cells by using green fluorescent protein

In the current model for bacterial cell division, FtsZ protein forms a ring that marks the division plane, creating a cytoskeletal framework for the subsequent action of other proteins such as FtsA. This putative protein complex ultimately generates the division septum. Herein we report that FtsZ and FtsA proteins tagged with green fluorescent protein (GEP) colocalize to division-site ring-like structures in living bacterial cells in a visible space between the segregated nucleoids. Cells with higher levels of FtsZ-GFP or with FtsA-GFP plus excess wild-type FtsZ were inhibited for cell division and often exhibited bright fluorescent spiral tubules that spanned the length of the filamentous cells. This suggests that FtsZ may switch from a septation-competent localized ring to an unlocalized spiral under some conditions and that FtsA can bind to FtsZ in both conformations. FtsZ-GFP also formed nonproductive but localized aggregates at a higher concentration that could represent FtsZ nucleation sites. The general domain structure of FtsZ-GFP resembles that of tubulin, since the C terminus of FtsZ is not required for polymerization but may regulate polymerization state. The N-terminal portion of Rhizobium FtsZ polymerized in Escherichia coli and appeared to copolymerize with E. coli FtsZ, suggesting a degree of interspecies functional conservation. Analysis of several deletions of FtsA-GFP suggests that multiple segments of FtsA are important for its localization to the FtsZ ring.