Bacterial cell division: regulating Z-ring formation

Leveraging Engineered Pseudomonas putida Minicells for Bioconversion of Organic Acids into Short-Chain Methyl Ketones

Methyl ketones, key building blocks widely used in diverse industrial applications, largely depend on oil-derived chemical methods for their production. Here, we investigated biobased production alternatives for short-chain ketones, adapting the solvent-tolerant soil bacterium Pseudomonas putida as a host for ketone biosynthesis either by whole-cell biocatalysis or using engineered minicells, chromosome-free bacterial vesicles. Organic acids (acetate, propanoate and butanoate) were selected as the main carbon substrate to drive the biosynthesis of acetone, butanone and 2-pentanone. Pathway optimization identified efficient enzyme variants from Clostridium acetobutylicum and Escherichia coli, tested with both constitutive and inducible expression of the cognate genes. By implementing these optimized pathways in P. putida minicells, which can be prepared through a simple three-step purification protocol, the feedstock was converted into the target short-chain methyl ketones. These results highlight the value of combining morphology and pathway engineering of noncanonical bacterial hosts to establish alternative bioprocesses for toxic chemicals that are difficult to produce by conventional approaches.

Minicell-forming Escherichia coli mutant with increased chemical production capacity and tolerance to toxic compounds

Minicell, a small spherical form of bacterium produced by abnormal fission, possesses cytoplasmic constituents similar to those of the parental cell, except for genomic DNA. E. coli strains were engineered to produce minicells and value-added chemicals. Minicell-forming mutants showed enhanced tolerance to toxic chemicals and a higher intracellular NADH/NAD⁺ ratio than the wild-type. When toxic chemicals such as isobutanol, isobutyraldehyde, and isobutyl acetate were produced in this mutant, the titers increased by 67 %, 175 %, and 214 %, respectively. In addition, morphological changes and membrane dispersion mechanisms in minicell-forming mutants improved lycopene production by 259 %. This increase in production capacity was more pronounced when biomass hydrolysate was used as the substrate. Isobutanol and lycopene production also increased by 92 % and 295 %, respectively, on using the substrate in the mutant. It suggests that minicell-forming mutants are an excellent platform for biochemical production.

Escherichia coli minicells with targeted enzymes as bioreactors for producing toxic compounds

Formed by aberrant cell division, minicells possess functional metabolism despite their inability to grow and divide. Minicells exhibit not only superior stability when compared with bacterial cells but also exceptional tolerance-characteristics that are essential for a de novo bioreactor platform. Accordingly, we engineered minicells to accumulate protein, ensuring sufficient production capability. When tested with chemicals regarded as toxic against cells, the engineered minicells produced titers of C6-C10 alcohols and esters, far surpassing the corresponding production from bacterial cells. Additionally, microbial autoinducer production that is limited in expanding bacterial population was conducted in the minicells. Because bacterial population growth was nonexistent, the minicells produced autoinducers in constant amounts, which allowed precise control of the bacterial population having autoinducer-responsive gene circuits. When bacterial population growth was nonexistent, the minicells produced autoinducers in constant amounts, which allowed precise control of the bacterial population having autoinducer-based gene circuits with the minicells. This study demonstrates the potential of minicells as bioreactors suitable for products with known limitations in microbial production, thus providing new possibilities for bioreactor engineering.

Thermo-Statistical Effects of Inclusions on Vesicles: Division into Multispheres and Polyhedral Deformation

The construction of simple cellular models has attracted much attention as a way to explore the origin of life or elucidate the mechanisms of cell division. In the absence of complex regulatory systems, some bacteria spontaneously divide through thermostatistically elucidated mechanisms, and incorporating these simple physical principles could help to construct primitive or artificial cells. Because thermodynamic interactions play an essential role in such mechanisms, this review discusses the thermodynamic aspects of spontaneous division models of vesicles that contain a high density of inclusions, with their membrane serving as a boundary. Vesicles with highly dense inclusions are deformed according to the volume-to-area ratio. The phase separation of beads at specific intermediate volume fractions and the associated polyhedral deformation of the membrane are considered in relation to the Alder transition. Current advances in the development of a membrane-growth vesicular model are summarized. The thermostatistical understanding of these mechanisms could become a cornerstone for the construction of vesicular models that display spontaneous cell division.

Catching a Walker in the Act-DNA Partitioning by ParA Family of Proteins

Partitioning the replicated genetic material is a crucial process in the cell cycle program of any life form. In bacteria, many plasmids utilize cytoskeletal proteins that include ParM and TubZ, the ancestors of the eukaryotic actin and tubulin, respectively, to segregate the plasmids into the daughter cells. Another distinct class of cytoskeletal proteins, known as the Walker A type Cytoskeletal ATPases (WACA), is unique to Bacteria and Archaea. ParA, a WACA family protein, is involved in DNA partitioning and is more widespread. A centromere-like sequence parS, in the DNA is bound by ParB, an adaptor protein with CTPase activity to form the segregation complex. The ParA ATPase, interacts with the segregation complex and partitions the DNA into the daughter cells. Furthermore, the Walker A motif-containing ParA superfamily of proteins is associated with a diverse set of functions ranging from DNA segregation to cell division, cell polarity, chemotaxis cluster assembly, cellulose biosynthesis and carboxysome maintenance. Unifying principles underlying the varied range of cellular roles in which the ParA superfamily of proteins function are outlined. Here, we provide an overview of the recent findings on the structure and function of the ParB adaptor protein and review the current models and mechanisms by which the ParA family of proteins function in the partitioning of the replicated DNA into the newly born daughter cells.

Filamentous Thermosensitive Mutant Z: An Appealing Target for Emerging Pathogens and a Trek on Its Natural Inhibitors

Antibiotic resistance is a major emerging issue in the health care sector, as highlighted by the WHO. Filamentous Thermosensitive mutant Z (Fts-Z) is gaining significant attention in the scientific community as a potential anti-bacterial target for fighting antibiotic resistance among several pathogenic bacteria. The Fts-Z plays a key role in bacterial cell division by allowing Z ring formation. Several in vitro and in silico experiments have demonstrated that inhibition of Fts-Z can lead to filamentous growth of the cells, and finally, cell death occurs. Many natural compounds that have successfully inhibited Fts-Z are also studied. This review article intended to highlight the structural-functional aspect of Fts-Z that leads to Z-ring formation and its contribution to the biochemistry and physiology of cells. The current trend of natural inhibitors of Fts-Z protein is also covered.

The lack of the cell division protein FtsZ induced generation of giant cells under acidic stress in cyanobacterium Synechocystis sp. PCC6803

Bacteria exposed to environmental stresses often exhibit superior acclimation abilities to environmental change. Acid treatment causes an increase in the cell length of the cyanobacterium Synechocystis sp. PCC6803 under light conditions. We aimed to elucidate the relationship between acidic stress and cell enlargement. After being synchronized under dark conditions, the cells were cultivated at different pH (pH 8.0 or pH 6.0) levels under light conditions. Synechocystis 6803 cells exhibited only cell growth occurred (cell volume expansion) and slow proliferation under the acidic condition. In the recovery experiment of the enlarged cells, they proliferated normally at pH 8.0, and the cell lengths decreased to the normal cell size under light conditions. Inhibition of cell division might be caused by acidic stress. To understand the effect of acidic stress on cell division, we evaluated the expression of FtsZ via Western blotting. The FtsZ concentration in cells was lower at pH 6.0 than at pH 8.0 and was not sufficient for cell division in the photoautotrophic conditions. ClpXP is well known as a regulator of the Z-ring dynamics in E. coli. The transcriptional level of four clpXP genes was upregulated approximately threefold at pH 6.0 after 24 h compared with that in cells grown at pH 8.0. The lack of FtsZ may be caused by the upregulation of clpXP expression under acidic condition. Therefore, ClpXP may participate in the degradation of FtsZ and be involved in the regulation of cell division via FtsZ under acidic stress in Synechocystis 6803.

DivIVA Regulates Its Expression and the Orientation of New Septum Growth in Deinococcus radiodurans

In rod-shaped Gram-negative bacteria, FtsZ localization at midcell position is regulated by the gradient of MinCDE complex across the poles. In round-shaped bacteria, which lack predefined poles, the next plane of cell division is perpendicular to the previous plane, and determination of the FtsZ assembly site is still intriguing. Deinococcus radiodurans, a coccus bacterium, is characterized by its extraordinary resistance to DNA damage. DivIVA, a putative component of the Min system in this bacterium, interacts with cognate cell division and genome segregation proteins. Here, we report that deletion of a chromosomal copy of DivIVA was possible only when the wild-type copy of DivIVA was expressed in trans on a plasmid. However, deletion of the C-terminal domain (CTD) of DivIVA (CTD mutant) was possible but produced distinguishable phenotypes, like smaller cells, slower growth, and tilted septum orientation, in D. radiodurans. In trans expression of DivIVA in the CTD mutant could restore these features of the wild type. Interestingly, the overexpression of DivIVA led to delayed separation of tetrads from an octet state in both trans-complemented divIVA-mutant and wild-type cells. The CTD mutant showed upregulation of the yggS-divIVAN operon. Both the wild type and CTD mutant formed FtsZ foci; however, unlike wild type, the position of foci in the mutant cells was found to be away from conjectural midcell position in cocci. Notably, DivIVA-red fluorescent protein (DivIVA-RFP) localizes to the septum during cell division at the new division site. These results suggested that DivIVA is an essential protein in D. radiodurans, and its C-terminal domain plays an important role in the regulation of its expression and orientation of new septal growth in this bacterium. IMPORTANCE In rod-shaped Gram-negative bacteria, the midcell position for binary fission is relatively easy to model. In cocci that do not have predefined poles, the plane of next cell division is shown to be perpendicular to the previous plane. However, the molecular basis of perpendicularity is not known in cocci. The DivIVA protein of Deinococcus radiodurans, a coccus bacterium, physically interacts with the septum and establishes macromolecular interactions with genome segregation proteins through its N-terminal domain and with MinC through the C-terminal domain. Here, we have brought forth some evidence to suggest that DivIVA is essential for growth and plays an important role in cell polarity determination, and its C-terminal domain plays a crucial role in the growth of new septa in the correct orientation as well as in the regulation of DivIVA expression.

Identification of a Genetically Linked but Functionally Independent Two-Component System Important for Cell Division of the Rice Pathogen Burkholderia glumae

Bacterial two-component regulatory systems control the expression of sets of genes to coordinate physiological functions in response to environmental cues. Here, we report a genetically linked but functionally unpaired two-component system (TCS) comprising the sensor kinase GluS (BGLU_1G13350) and the response regulator GluR (BGLU_1G13360), which is critical for cell division in the rice pathogen Burkholderia glumae BGR1. The gluR null mutant, unlike the gluS mutant, formed filamentous cells in Lysogeny Broth medium and was sensitive to exposure to 42°C. Expression of genes responsible for cell division and cell-wall (dcw) biosynthesis in the gluR mutant was elevated at transcription levels compared with the wild type. GluR-His bound to the putative promoter regions of ftsA and ftsZ is involved in septum formation, indicating that repression of genes in the dcw cluster by GluR is critical for cell division in B. glumae. The gluR mutant did not form filamentous cells in M9 minimal medium, whereas exogenous addition of glutamine or glutamate to the medium induced filamentous cell formation. These results indicate that glutamine and glutamate influence GluR-mediated cell division in B. glumae, suggesting that GluR controls cell division of B. glumae in a nutrition-dependent manner. These findings provide insight into how the recognition of external signals by TCS affects the sophisticated molecular mechanisms involved in controlling bacterial cell division.

Superior antibacterial activity of gallium based liquid metals due to Ga3+ induced intracellular ROS generation

Gallium metals demonstrate enhanced antibacterial activity compared to gallium nitrate with the same gallium ion concentration.

Shaving and breaking bacterial chains with a viscous flow

Some food and ferment manufacturing steps such as spray-drying result in the application of viscous stresses to bacteria. This study explores how a viscous flow impacts both bacterial adhesion functionality and bacterial cell organization using a combined experimental and modeling approach. As a model organism we study Lactobacillus rhamnosus GG (LGG) "wild type" (WT), known to feature strong adhesive affinities towards beta-lactoglobulin thanks to pili produced by the bacteria on cell surfaces, along with three cell-surface mutant strains. Applying repeated flows with high shear-rates reduces bacterial adhesive abilities up to 20% for LGG WT. Bacterial chains are also broken by this process, into 2-cell chains at low industrial shear rates, and into single cells at very high shear rates. To rationalize the experimental observations we study numerically and analytically the Stokes equations describing viscous fluid flow around a chain of elastically connected spheroidal cell bodies. In this model setting we examine qualitatively the relationship between surface traction (force per unit area), a proxy for pili removal rate, and bacterial chain length (number of cells). Longer chains result in higher maximal surface tractions, particularly at the chain extremities, while inner cells enjoy a small protection from surface tractions due to hydrodynamic interactions with their neighbors. Chain rupture therefore may act as a mechanism to preserve surface adhesive functionality in bacteria.

Transcriptomic analysis of smooth versus rough Brucella melitensis Rev.1 vaccine strains reveals insights into virulence attenuation

Brucella melitensis Rev.1 is the live attenuated Elberg-originated vaccine strain of the facultative intracellular Brucella species, and is widely used to control brucellosis in small ruminants. However, Rev.1 may cause abortions in small ruminants that have been vaccinated during the last trimester of gestation, it is pathogenic to humans, and it induces antibodies directed at the O-polysaccharide (O-PS) of the smooth lipopolysaccharide, thus making it difficult to distinguish between vaccinated and infected animals. Rough Brucella strains, which lack O-PS and are considered less pathogenic, have been introduced to address these drawbacks; however, as Rev.1 confers a much better immunity than the rough mutants, it is still considered the reference vaccine for the prophylaxis of brucellosis in small ruminants. Therefore, developing an improved vaccine strain, which lacks the Rev.1 drawbacks, is a highly evaluated task, which requires a better understanding of the molecular mechanisms underlying the virulence attenuation of Rev.1 smooth strains and of natural Rev.1 rough strains, which are currently only partly understood. As the acidification of the Brucella-containing vacuole during the initial stages of infection is crucial for their survival, identifying the genes that contribute to their survival in an acidic environment versus a normal environment will greatly assist our understanding of the molecular pathogenic mechanisms and the attenuated virulence of the Rev.1 strain. Here, we compared the transcriptomes of the smooth and natural rough Rev.1 strains, each grown under either normal or acidic conditions. We found 12 key genes that are significantly downregulated in the Rev.1 rough strains under normal pH, as compared with Rev.1 smooth strains, and six highly important genes that are significantly upregulated in the smooth strains under acidic conditions, as compared with Rev.1 rough strains. All 18 differentially expressed genes are associated with bacterial virulence and survival and may explain the attenuated virulence of the rough Rev.1 strains versus smooth Rev.1 strains, thus providing new insights into the virulence attenuation mechanisms of Brucella. These highly important candidate genes may facilitate the design of new and improved brucellosis vaccines.

Microfluidic time-lapse analysis and reevaluation of the Bacillus subtilis cell cycle

Recent studies taking advantage of automated single-cell time-lapse analysis have reignited interest in the bacterial cell cycle. Several studies have highlighted alternative models, such as Sizer and Adder, which differ essentially in relation to whether cells can measure their present size or their amount of growth since birth. Most of the recent work has been done with Escherichia coli. We set out to study the well-characterized Gram-positive bacterium, Bacillus subtilis, at the single-cell level, using an accurate fluorescent marker for division as well as a marker for completion of chromosome replication. Our results are consistent with the Adder model in both fast and slow growth conditions tested, and with Sizer but only at the slower growth rate. We also find that cell size variation arises not only from the expected variation in size at division but also that division site offset from mid-cell contributes to a significant degree. Finally, although traditional cell cycle models imply a strong connection between the termination of a round of replication and subsequent division, we find that at the single-cell level these events are largely disconnected.

Wahab H. Antituberculosis activity, phytochemical identification of Costus speciosus (J. Koenig) Sm., Cymbopogon citratus (DC. Ex Nees) Stapf., and Tabernaemontana coronaria (L.) Willd. and their effects on the growth kinetics and cellular integrity of Mycobacterium tuberculosis H37Rv

Background

Costus speciosus, Cymbopogon citratus, and Tabernaemontana coronaria are herbal plants traditionally used as remedies for symptoms of tuberculosis (TB) including cough. The aims of the present study were to evaluate the in vitro anti-TB activity of different solvent partitions of these plants, to identify the phytochemical compounds, and to assess the effects of the most active partitions on the growth kinetics and cellular integrity of the tubercle organism.

Methods

The in vitro anti-TB activity of different solvent partitions of the plant materials was determined against M. tuberculosis H37Rv using a tetrazolium colorimetric microdilution assay. The phytochemical compounds in the most active partition of each plant were identified using gas chromatography-mass spectrometry (GC-MS) analysis. The effects of these partitions on the growth kinetics of the mycobacteria were evaluated over 7-day treatment period in a batch culture system. Their effects on the mycobacterial cellular integrity were observed under a scanning electron microscope (SEM).

Results

The respective n-hexane partition of C. speciosus, C. citratus, and T. coronaria exhibited the highest anti-TB activity with minimum inhibitory concentrations (MICs) of 100–200 μg/mL and minimum bactericidal concentration (MBC) of 200 μg/mL. GC-MS phytochemical analysis of these active partitions revealed that majority of the identified compounds belonged to lipophilic fatty acid groups. The active partitions of C. speciosus and T. coronaria exhibited high cidal activity in relation to time, killing more than 99% of the cell population. SEM observations showed that these active plant partitions caused multiple structural changes indicating massive cellular damages.

Conclusions

The n-hexane partition of the plant materials exhibited promising in vitro anti-TB activity against M. tuberculosis H37Rv. Their anti-TB activity was supported by their destructive effects on the integrity of the mycobacterial cellular structure.

A Specialized Peptidoglycan Synthase Promotes Salmonella Cell Division inside Host Cells

Bacterial cell division has been studied extensively under laboratory conditions. Despite being a key event in the bacterial cell cycle, cell division has not been explored in vivo in bacterial pathogens interacting with their hosts. We discovered in Salmonella enterica serovar Typhimurium a gene absent in nonpathogenic bacteria and encoding a peptidoglycan synthase with 63% identity to penicillin-binding protein 3 (PBP3). PBP3 is an essential cell division-specific peptidoglycan synthase that builds the septum required to separate daughter cells. Since S. Typhimurium carries genes that encode a PBP3 paralog—which we named PBP3_SAL—and PBP3, we hypothesized that there are different cell division events in host and nonhost environments. To test this, we generated S. Typhimurium isogenic mutants lacking PBP3_SAL or the hitherto considered essential PBP3. While PBP3 alone promotes cell division under all conditions tested, the mutant producing only PBP3_SAL proliferates under acidic conditions (pH ≤ 5.8) but does not divide at neutral pH. PBP3_SAL production is tightly regulated with increased levels as bacteria grow in media acidified up to pH 4.0 and in intracellular bacteria infecting eukaryotic cells. PBP3_SAL activity is also strictly dependent on acidic pH, as shown by beta-lactam antibiotic binding assays. Live-cell imaging microscopy revealed that PBP3_SAL alone is sufficient for S. Typhimurium to divide within phagosomes of the eukaryotic cell. Additionally, we detected much larger amounts of PBP3_SAL than those of PBP3 in vivo in bacteria colonizing mouse target organs. Therefore, PBP3_SAL evolved in S. Typhimurium as a specialized peptidoglycan synthase promoting cell division in the acidic intraphagosomal environment.

During bacterial cell division, daughter cells separate by a transversal structure known as the division septum. The septum is a continuum of the cell wall and therefore is composed of membrane(s) and a peptidoglycan layer. To date, actively growing bacteria were reported to have only a “cell division-specific” peptidoglycan synthase required for the last steps of septum formation and consequently, essential for bacterial life. Here, we discovered that Salmonella enterica has two peptidoglycan synthases capable of synthesizing the division septum. One of these enzymes, PBP3_SAL, is present only in bacterial pathogens and evolved in Salmonella to function exclusively in acidic environments. PBP3_SAL is used preferentially by Salmonella to promote cell division in vivo in mouse target organs and inside acidified phagosomes. Our data challenge the concept of only one essential cell division-specific peptidoglycan synthase and demonstrate that pathogens can divide in defined host locations using alternative mechanisms.

Engineering Rugged Field Assays to Detect Hazardous Chemicals Using Spore-Based Bacterial Biosensors

Bacterial whole cell-based biosensors have been genetically engineered to achieve selective and reliable detection of a wide range of hazardous chemicals. Although whole-cell biosensors demonstrate many advantages for field-based detection of target analytes, there are still some challenges that need to be addressed. Most notably, their often modest shelf life and need for special handling and storage make them challenging to use in situations where access to reagents, instrumentation, and expertise are limited. These problems can be circumvented by developing biosensors in Bacillus spores, which can be engineered to address all of these concerns. In its sporulated state, a whole cell-based biosensor has a remarkably long life span and is exceptionally resistant to environmental insult. When these spores are germinated for use in analytical techniques, they show no loss in performance, even after long periods of storage under harsh conditions. In this chapter, we will discuss the development and use of whole cell-based sensors, their adaptation to spore-based biosensors, their current applications, and future directions in the field.

WhmD promotes the assembly of Mycobacterium smegmatis FtsZ: A possible role of WhmD in bacterial cell division

WhmD is considered to have a role in the septation and division of Mycobacterium smegmatis cells. Since FtsZ is the central protein of the septum, we determined the effect of WhmD on the assembly of Mycobacterium smegmatis FtsZ (MsFtsZ) in vitro. WhmD increased both the rate and extent of the assembly of MsFtsZ in vitro. WhmD also increased the amount of polymerized MsFtsZ as evident from a sedimentation assay. Further, the assembly promoting activity of WhmD occurred in the presence of GTP. MsFtsZ polymerized to form thin filaments in the absence of WhmD while MsFtsZ formed thick filaments in the presence of WhmD suggesting that WhmD enhanced the bundling of MsFtsZ filaments. Interestingly, WhmD neither suppressed the dilution-induced disassembly of FtsZ filaments nor significantly altered the GTPase activity of FtsZ. Using size exclusion chromatography, circular dichroism and fluorescence spectroscopy, WhmD was found to bind to MsFtsZ in vitro. The results showed that WhmD can promote the assembly of FtsZ and indicated that WhmD may play a role in the division of M. smegmatis cells by assisting the polymerization of FtsZ.

Resveratrol antibacterial activity against Escherichia coli is mediated by Z-ring formation inhibition via suppression of FtsZ expression

Resveratrol exhibits a potent antimicrobial activity. However, the mechanism underlying its antibacterial activity has not been shown. In this study, the antibacterial mechanism of resveratrol was investigated. To investigate induction of the SOS response, a strain containing the lacZ⁺gene under the control of an SOS-inducible sulA promoter was constructed. DNA damage was measured by pulse-field gel electrophoresis (PFGE). After resveratrol treatment, the cells were observed by confocal microscopy. For the RNA silencing assay, ftsZ-specific antisense peptide nucleic acid (PNA) was used. Reactive oxygen species (ROS) production increased in Escherichia coli after resveratrol treatment; however, cell growth was not recovered by ROS quenching, indicating that, in this experiment, ROS formation and cell death following resveratrol treatment were not directly correlated. Resveratrol treatment increased DNA fragmentation in cells, while SOS response-related gene expression levels increased in a dose-dependent manner. Cell elongation was observed after resveratrol treatment. Elongation was induced by inhibiting FtsZ, an essential cell-division protein in prokaryotes, and resulted in significant inhibition of Z-ring the formation in E. coli. The expression of ftsZ mRNA was suppressed by resveratrol. Our results indicate that resveratrol inhibits bacterial cell growth by suppressing FtsZ expression and Z-ring formation.

Eukaryote-like Ser/Thr protein kinase PrkA modulates sporulation via regulating the transcriptional factor σ(K) in Bacillus subtilis

Protein kinase A (PrkA), also known as AMP-activated protein kinase, functions as a serine/threonine protein kinase (STPK), has been shown to be involved in a variety of important biologic processes, including pathogenesis of many important diseases in mammals. However, the biological functions of PrkA are less known in prokaryote cells. Here, we explored the function of PrkA as well as its underlying molecular mechanisms using the model bacterium Bacillus subtilis168. When PrkA is inhibited by 9-β-D-arabinofuranosyladenine (ara-A) in the wild type strain or deleted in the ΔprkA mutant strain, we observed sporulation defects in B. subtilis 168, suggesting that PrkA functions as a sporulation-related protein. Transcriptional analysis using the lacZ reporter gene demonstrated that deletion of prkA significantly reduced the expression of the transcriptional factor σ(K) and its downstream genes. Complementation of sigK gene in prkA knockout mutant partially rescued the phenotype of ΔprkA, further supporting the hypothesis that the decreased σ(K) expression should be one of the reasons for the sporulation defect resulting from prkA disruption. Finally, our data confirmed that Hpr (ScoC) negatively controlled the expression of transcriptional factor σ(K), and thus PrkA accelerated sporulation and the expression of σ(K) by suppression of Hpr (ScoC). Taken together, our study discovered a novel function of the eukaryotic-like STPK PrkA in spore development as well as its underlying molecular mechanism in B. subtilis.

LocZ is a new cell division protein involved in proper septum placement in Streptococcus pneumoniae

How bacteria control proper septum placement at midcell, to guarantee the generation of identical daughter cells, is still largely unknown. Although different systems involved in the selection of the division site have been described in selected species, these do not appear to be widely conserved. Here, we report that LocZ (Spr0334), a newly identified cell division protein, is involved in proper septum placement in Streptococcus pneumoniae. We show that locZ is not essential but that its deletion results in cell division defects and shape deformation, causing cells to divide asymmetrically and generate unequally sized, occasionally anucleated, daughter cells. LocZ has a unique localization profile. It arrives early at midcell, before FtsZ and FtsA, and leaves the septum early, apparently moving along with the equatorial rings that mark the future division sites. Consistently, cells lacking LocZ also show misplacement of the Z-ring, suggesting that it could act as a positive regulator to determine septum placement. LocZ was identified as a substrate of the Ser/Thr protein kinase StkP, which regulates cell division in S. pneumoniae. Interestingly, homologues of LocZ are found only in streptococci, lactococci, and enterococci, indicating that this close phylogenetically related group of bacteria evolved a specific solution to spatially regulate cell division.

IMPORTANCE

Bacterial cell division is a highly ordered process regulated in time and space. Recently, we reported that the Ser/Thr protein kinase StkP regulates cell division in Streptococcus pneumoniae, through phosphorylation of several key proteins. Here, we characterized one of the StkP substrates, Spr0334, which we named LocZ. We show that LocZ is a new cell division protein important for proper septum placement and likely functions as a marker of the cell division site. Consistently, LocZ supports proper Z-ring positioning at midcell. LocZ is conserved only among streptococci, lactococci, and enterococci, which lack homologues of the Min and nucleoid occlusion effectors, indicating that these bacteria adapted a unique mechanism to find their middle, reflecting their specific shape and symmetry.

Bacterial cell division regulation by Ser/Thr kinases: a structural perspective

Recent genetic, biochemical and structural studies have established that eukaryotic-like Ser/Thr protein-kinases are critical mediators of developmental changes and host pathogen interactions in bacteria. Although with lower abundance compared to their homologues from eukaryotes, Ser/Thr protein-kinases are widespread in gram-positive bacteria. These data underline a key role of reversible Ser/Thr phosphorylation in bacterial physiology and virulence. Numerous studies have revealed how phosphorylation/dephosphorylation of Ser/Thr protein-kinases governs cell division and cell wall biosynthesis and that Ser/Thr protein kinases are responsible for distinct phenotypes, dependent on different environmental signals. In this review we discuss the current understandings of Ser/Thr protein-kinases functional processes based on structural data.

The role of lipid domains in bacterial cell processes

Membranes are vital structures for cellular life forms. As thin, hydrophobic films, they provide a physical barrier separating the aqueous cytoplasm from the outside world or from the interiors of other cellular compartments. They maintain a selective permeability for the import and export of water-soluble compounds, enabling the living cell to maintain a stable chemical environment for biological processes. Cell membranes are primarily composed of two crucial substances, lipids and proteins. Bacterial membranes can sense environmental changes or communication signals from other cells and they support different cell processes, including cell division, differentiation, protein secretion and supplementary protein functions. The original fluid mosaic model of membrane structure has been recently revised because it has become apparent that domains of different lipid composition are present in both eukaryotic and prokaryotic cell membranes. In this review, we summarize different aspects of phospholipid domain formation in bacterial membranes, mainly in Gram-negative Escherichia coli and Gram-positive Bacillus subtilis. We describe the role of these lipid domains in membrane dynamics and the localization of specific proteins and protein complexes in relation to the regulation of cellular function.

Localization of FtsZ in Helicobacter pylori and consequences for cell division

Of the various kinds of cell division, the most common mode is binary fission, the division of a cell into two morphologically identical daughter cells. However, in the case of asymmetric cell division, Caulobacter crescentus produces two morphologically and functionally distinct cell types. Here, we have studied cell cycle progression of the human pathogen Helicobacter pylori using a functional green fluorescent protein (GFP) fusion of FtsZ protein and membrane staining. In small cells, representing newly divided cells, FtsZ localizes to a single cell pole. During the cell cycle, spiral intermediates are formed until an FtsZ ring is positioned with very little precision, such that central as well as acentral rings can be observed. Daughter cells showed considerably different sizes, suggesting that H. pylori divides asymmetrically. Fluorescence recovery after photobleaching (FRAP) analyses demonstrate that the H. pylori FtsZ ring is about as dynamic as that of Escherichia coli but that polar assemblies show less turnover. Strikingly, our results demonstrate that H. pylori cell division follows a different route from that in E. coli and Bacillus subtilis. It is also different from that in C. crescentus, where cytokinesis regulation proteins like MipZ play a role. Therefore, this report provides the first cell-biological analysis of FtsZ dynamics in the human pathogen H. pylori and even in epsilonproteobacteria to our knowledge. In addition, analysis of the filament architecture of H. pylori and E. coli FtsZ filaments in the heterologous system of Drosophila melanogaster S2 Schneider cells revealed that both have different filamentation properties in vivo, suggesting a unique intrinsic characteristic of each protein.

Physical and biochemical nature of the bacterial cytoplasm: movement and localization of mRNA and the 30S subunits of ribosomes

There is a paucity of knowledge on how mRNA transcripts in the spatially crowded, but molecularly organized bacterial cytoplasm contact the 30S ribosomal subunits. Does simple diffusion in the cytoplasm account for transcript-ribosome interactions given that a large number of ribosomes (e.g., about 72,000 in Escherichia coli during exponential growth) can be present in the cytoplasm? Or are undiscovered mechanisms present where specific transcripts are directed to specific ribosomes at specific cytoplasmic locations, while others are mobilized in a random manner? Moreover, is it possible that cytoplasmic mobilization occurs in bacteria, driven possibly by thermal infrared (IR) radiation and the generation of exclusion zone (EZ) water? These aspects will be discussed in this article and hypotheses presented.

Characterization of Caulobacter crescentus FtsZ protein using dynamic light scattering

The self-assembly of the tubulin homologue FtsZ at the mid-cell is a critical step in bacterial cell division. We introduce dynamic light scattering (DLS) spectroscopy as a new method to study the polymerization kinetics of FtsZ in solution. Analysis of the DLS data indicates that the FtsZ polymers are remarkably monodisperse in length, independent of the concentrations of GTP, GDP, and FtsZ monomers. Measurements of the diffusion coefficient of the polymers demonstrate that their length is remarkably stable until the free GTP is consumed. We estimated the mean size of the FtsZ polymers within this interval of stable length to be between 9 and 18 monomers. The rates of FtsZ polymerization and depolymerization are likely influenced by the concentration of GDP, as the repeated addition of GTP to FtsZ increased the rate of polymerization and slowed down depolymerization. Increasing the FtsZ concentration did not change the size of FtsZ polymers; however, it increased the rate of the depolymerization reaction by depleting free GTP. Using transmission electron microscopy we observed that FtsZ forms linear polymers in solutions which rapidly convert to large bundles upon contact with surfaces at time scales as short as several seconds. Finally, the best studied small molecule that binds to FtsZ, PC190723, had no stabilizing effect on Caulobacter crescentus FtsZ filaments in vitro, which complements previous studies with Escherichia coli FtsZ and confirms that this class of small molecules binds Gram-negative FtsZ weakly.

The Min system and nucleoid occlusion are not required for identifying the division site in Bacillus subtilis but ensure its efficient utilization

Precise temporal and spatial control of cell division is essential for progeny survival. The current general view is that precise positioning of the division site at midcell in rod-shaped bacteria is a result of the combined action of the Min system and nucleoid (chromosome) occlusion. Both systems prevent assembly of the cytokinetic Z ring at inappropriate places in the cell, restricting Z rings to the correct site at midcell. Here we show that in the bacterium Bacillus subtilis Z rings are positioned precisely at midcell in the complete absence of both these systems, revealing the existence of a mechanism independent of Min and nucleoid occlusion that identifies midcell in this organism. We further show that Z ring assembly at midcell is delayed in the absence of Min and Noc proteins, while at the same time FtsZ accumulates at other potential division sites. This suggests that a major role for Min and Noc is to ensure efficient utilization of the midcell division site by preventing Z ring assembly at potential division sites, including the cell poles. Our data lead us to propose a model in which spatial regulation of division in B. subtilis involves identification of the division site at midcell that requires Min and nucleoid occlusion to ensure efficient Z ring assembly there and only there, at the right time in the cell cycle.

How organisms regulate biological processes so that they occur at the correct place within the cell is a fundamental question in research. Spatial regulation of cell division is vital to ensure equal partitioning of DNA into newborn cells. Correct positioning of the division site at the cell centre in rod-shaped bacteria is generally believed to occur via the combined action of two factors: (i) nucleoid (chromosome) occlusion and (ii) a set of proteins known collectively as the Min system. The earliest stage in bacterial cell division is the assembly of a ring, called the Z ring, at the division site. Nucleoid occlusion and Min work by preventing Z ring assembly at all sites along the cell other than the cell centre. Here we make the surprising discovery that, in the absence of both these factors, Z rings are positioned correctly at the division site, but there is a delay in this process and it is less efficient. We propose that a separate mechanism identifies the division site at midcell in rod-shaped bacteria, and nucleoid occlusion and Min ensure that the Z ring forms there and only there, at the right time and every time.

The relationship between a coiled morphology and Mbl in alkaliphilic Bacillus halodurans C-125 at neutral pH values

The facultative alkaliphilic Bacillus halodurans C-125 can grow in a pH range from 6.8 to 10.8. The morphology of the cells grown at pH values above 7.5 is rod shaped, whereas, that gown at pH values less than 7.5 is coiled. Cytoplasmic membrane staining revealed that this coiled morphology was formed not by one filamentous cell, but by many chained bent/non-bent cells. Prokaryotic actin and tubulin homologs (MreB, Mbl MreBH, and FtsZ, respectively) are known to function as bacterial cytoskeleton proteins. The transcription levels of ftsZ, mreB, and mreBH genes were hardly affected by growth pH. However, the level of the mbl gene was significantly decreased at neutral pH values. Moreover, the expression level of the Mbl protein at pH 7.0 was about one-fourth of that at pH 10. Immunofluorescence microscopy (IFM) showed that the Mbl protein was localized as a helical structure in the rod-shaped cell grown at pH 10, whereas a helical structure was not observed in the cells grown at pH 7.0. Fluorescent vancomycin staining showed insertion of new peptidoglycan strands of sidewalls occurred in the cells grown at pH 7.0. These data suggested that a decrease in the expression level of the Mbl protein can influence the morphology of B. halodurans C-125 grown at pH 7.0 without influencing insertion of new peptidoglycan strands.

Growth, cell division and sporulation in mycobacteria

Bacteria have the ability to adapt to different growth conditions and to survive in various environments. They have also the capacity to enter into dormant states and some bacteria form spores when exposed to stresses such as starvation and oxygen deprivation. Sporulation has been demonstrated in a number of different bacteria but Mycobacterium spp. have been considered to be non-sporulating bacteria. We recently provided evidence that Mycobacterium marinum and likely also Mycobacterium bovis bacillus Calmette–Guérin can form spores. Mycobacterial spores were detected in old cultures and our findings suggest that sporulation might be an adaptation of lifestyle for mycobacteria under stress. Here we will discuss our current understanding of growth, cell division, and sporulation in mycobacteria.

Temporal controls of the asymmetric cell division cycle in Caulobacter crescentus

The asymmetric cell division cycle of Caulobacter crescentus is orchestrated by an elaborate gene-protein regulatory network, centered on three major control proteins, DnaA, GcrA and CtrA. The regulatory network is cast into a quantitative computational model to investigate in a systematic fashion how these three proteins control the relevant genetic, biochemical and physiological properties of proliferating bacteria. Different controls for both swarmer and stalked cell cycles are represented in the mathematical scheme. The model is validated against observed phenotypes of wild-type cells and relevant mutants, and it predicts the phenotypes of novel mutants and of known mutants under novel experimental conditions. Because the cell cycle control proteins of Caulobacter are conserved across many species of alpha-proteobacteria, the model we are proposing here may be applicable to other genera of importance to agriculture and medicine (e.g., Rhizobium, Brucella).

Lipid spirals in Bacillus subtilis and their role in cell division

The fluid mosaic model of membrane structure has been revised in recent years as it has become evident that domains of different lipid composition are present in eukaryotic and prokaryotic cells. Using membrane binding fluorescent dyes, we demonstrate the presence of lipid spirals extending along the long axis of cells of the rod-shaped bacterium Bacillus subtilis. These spiral structures are absent from cells in which the synthesis of phosphatidylglycerol is disrupted, suggesting an enrichment in anionic phospholipids. Green fluorescent protein fusions of the cell division protein MinD also form spiral structures and these were shown by fluorescence resonance energy transfer to be coincident with the lipid spirals. These data indicate a higher level of membrane lipid organization than previously observed and a primary role for lipid spirals in determining the site of cell division in bacterial cells.

New model for assembly dynamics of bacterial tubulin in relation to the stages of DNA replication

How living cells receive their genome through cell division has been one of the important questions of biology. In prokaryotes, cell division starts with formation of a ring-shaped microtubule-like structure, FtsZ-ring, at the potential division site. All the previous models suggested that FtsZ-ring is formed coupling to termination or far after initiation of DNA replication. In contrast, we demonstrated that a close communication with DNA replication is maintained throughout the cell cycle. FtsZ starts to assemble to the cell center coupling to initiation of DNA replication, and stabilizes as FtsZ-ring at its termination, but does not constrict before separation of nucleoids. This combination of a positive and a negative control would guarantee that a successful replication event would inevitably induce one cell division such that each of the daughter cells would receive one and only one daughter nucleoid.

The ftsA* gain-of-function allele of Escherichia coli and its effects on the stability and dynamics of the Z ring

Formation of the FtsZ ring (Z ring) in Escherichia coli is the first step in the assembly of the divisome, a protein machine required for cell division. Although the biochemical functions of most divisome proteins are unknown, several, including ZipA, FtsA and FtsK, have overlapping roles in ensuring that the Z ring assembles at the cytoplasmic membrane, and that it is active. As shown previously, a single amino acid change in FtsA, R286W, also called FtsA*, bypasses the requirement for either ZipA or FtsK in cell division. In this study, the properties of FtsA* were investigated further, with the eventual goal of understanding the molecular mechanism behind the bypass. Compared to wild-type FtsA, the presence of FtsA* resulted in a modest but significant decrease in the mean length of cells in the population, accelerated the reassembly of Z rings, and suppressed the cell-division block caused by excessively high levels of FtsZ. These effects were not mediated by Z-ring remodelling, because FtsA* did not alter the kinetics of FtsZ turnover within the Z ring, as measured by fluorescence recovery after photobleaching. FtsA* was also unable to permit normal cell division at below normal levels of FtsZ, or after thermoinactivation of ftsZ84(ts). However, turnover of FtsA* in the ring was somewhat faster than that of wild-type FtsA, and overexpressed FtsA* did not inhibit cell division as efficiently as wild-type FtsA. Finally, FtsA* interacted more strongly with FtsZ compared with FtsA in a yeast two-hybrid system. These results suggest that FtsA* interacts with FtsZ in a markedly different way compared with FtsA.

Division site recognition inEscherichia coliandBacillus subtilis

The process of cell division has been intensively studied at the molecular level for decades but some basic questions remain unanswered. The mechanisms of cell division are probably best characterized in the rod-shaped bacteria Escherichia coli and Bacillus subtilis. Many of the key players are known, but detailed descriptions of the molecular mechanisms which determine where, how and when cells form the division septum are lacking. Different models have been proposed to account for the high precision with which the septum is constructed at the midcell and these models have been evaluated and refined against new data emerging from the fast improving methodologies of cell biology. This review summarizes important advances in our understanding of how the cell positions the division septum, whether it be vegetative or asymmetric. It also describes how the asymmetric septum forms and how this septation event is linked to chromosome segregation and subsequent asymmetric gene expression during spore formation in B. subtilis.

Differences in the degree of inhibition of NDP reductase by chemical inactivation and by the thermosensitive mutation nrdA101 in Escherichia coli suggest an effect on chromosome segregation

NDP reductase activity can be inhibited either by treatment with hydroxyurea or by incubation of an nrdA_ts mutant strain at the non-permissive temperature. Both methods inhibit replication, but experiments on these two types of inhibition yielded very different results. The chemical treatment immediately inhibited DNA synthesis but did not affect the cell and nucleoid appearance, while the incubation of an nrdA101 mutant strain at the non-permissive temperature inhibited DNA synthesis after more than 50 min, and resulted in aberrant chromosome segregation, long filaments, and a high frequency of anucleate cells. These phenotypes are not induced by SOS. In view of these results, we suggest there is an indirect relationship between NDP reductase and the chromosome segregation machinery through the maintenance of the proposed replication hyperstructure.

Bacillus subtilis EzrA and FtsL synergistically regulate FtsZ ring dynamics during cell division

Previous work has shown that the Bacillus subtilis EzrA protein directly inhibits FtsZ ring assembly, which is required for normal cell division, and that loss of EzrA results in hyperstabilization of the FtsZ polymer in vivo. Here, it was found that in ezrA-disrupted cells, artificial expression of YneA, which suppresses cell division during the SOS response, and disruption of noc (yyaA), which acts as an effector of nucleoid occlusion, resulted in accumulation of multiple non-constricting FtsZ rings, inhibition of cell division, and synthetic lethality. Overexpression of the essential cell division protein FtsL suppressed the effect of ezrA disruption. FtsL overexpression recovered the delayed FtsZ ring constriction seen in ezrA-disrupted wild-type cells. Conversely, the absence of EzrA caused lethality in cells producing a lower amount of FtsL than wild-type cells. It has previously been reported that FtsL is recruited to the division site during the later stages of cell division, although its exact role is currently unknown. The results of this study suggest that FtsL and EzrA synergistically regulate the FtsZ ring constriction in B. subtilis. Interestingly, FtsL overexpression also suppressed the cell division inhibition due to YneA expression or Noc inactivation in ezrA-disrupted cells.

Influence of CrgA on assembly of the cell division protein FtsZ during development of Streptomyces coelicolor

The product of the crgA gene of Streptomyces coelicolor represents a novel family of small proteins. A single orthologous gene is located close to the origin of replication of all fully sequenced actinomycete genomes and borders a conserved gene cluster implicated in cell growth and division. In S. coelicolor, CrgA is important for coordinating growth and cell division in sporogenic hyphae. In this study, we demonstrate that CrgA is an integral membrane protein whose peak expression is coordinated with the onset of development of aerial hyphae. The protein localizes to discrete foci away from growing hyphal tips. Upon overexpression, CrgA localizes to apical syncytial cells of aerial hyphae and inhibits the formation of productive cytokinetic rings of the bacterial tubulin homolog FtsZ, leading to proteolytic turnover of this major cell division determinant. In the absence of known prokaryotic cell division inhibitors in actinomycetes, CrgA may have an important conserved function influencing Z-ring formation in these bacteria.

Probing the domain structure of FtsZ by random truncation and insertion of GFP

Random transposon-mediated mutagenesis has been used to create truncations and insertions of green fluorescent protein (GFP), and Venus-yellow fluorescent protein (YFP), in Escherichia coli FtsZ. Sixteen unique insertions were obtained, and one of them, in the poorly conserved C-terminal spacer, was functional for cell division with the Venus-YFP insert. The insertion of enhanced GFP (eGFP) at this same site was not functional; Venus-YFP was found to be superior to eGFP in other respects too. Testing the constructs for dominant negative effects led to the following general conclusion. The N-terminal domain, aa 1-195, is an independently folding domain that can poison Z-ring function when expressed without a functional C-terminal domain. The effects were weak, requiring expression of the mutant at 3-5 times the level of wild-type FtsZ. The C-terminal domain, aa 195-383, was also independently folding, but had no activity in vivo. The differential activity of the N- and C-terminal domains suggests that FtsZ protofilament assembly is directional, with subunits adding primarily at the bottom of the protofilament. Directional assembly could occur by either a treadmilling or a dynamic instability mechanism.

Cell biology of mitochondrial dynamics

Mitochondria are the product of an ancient endosymbiotic event between an alpha-proteobacterium and an archael host. An early barrier to overcome in this relationship was the control of the bacterium's proliferation within the host. Undoubtedly, the bacterium (or protomitochondrion) would have used its own cell division apparatus to divide at first and, today a remnant of this system remains in some "ancient" and diverse eukaryotes such as algae and amoebae, the most conserved and widespread of all bacterial division proteins, FtsZ. In many of the eukaryotes that still use FtsZ to constrict the mitochondria from the inside, the mitochondria still resemble bacteria in shape and size. Eukaryotes, however, have a mitochondrial morphology that is often highly fluid, and in their tubular networks of mitochondria, division is clearly complemented by mitochondrial fusion. FtsZ is no longer used by these complex eukaryotes, and may have been replaced by other proteins better suited to sustaining complex mitochondrial networks. Although proteins that divide mitochondria from the inside are just beginning to be characterized in higher eukaryotes, many division proteins are known to act on the outside of the organelle. The most widespread of these are the dynamin-like proteins, which appear to have been recruited very early in the evolution of mitochondria. The essential nature of mitochondria dictates that their loss is intolerable to human cells, and that mutations disrupting mitochondrial division are more likely to be fatal than result in disease. To date, only one disease (Charcot-Marie-Tooth disease 2A) has been mapped to a gene that is required for mitochondrial division, whereas two other diseases can be attributed to mutations in mitochondrial fusion genes. Apart from playing a role in regulating the morphology, which might be important for efficient ATP production, research has indicated that the mitochondrial division and fusion proteins can also be important during apoptosis; mitochondrial fragmentation is an early triggering (and under many stimuli, essential) step in the pathway to cell suicide.

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.

Dancing around the divisome: asymmetric chromosome segregation in Escherichia coli

By simultaneously tracking pairs of specific genetic regions and divisome proteins in live Escherichia coli, we develop a new scheme for the relationship between DNA replication-segregation, chromosome organization, and cell division. A remarkable asymmetric pattern of segregation of different loci in the replication termination region (ter) suggests that individual replichores segregate to distinct nucleoid positions, consistent with an asymmetric segregation of leading and lagging strand templates after replication. Cells growing with a generation time of 100 min are born with a nonreplicating chromosome and have their origin region close to mid-cell and their ter polar. After replication initiation, the two newly replicated origin regions move away from mid-cell to opposite cell halves. By mid-S phase, FtsZ forms a ring at mid-cell at the time of initiation of nucleoid separation; ter remains polar. In the latter half of S phase, ter moves quickly toward mid-cell. FtsK, which coordinates the late stages of chromosome segregation with cell division, forms a ring coincident with the FtsZ ring as S phase completes, approximately 50 min after its initiation. As ter duplicates at mid-cell, sister nucleoid separation appears complete. After initiation of invagination, the FtsZ ring disassembles, leaving FtsK to complete chromosome segregation and cytokinesis.

SlmA, a nucleoid-associated, FtsZ binding protein required for blocking septal ring assembly over Chromosomes in E. coli

Cell division in Escherichia coli begins with assembly of the tubulin-like FtsZ protein into a ring structure just underneath the cell membrane. Spatial control over Z ring assembly is achieved by two partially redundant negative regulatory systems, the Min system and nucleoid occlusion (NO), which cooperate to position the division site at midcell. In contrast to the well-studied Min system, almost nothing is known about how Z ring assembly is blocked in the vicinity of nucleoids to effect NO. Reasoning that Min function might become essential in cells impaired for NO, we screened for mutations synthetically lethal with a defective Min system (slm mutants). By using this approach, we identified SlmA (Ttk) as the first NO factor in E. coli. Our combined genetic, cytological, and biochemical results suggest that SlmA is a DNA-associated division inhibitor that is directly involved in preventing Z ring assembly on portions of the membrane surrounding the nucleoid.

Dynamics of FtsZ assembly during sporulation in Streptomyces coelicolor A3(2)

FtsZ, the bacterial tubulin homologue, is the main player in at least two distinct processes of cell division during the development of Streptomyces coelicolor A3(2). It forms cytokinetic rings and is required for the formation of both the widely spaced hyphal cross walls in the substrate mycelium and the specialized septation that converts sporogenic aerial hyphae into spores. The latter developmentally controlled septation involves the coordinated assembly of large numbers of FtsZ rings in each sporulating hyphal cell. We used an FtsZ-enhanced green fluorescent protein (EGFP) translational fusion to visualize the progression of FtsZ ring assembly in vivo during sporulation of aerial hyphae. This revealed that the regular placement of multiple FtsZ rings and initiation of cytokinesis was preceded by a protracted phase during which spiral-shaped FtsZ intermediates were detected along the length of the aerial hyphal cell. Time course experiments indicated that they were remodeled and gradually replaced by regularly spaced FtsZ rings. Such spiral-shaped filaments could also be detected with immunofluorescence microscopy using an antiserum against FtsZ. Based on our observations, we propose a model for the progression of Z-ring assembly during sporulation of S. coelicolor. Furthermore, mutants lacking the developmental regulatory genes whiA, whiB, whiG, whiH, and whiI were investigated. They failed in up-regulation of the expression of FtsZ-EGFP in aerial hyphae, which is consistent with the known effects of these genes on ftsZ transcription.

Transcriptional analysis of the principal cell division gene, ftsZ, of Mycobacterium tuberculosis

Multiple promoters drive the expression of the principal cell division gene, ftsZ, in bacterial systems. Primer extension analysis of total RNA from Mycobacterium tuberculosis and a Mycobacterium smegmatis transformant containing 1.117 kb of the upstream region of M. tuberculosis ftsZ and promoter fusion studies identified six ftsZ transcripts and their promoters in the ftsQ open reading frame and ftsQ-ftsZ intergenic region. The presence of multiple promoters reflects the requirement to maintain a high basal level of, or to differentially regulate, FtsZ expression during different growth conditions of the pathogen in vivo.

Identification of a polar targeting determinant for Bacillus subtilis DivIVA

The Bacillus subtilis protein DivIVA controls both the positioning of the vegetative cell division site and the polar attachment of the chromosome during sporulation. In vegetative growth DivIVA attracts the bipartite cell division inhibitor MinCD away from the cell centre and towards the cell pole. This process ensures the inactivation of old polar division sites and leaves the cell centre free for the assembly of a new cell division complex. During sporulation MinCD and DivIVA levels fall, but DivIVA remains at the cell poles and becomes involved in the migration of the chromosomes to the pole. In order to investigate polar targeting of DivIVA, we undertook a mutational analysis of the 164-amino-acid protein. These studies identified one mutant (divIVA(R18C)) that could not localize to the cell pole but which retained the ability to support both vegetative growth and 50% sporulation efficiency. Further analysis revealed that, in the absence of polar targeting, DivIVA(R18C) localized to the nucleoid during vegetative growth in a Spo0J/Soj-dependent manner and required Spo0J/Soj and MinD to orientate the chromosomes correctly during sporulation. We demonstrate that polar targeting of DivIVA(R18C) is not essential during vegetative growth because the mutant can recognize the cell division site and influences the localization of MinD. Similarly we show that DivIVA(R18C) can function during sporulation because it can support the Spo0J/Soj orientation of the chromosome. In addition, we establish that both residues 18 and 19 constitute a DivIVA polar targeting determinant.

The midcell replication factory in Bacillus subtilis is highly mobile: implications for coordinating chromosome replication with other cell cycle events

During vegetative growth, rod-shaped bacterial cells such as Escherichia coli and Bacillus subtilis divide precisely at midcell. It is the Z ring that defines the position of the division site. We previously demonstrated that the early stages of chromosome replication are linked to midcell Z ring assembly in B. subtilis and proposed a direct role for the centrally located replication factory in masking and subsequently unmasking the midcell site for Z ring assembly. We now show that the replication factory is significantly more scattered about the cell centre than the Z ring in both vegetative cells and outgrown spores of B. subtilis. This finding is inconsistent with the midcell replication factory acting as a direct physical block to Z ring assembly. Time-lapse experiments demonstrated that the lower precision of replication factory positioning results from its high mobility around the cell centre. Various aspects of this mobility are presented and the results are discussed in the light of current views on the determinants of positional information required for accurate chromosome segregation and cell division.

Alternatives to binary fission in bacteria

Whereas most prokaryotes rely on binary fission for propagation, many species use alternative mechanisms, which include multiple offspring formation and budding, to reproduce. In some bacterial species, these eccentric reproductive strategies are essential for propagation, whereas in others the programmes are used conditionally. Although there are tantalizing images and morphological descriptions of these atypical developmental processes, none of these reproductive structures are characterized at the molecular genetic level. Now, with newly available analytical techniques, model systems to study these alternative reproductive programmes are being developed.

Diversity and versatility of lipid-protein interactions revealed by molecular genetic approaches

The diversity in structures and physical properties of lipids provides a wide variety of possible interactions with proteins that affect their assembly, organization, and function either at the surface of or within membranes. Because lipids have no catalytic activity, it has been challenging to define many of their precise functions in vivo in molecular terms. Those processes responsive to lipids are attuned to the native lipid environment for optimal function, but evidence that lipids with similar properties or even detergents can sometimes partially replace the natural lipid environment has led to uncertainty as to the requirement for specific lipids. The development of strains of microorganisms in which membrane lipid composition can be genetically manipulated in viable cells has provided a set of reagents to probe lipid functions. These mutants have uncovered previously unrecognized roles for lipids and provided in vivo verification for putative functions described in vitro. In this review, we summarize how these reagent strains have provided new insight into the function of lipids. The role of specific lipids in membrane protein folding and topological organization is reviewed. The evidence is summarized for the involvement of anionic lipid-enriched domains in the organization of amphitropic proteins on the membrane surface into molecular machines involved in DNA replication and cell division.