Bacterial cell division: a moving MinE sweeper boggles the MinD

The MinCDE Cell Division System Participates in the Regulation of Type III Secretion System (T3SS) Genes, Bacterial Virulence, and Motility in Xanthomonas oryzae pv. oryzae

Xanthomonas oryzae pv. oryzae (Xoo) causes bacterial leaf blight (BLB) in rice, which is one of the most severe bacterial diseases in rice in some Asian countries. The type III secretion system (T3SS) of Xoo encoded by the hypersensitive response and pathogenicity (hrp) genes is essential for its pathogenicity in host rice. Here, we identified the Min system (MinC, MinD, and MinE), a negative regulatory system for bacterial cell division encoded by minC, minD, and minE genes, which is involved in negative regulation of hrp genes (hrpB1 and hrpF) in Xoo. We found that the deletion of minC, minD, and minCDE resulted in enhanced hrpB1 and hrpF expression, which is dependent on two key hrp regulators HrpG and HrpX. The minC, minD, and minCDE mutants exhibited elongated cell lengths, and the classic Min system-defective cell morphology including minicells and short filamentations. Mutation of minC in Xoo resulted in significantly impaired virulence in host rice, swimming motility, and enhanced biofilm formation. Our transcriptome profiling also indicated some virulence genes were differentially expressed in the minC mutants. To our knowledge, this is the first report about the Min system participating in the regulation of T3SS expression. It sheds light on the understanding of Xoo virulence mechanisms.

Comparison of Auxenochlorella protothecoides and Chlorella spp. Chloroplast Genomes: Evidence for Endosymbiosis and Horizontal Virus-like Gene Transfer

Resequencing of the chloroplast genome (cpDNA) of Auxenochlorella protothecoides UTEX 25 was completed (GenBank Accession no. KC631634.1), revealing a genome size of 84,576 base pairs and 30.8% GC content, consistent with features reported for the previously sequenced A. protothecoides 0710, (GenBank Accession no. KC843975). The A. protothecoides UTEX 25 cpDNA encoded 78 predicted open reading frames, 32 tRNAs, and 4 rRNAs, making it smaller and more compact than the cpDNA genome of C. variabilis (124,579 bp) and C. vulgaris (150,613 bp). By comparison, the compact genome size of A. protothecoides was attributable primarily to a lower intergenic sequence content. The cpDNA coding regions of all known Chlorella species were found to be organized in conserved colinear blocks, with some rearrangements. The Auxenochlorella and Chlorella species genome structure and composition were similar, and of particular interest were genes influencing photosynthetic efficiency, i.e., chlorophyll synthesis and photosystem subunit I and II genes, consistent with other biofuel species of interest. Phylogenetic analysis revealed that Prototheca cutis is the closest known A. protothecoides relative, followed by members of the genus Chlorella. The cpDNA of A. protothecoides encodes 37 genes that are highly homologous to representative cyanobacteria species, including rrn16, rrn23, and psbA, corroborating a well-recognized symbiosis. Several putative coding regions were identified that shared high nucleotide sequence identity with virus-like sequences, suggestive of horizontal gene transfer. Despite these predictions, no corresponding transcripts were obtained by RT-PCR amplification, indicating they are unlikely to be expressed in the extant lineage.

Eugene P. Kennedy's Legacy: Defining Bacterial Phospholipid Pathways and Function

In the 1950's and 1960's Eugene P. Kennedy laid out the blueprint for phospholipid biosynthesis in somatic cells and Escherichia coli, which have been coined the Kennedy Pathways for phospholipid biosynthesis. His research group continued to make seminal contributions in the area of phospholipids until his retirement in the early 1990's. During these years he mentored many young scientists that continued to build on his early discoveries and who also mentored additional scientists that continue to make important contributions in areas related to phospholipids and membrane biogenesis. This review will focus on the initial E. coli Kennedy Pathways and how his early contributions have laid the foundation for our current understanding of bacterial phospholipid genetics, biochemistry and function as carried on by his scientific progeny and others who have been inspired to study microbial phospholipids.

Proteomic Analysis of Stationary Growth Stage Adaptation and Nutritional Deficiency Response of Brucella abortus

Brucellosis, an important bacterial zoonosis caused by Brucella species, has drawn increasing attention worldwide. As an intracellular pathogen, the ability of Brucella to deal with stress within the host cell is closely related to its virulence. Due to the similarity between the survival pressure on Brucella within host cells and that during the stationary phase, a label-free proteomics approach was used to study the adaptive response of Brucella abortus in the stationary stage to reveal the possible intracellular adaptation mechanism in this study. A total of 182 downregulated and 140 upregulated proteins were found in the stationary-phase B. abortus. B. abortus adapted to adverse environmental changes by regulating virulence, reproduction, transcription, translation, stress response, and energy production. In addition, both exponential- and stationary-phase B. abortus were treated with short-term starvation. The exponential B. abortus restricted cell reproduction and energy utilization and enhanced material transport in response to nutritional stress. Compared with the exponential phase, stationary Brucella adjusted their protein expression to a lesser extent under starvation. Therefore, B. abortus in the two growth stages significantly differed in the regulation of protein expression in response to the same stress. Overall, we outlined the adaptive mechanisms that B. abortus may employ during growth and compared the differences between exponential- and stationary-phase B. abortus in response to starvation.

Physics of bacterial morphogenesis

Bacterial cells utilize three-dimensional (3D) protein assemblies to perform important cellular functions such as growth, division, chemoreception, and motility. These assemblies are composed of mechanoproteins that can mechanically deform and exert force. Sometimes, small-nucleotide hydrolysis is coupled to mechanical deformations. In this review, we describe the general principle for an understanding of the coupling of mechanics with chemistry in mechanochemical systems. We apply this principle to understand bacterial cell shape and morphogenesis and how mechanical forces can influence peptidoglycan cell wall growth. We review a model that can potentially reconcile the growth dynamics of the cell wall with the role of cytoskeletal proteins such as MreB and crescentin. We also review the application of mechanochemical principles to understand the assembly and constriction of the FtsZ ring. A number of potential mechanisms are proposed, and important questions are discussed.

Protein-membrane interactions: the virtue of minimal systems in systems biology

The plasma membrane of cells can be viewed as a highly dynamic, regulated, heterogeneous environment with multiple functions. It constitutes the boundary of the cell, encapsulating all its components. Proteins interact with the membrane in many ways to accommodate essential processes, such as membrane trafficking, membrane protrusions, cytokinesis, signaling, and cell-cell communication. A vast amount of literature has already fostered our current understanding of membrane-protein interactions. However, many phenomena still remain to be understood, e.g., the exact mechanisms of how certain proteins cause or assist membrane transformations. Systems biology aims to predict biological processes on the basis of the set of molecules involved. Many key processes arise from interactions with the lipid membrane. Protein interactome maps do not consider such specific interactions, and thus cannot predict precise outcomes of the interactions of the involved proteins. These can only be inferred from experimental approaches. We describe examples of how an emergent behavior of protein-membrane interactions has been demonstrated by the use of minimal systems. These studies contribute to a deeper understanding of protein interactomes involving membranes and complement other approaches of systems biology.

A plant MinD homologue rescues Escherichia coli HL1 mutant (DeltaMinDE) in the absence of MinE

Background

In E. coli, the Min operon (MinCDE) plays a key role in determining the site of cell division. MinE oscillates from the middle to one pole or another to drive the MinCD complex to the end of the cell. The MinCD complex prevents FtsZ ring formation and the subsequent cell division at cell ends. In Arabidopsis thaliana, a homologue of MinD has been shown to be involved in the positioning of chloroplast division site.

Results

To learn whether the MinD homologue in plants is functional in bacteria, AtMinD was expressed in E. coli. Surprisingly, AtMinD can rescue the minicell phenotype of E. coli HL1 mutant (ΔMinDE) in the absence of EcMinE. This rescue requires EcMinC. AtMinD was localized to puncta at the poles of E. coli cells and puncta in chloroplasts without oscillation. AtMinD expressed in the HL1 mutant can cause a punctate localization pattern of GFP-EcMinC at cell ends. Yeast two hybrid and BiFC analysis showed that AtMinD can interact with EcMinC.

Conclusion

Similar to the MinD in Bacillus subtilis, AtMinD is localized to the polar region in E. coli and interacts with EcMinC to confine EcFtsZ polymerization and cell division at the midpoint of the cell.

FtsZ polymer-bundling by the Escherichia coli ZapA orthologue, YgfE, involves a conformational change in bound GTP

Cell division is a fundamental process for both eukaryotic and prokaryotic cells. In bacteria, cell division is driven by a dynamic, ring-shaped, cytoskeletal element (the Z-ring) made up of polymers of the tubulin-like protein FtsZ. It is thought that lateral associations between FtsZ polymers are important for function of the Z-ring in vivo, and that these interactions are regulated by accessory cell division proteins such as ZipA, EzrA and ZapA. We demonstrate that the putative Escherichia coli ZapA orthologue, YgfE, exists in a dimer/tetramer equilibrium in solution, binds to FtsZ polymers, strongly promotes FtsZ polymer bundling and is a potent inhibitor of the FtsZ GTPase activity. We use linear dichroism, a technique that allows structure analysis of molecules within linear polymers, to reveal a specific conformational change in GTP bound to FtsZ polymers, upon bundling by YgfE. We show that the consequences of FtsZ polymer bundling by YgfE and divalent cations are very similar in terms of GTPase activity, bundle morphology and GTP orientation and therefore propose that this conformational change in bound GTP reveals a general mechanism of FtsZ bundling.

Microfabrication meets microbiology

This Review summarizes methods for constructing systems and structures at micron or submicron scales that have applications in microbiology. These tools make it possible to manipulate individual cells and their immediate extracellular environments and have the capability to transform the study of microbial physiology and behaviour. Because of their simplicity, low cost and use in microfabrication, we focus on the application of soft lithographic techniques to the study of microorganisms, and describe several key areas in microbiology in which the development of new microfabricated materials and tools can have a crucial role.

Regulation of polar flagellar number by the flhF and flhG genes in Vibrio alginolyticus

The number and location of bacterial flagella vary with the species. The Vibrio alginolyticus cell has a single polar flagellum, which is driven by sodium ions. We selected mutants on the basis of reduced swarming ability on soft agar plates. Among them, we found two mutants with multiple polar flagella, and named them KK148 and NMB155. In Pseudomonas species, it is known that FlhF and FleN, which are FtsY and MinD homologs, respectively, are involved in regulation of flagellar placement and number, respectively. We cloned homologous genes of V. alginolyticus, flhF and flhG. KK148 cells had a nonsense mutation in flhG; cells expressing transgenic flhG recovered the swarming ability and had a reduced number of polar flagella. NMB155 cells did not have a mutation in either flhF or flhG. In wild-type cells, expression of flhF increased the number of polar flagella; in contrast, expression of flhG reduced both the number of polar flagella and the swarming ability. These results suggest that FlhG negatively regulates the number of polar flagella in V. alginolyticus. KK148 cells expressing both flhF and flhG exhibited fewer polar flagella and better swarming ability than KK148 cells expressing flhG alone, suggesting that FlhG acts with FlhF.

Of the morphogenes that make a ring, a rod and a sphere in Escherichia coli

In 1993, William Donachie wrote "The success of molecular genetics in the study of bacterial cell division has been so great that we find ourselves, armed with much greater knowledge of detail, confronted once again with the same naive questions that we set to answer in the first place". Indeed, attempts to answer the apparently simple question of how a bacterial cell divides have led to a wealth of new knowledge, in particular over the past decade and a half. And while some questions have been answered to a great extent since the early reports of isolation of division mutants of Escherichia coli, some key pieces of the puzzle remain elusive. In addition to it being a fundamental process in bacteria that merits investigation in its own right, studying the process of cell division offers an abundance of new targets for the development of new antibacterial compounds that act directly against key division proteins and other components of the cytoskeleton, which are encoded by the morphogenes of E. coli. This review aims to present the reader with a snapshot summary of the key players in E. coli morphogenesis with emphasis on cell division and the rod to sphere transition.

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

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

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.

Division accuracy in a stochastic model of Min oscillations in Escherichia coli

Accurate cell division in Escherichia coli requires the Min proteins MinC, MinD, and MinE as well as the presence of nucleoids. MinD and MinE exhibit spatial oscillations, moving from pole to pole of the bacterium, resulting in an average MinD concentration that is low at the center of the cell and high at the poles. This concentration minimum is thought to signal the site of cell division. Deterministic models of the Min oscillations reproduce many observed features of the system, including the concentration minimum of MinD. However, there are only a few thousand Min proteins in a bacterium, so stochastic effects are likely to play an important role. Here, we show that Monte Carlo simulations with a large number of proteins agree well with the results from a deterministic treatment of the equations. The location of minimum local MinD concentration is too variable to account for cell division accuracy in wild-type, but is consistent with the accuracy of cell division in cells without nucleoids. This finding confirms the need to include additional mechanisms, such as reciprocal interactions with the cell division ring or positioning of the nucleoids, to explain wild-type accuracy.

A conserved polar region in the cell division site determinant MinD is required for responding to MinE-induced oscillation but not for localization within coiled arrays

A region in the cell division site determinant MinD required for stimulation by MinE and which determines MinD topological specificity along coil-like structures has been identified. Structural modeling of dimeric MinD and sequence alignment of 24 MinD proteins revealed a conserved polar region in Gram-negative bacterial MinD proteins, corresponding to residues 92-94 of Neisseria gonorrhoeae MinD (MinD(Ng)). Using MinD(Ng) as a paradigm for MinD functionality in Gram-negative organisms, mutation of these conserved residues did not abrogate MinD(Ng) self-association, nor its interaction with MinE(Ng) and the cell division inhibitor MinC. Although the MinD(Ng) mutant dimerized in the presence of ATP, its ATPase activity was not stimulated by MinE(Ng), unlike wild-type MinD(Ng). GFP fusions to either MinD(Ng) or to Escherichia coli MinD bearing simultaneous or individual mutations to residues 92-94 localized within coiled arrays along the E. coli inner cell periphery, similar to wild-type GFP-MinD. However, unlike wild-type GFP-fusions, the mutant proteins were distributed uniformly throughout the array, despite the presence of MinE, which normally imparts topological specificity to MinD by inducing the latter to oscillate from pole-to-pole and away from midcell. Hence, despite localizing along the inner cell periphery as a polymeric structure, the mutant MinD proteins in this study have lost the ability to be efficiently stimulated by MinE(Ng), resulting in a loss of distinct pole-to-pole oscillation.

Solution structure of the hypothetical protein Mth677 from Methanobacterium thermoautotrophicum: a novel alpha+beta fold

The structure of Mth677, a hypothetical protein from Methanobacterium thermoautotrophicum (Mth), has been determined by using heteronuclear nuclear magnetic resonance (NMR) methods on a double-labeled (15)N-(13)C sample. Mth677 adopts a novel alpha+beta fold, consisting of two alpha-helices (one N terminal and one C terminal) packed on the same side of a central beta-hairpin. This structure is likely shared by its three orthologs, detected in three other Archaebacteria. There are no clear features in the sequences of these proteins or in the genome organization of Mth to make a reliable functional assignment to this protein. However, the structural similarity to Escherichia coli MinE, the protein which controls that division occurs at the midcell site, lends support to the proposal that Mth677 might be, in Mth, the counterpart of the topological specificity domain of MinE in E. coli.

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.

A biochemical oscillator explains several aspects of Myxococcus xanthus behavior during development

During development, Myxococcus xanthus cells produce a series of spatial patterns by coordinating their motion through a contact-dependent signal, the C-signal. C-signaling modulates the frequency at which cells reverse their gliding direction. It does this by interacting with the Frz system (a homolog of the Escherichia coli chemosensory system) via a cascade of covalent modifications. Here we show that introducing a negative feedback into this cascade results in oscillatory behavior of the signaling circuit. The model explains several aspects of M. xanthus behavior during development, including the nonrandom distribution of reversal times, and the differences in response of the reversal frequency to both moderate and high levels of C-signaling at different developmental stages. We also propose experiments to test the model.

Rippling of myxobacteria

Myxobacteria colonies during their aggregation phase propagate complex waves over their surface. These waves are fundamentally different from the analogous phenomenon in diffusion-reaction systems or in populations of Dictyostelium discoideum where colliding waves annhilate. Myxobacterial waves appear to pass through one another, analogous to solitons. Moreover, individual bacteria oscillate back and forth, exhibiting no net mass transfer. A mathematical model can explain virtually all of the experimentally observed properties of these waves and draw several conclusions about the properties of the intercelular signaling system.

Essential role of DivIVA in polar growth and morphogenesis in Streptomyces coelicolor A3(2)

Streptomycetes grow by cell wall extension at hyphal tips. The molecular basis for such polar growth in prokaryotes is largely unknown. It is reported here that DivIVASC, the Streptomyces coelicolor homologue of the Bacillus subtilis protein DivIVA, is essential and directly involved in hyphal tip growth and morphogenesis. A DivIVASC-EGFP hybrid was distinctively localized to hyphal tips and lateral branches. Reduction of divIVASC expression to about 10% of the normal level produced a phenotype strikingly similar to that of many tip growth mutants in fungi, including irregular curly hyphae and apical branching. Overexpression of the gene dramatically perturbed determination of cell shape at the growing tips. Furthermore, staining of nascent peptidoglycan with a fluorescent vancomycin conjugate revealed that induction of overexpression in normal hyphae disturbed tip growth, and gave rise to several new sites of cell wall assembly, effectively causing hyperbranching. The results show that DivIVASC is a novel bacterial morphogene, and it is localized at or very close to the apical sites of peptidoglycan assembly in Streptomyces hyphae.

Effects of phospholipid composition on MinD-membrane interactions in vitro and in vivo

The peripheral membrane ATPase MinD is a component of the Min system responsible for correct placement of the division site in Escherichia coli cells. By rapidly migrating from one cell pole to the other, MinD helps to block unwanted septation events at the poles. MinD is an amphitropic protein that is localized to the membrane in its ATP-bound form. A C-terminal domain essential for membrane localization is predicted to be an amphipathic alpha-helix with hydrophobic residues interacting with lipid acyl chains and cationic residues on the opposite face of the helix interacting with the head groups of anionic phospholipids (Szeto, T. H., Rowland, S. L., Rothfield, L. I., and King, G. F. (2002) Proc. Natl. Acad. Sci. U. S. A. 99, 15693-15698). To investigate whether E. coli MinD displays a preference for anionic phospholipids, we first examined the localization dynamics of a green fluorescent protein-tagged derivative of MinD expressed in a mutant of E. coli that lacks phosphatidylethanolamine. In these cells, which contain only anionic phospholipids (phosphatidylglycerol and cardiolipin), green fluorescent protein-MinD assembled into dynamic focal clusters instead of the broad zones typical of cells with normal phospholipid content. In experiments with liposomes composed of only zwitterionic, only anionic, or a mixture of anionic and zwitterionic phospholipids, purified MinD bound to these liposomes in the presence of ATP with positive cooperativity with respect to the protein concentration and exhibited Hill coefficients of about 2. Oligomerization of MinD on the liposome surface also was detected by fluorescence resonance energy transfer between MinD molecules labeled with different fluorescent probes. The affinity of MinD-ATP for anionic liposomes as well as liposomes composed of both anionic and zwitterionic phospholipids increased 9- and 2-fold, respectively, relative to zwitterionic liposomes. The degree of acyl chain unsaturation contributed positively to binding strength. These results suggest that MinD has a preference for anionic phospholipids and that MinD oscillation behavior, and therefore cell division site selection, may be regulated by membrane phospholipid composition.

A mechanism for ParB-dependent waves of ParA, a protein related to DNA segregation during cell division in prokaryotes

Prokaryotic plasmids encode partitioning (par) loci involved in segregation of DNA to daughter cells at cell division. A functional fusion protein consisting of Walker-type ParA ATPase and green fluorescent protein (Gfp) oscillates back and forth within nucleoid regions with a wave period of about 20 minutes. A model is discussed which is based on cooperative non-specific binding of ParA to the nucleoid, and local ParB initiated generation of ParA oligomer degradation products, which act autocatalytically on the degradation reaction. The model yields self-initiated spontaneous pattern formation, based on Turing's mechanism, and these patterns are destroyed by the degradation products, only to initiate a new pattern at the opposite nucleoid region. A recurrent wave thus emerges. This may be a particular example of a more general class of pattern forming mechanisms, based on protein oligomerization upon a template (membranes, DNA a.o.) with resulting enhanced NTPase function in the oligomer state, which may bring the oligomer into an unstable internal state. An effector initializes destabilization of the oligomer to yield degradation products, which act as seeds for further degradation in an autocatalytic process. We discuss this mechanism in relation to recent models for MinDE oscillations in E.coli and to microtubule degradation in mitosis. The study points to an ancestral role for the presented pattern types in generating bipolarity in prokaryotes and eukaryotes.

The Min system is not required for precise placement of the midcell Z ring in Bacillus subtilis

In bacteria, the Min system plays a role in positioning the midcell division site by inhibiting the formation of the earliest precursor of cell division, the Z ring, at the cell poles. However, whether the Min system also contributes to establishing the precise placement of the midcell Z ring is unresolved. We show that the Z ring is positioned at midcell with a high degree of precision in Bacillus subtilis, and this is completely maintained in the absence of the Min system. Min is therefore not required for correct midcell Z ring placement in B. subtilis. Our results strongly support the idea that the primary role of the Min system is to block Z ring formation at the cell poles and that a separate mechanism must exist to ensure cell division occurs precisely at midcell.

Conservation of dynamic localization among MinD and MinE orthologues: oscillation of Neisseria gonorrhoeae proteins in Escherichia coli

Min proteins are involved in the correct placement of division septa in many bacterial species. In Escherichia coli (Ec) cells, these proteins oscillate from pole to pole, ostensibly to prevent unwanted polar septation. Here, we show that Min proteins from the coccus Neisseria gonorrhoeae (Ng) also oscillate in E. coli. Green fluorescent protein (GFP) fusions to gonococcal MinD and MinE localized dynamically in different E. coli backgrounds. GFP-MinDNg moved from pole to pole in rod-shaped E. coli cells with a 70 +/- 25 s localization cycle when MinENg was expressed in cis. The oscillation time of GFP-MinDNg was reduced when wild-type MinENg was replaced with MinENg carrying a R30D mutation, but lengthened by 15 s when activated by MinEEc. Several mutations in the N-terminal domain of MinDNg, including K16Q and 4- and 19-amino acid truncations, prevented oscillation; these MinDNg mutants showed decreased or lost interaction with themselves and MinENg. Like MinEEc-GFP, MinENg-GFP formed MinE rings and oscillated in E. coli cells when MinDEc was expressed in cis. Finally, in round E. coli cells, GFP-MinDNg appeared to move in a plane parallel to completed septa. This pattern of movement is predicted to be similar in gonococcal cells, which also divide in alternating perpendicular planes.

Exploring intracellular space: function of the Min system in round-shaped Escherichia coli

The MinCDE proteins help to select cell division sites in normal cylindrical Escherichia coli by oscillating along the long axis, preventing unwanted polar divisions. To determine how the Min system might function in cells with multiple potential division planes, we investigated its role in a round-cell rodA mutant. Round cells lacking MinCDE were viable, but growth, morphology and positioning of cell division sites were abnormal relative to Min+ cells. In round cells with a long axis, such as those undergoing cell division, green fluorescent protein (GFP) fusions to MinD almost always oscillated parallel to the long axis. However, perfect spheres or irregularly shaped cells exhibited MinD movement to and from multiple sites on the cell surface. A MinE-GFP fusion exhibited similar behavior. These results indicate that the Min proteins can potentially localize anywhere in the cell but tend to move a certain maximum distance from their previous assembly site, thus favoring movement along the cell's long axis. A new model for the spatial control of division planes by the Min system in round cells is proposed.