Stochastic nucleoid segregation dynamics as a source of the phenotypic variability in E. coli

Segregation of the replicating chromosome from a single to two nucleoid bodies is one of the major processes in growing bacterial cells. The segregation dynamics is tuned by intricate interactions with other cellular processes such as growth and division, ensuring flexibility in a changing environment. We hypothesize that the internal stochasticity of the segregation process may be the source of cell-to-cell phenotypic variability, in addition to the well-established gene expression noise and uneven partitioning of low copy number components. We compare dividing cell lineages with filamentous cells, where the lack of the diffusion barriers is expected to reduce the impact of other factors on the variability of nucleoid segregation dynamics. The nucleoid segregation was monitored using time-lapse microscopy in live E. coli cells grown in linear grooves. The main characteristics of the segregation process, namely, the synchrony of partitioning, rates of separation, and final positions, as well as the variability of these characteristics, were determined for dividing and filamentous lineages growing under the same conditions. Indeed, the gene expression noise was considerably homogenized along filaments as determined from the distribution of CFP and YFP stochastically expressed from the chromosome. We find that 1) the synchrony of nucleoid partitioning is progressively decreasing during consecutive cell cycles, but to a significantly lesser degree in filamentous than in dividing cells; 2) the mean partitioning rate of nucleoids is essentially the same in dividing and filamentous cells, displaying a substantial variability in both; and 3) nucleoids segregate to the same distances in dividing and filamentous cells. Variability in distances is increasing during successive cell cycles, but to a much lesser extent in filamentous cells. Our findings indicate that the variability of the chromosome segregation dynamics is reduced upon removal of boundaries between nucleoids, whereas the remaining variability is essentially inherent to the nucleoid itself.

Transient enhanced cell division by blocking DNA synthesis in Escherichia coli

Duplication of the bacterial nucleoid is necessary for cell division hence specific arrest of DNA replication inhibits divisions culminating in filamentation, nucleoid dispersion and appearance of a-nucleated cells. It is demonstrated here that during the first 10 min however, Escherichia coli enhanced residual divisions: the proportion of constricted cells doubled (to 40%), nucleoids contracted and cells remodelled dimensions: length decreased and width increased. The preliminary data provides further support to the existence of temporal and spatial couplings between the nucleoid/replisome and the sacculus/divisome, and is consistent with the idea that bacillary bacteria modulate width during the division process exclusively.

The transcription of pafA, encoding the prokaryotic ubiquitin-like protein ligase, is regulated by PafBC

AIM

Mycobacterium tuberculosis possesses an intracellular tagging and degradation system, which has emerged as a target for development of anti-tuberculosis agents. In this system, PafA is the ligase that marks proteins for degradation by their covalent modification with a protein modifier. Here, we studied pafA transcriptional regulation, which remained elusive despite its importance for M. tuberculosis virulence.

MATERIALS & METHODS

Working with Mycobacterium smegmatis, a mycobacterial model organism, we examined the involvement of the global regulators PafB and PafC in pafA regulation.

RESULTS

PafBC activated pafA transcription following DNA damage, resulting in efficient cellular recovery.

CONCLUSION

The results unraveled the involvement of PafBC in pafA transcription, and revealed the importance of proper PafA regulation in mycobacterial physiology.

HU content and dynamics in Escherichia coli during the cell cycle and at different growth rates

DNA-binding proteins play an important role in maintaining bacterial chromosome structure and functions. Heat-unstable (HU) histone-like protein is one of the most abundant of these proteins and participates in all major chromosome-related activities. Owing to its low sequence specificity, HU fusions with fluorescent proteins were used for general staining of the nucleoid, aiming to reveal its morphology and dynamics. We have exploited a single chromosomal copy of hupA-egfp fusion under the native promoter and used quantitative microscopy imaging to investigate the amount and dynamics of HUα in Escherichia coli cells. We found that in steady-state growing populations the cellular HUα content is proportional to the cell size, whereas its concentration is size independent. Single-cell live microscopy imaging confirmed that the amount of HUα exponentially increases during the cell cycle, but its concentration is maintained constant. This supports the existence of an auto-regulatory mechanism underlying the HUα cellular level, in addition to reflecting the gene copy number. Both the HUα amount and concentration strongly increase with the cell growth rate in different culture media. Unexpectedly, the HU/DNA stoichiometry also remarkably increases with the growth rate. This last finding may be attributed to a higher requirement for maintaining the chromosome structure in nucleoids with higher complexity.

Z-ring Structure and Constriction Dynamics in E. coli

The Z-ring plays a central role in bacterial division. It consists of FtsZ filaments, but the way these reorganize in the ring-like structure during septation remains largely unknown. Here, we measure the effective constriction dynamics of the ring. Using an oscillating optical trap, we can switch individual rod-shaped E. coli cells between horizontal and vertical orientations. In the vertical orientation, the fluorescent Z-ring image appears as a symmetric circular structure that renders itself to quantitative analysis. In the horizontal orientation, we use phase-contrast imaging to determine the extent of the cell constriction and obtain the effective time of division. We find evidence that the Z-ring constricts at a faster rate than the cell envelope such that its radial width (inwards from the cytoplasmic membrane) grows during septation. In this respect, our results differ from those recently obtained using photoactivated localization microscopy (PALM) where the radial width of the Z-ring was found to be approximately constant as the ring constricts. A possible reason for the different behavior of the constricting Z-rings could be the significant difference in the corresponding cell growth rates.

The membrane: transertion as an organizing principle in membrane heterogeneity

The bacterial membrane exhibits a significantly heterogeneous distribution of lipids and proteins. This heterogeneity results mainly from lipid-lipid, protein-protein, and lipid-protein associations which are orchestrated by the coupled transcription, translation and insertion of nascent proteins into and through membrane (transertion). Transertion is central not only to the individual assembly and disassembly of large physically linked groups of macromolecules (alias hyperstructures) but also to the interactions between these hyperstructures. We review here these interactions in the context of the processes in Bacillus subtilis and Escherichia coli of nutrient sensing, membrane synthesis, cytoskeletal dynamics, DNA replication, chromosome segregation, and cell division.

Three-dimensional structure of the Z-ring as a random network of FtsZ filaments

The spatial organization of the Z-ring, the central element of the bacterial division machinery, is not yet fully understood. Using optical tweezers and subpixel image analysis, we have recently shown that the radial width of the Z-ring in unconstricted Escherichia coli is about 100 nm. The relatively large width is consistent with the observations of others. Moreover, simulation of the experimental FtsZ distribution using the theoretical three-dimensional (3D) point spread function was strongly in favour of a toroidal rather than a thin cylindrical model of the Z-ring. Here, we show that the low density of FtsZ filaments in the ring coincides within experimental uncertainty with the critical density of a 3D random network of cylindrical sticks. This suggests that the Z-ring may consist of a percolating network of FtsZ filaments. Several factors that are expected to affect the polymerization state and the extent of self-interaction of FtsZ within the Z-ring, as well as the functional implications of its sparse toroidal structure, are discussed in terms of percolation theory.

Association of the chromosome replication initiator DnaA with the Escherichia coli inner membrane in vivo: quantity and mode of binding

DnaA initiates chromosome replication in most known bacteria and its activity is controlled so that this event occurs only once every cell division cycle. ATP in the active ATP-DnaA is hydrolyzed after initiation and the resulting ADP is replaced with ATP on the verge of the next initiation. Two putative recycling mechanisms depend on the binding of DnaA either to the membrane or to specific chromosomal sites, promoting nucleotide dissociation. While there is no doubt that DnaA interacts with artificial membranes in vitro, it is still controversial as to whether it binds the cytoplasmic membrane in vivo. In this work we looked for DnaA-membrane interaction in E. coli cells by employing cell fractionation with both native and fluorescent DnaA hybrids. We show that about 10% of cellular DnaA is reproducibly membrane-associated. This small fraction might be physiologically significant and represent the free DnaA available for initiation, rather than the vast majority bound to the datA reservoir. Using the combination of mCherry with a variety of DnaA fragments, we demonstrate that the membrane binding function is delocalized on the surface of the protein's domain III, rather than confined to a particular sequence. We propose a new binding-bending mechanism to explain the membrane-induced nucleotide release from DnaA. This mechanism would be fundamental to the initiation of replication.

Oriented imaging of 3D subcellular structures in bacterial cells using optical tweezers

Using oscillating optical tweezers, we show that controlled alignment of rod-shaped bacterial cells allows imaging fluorescently labeled three-dimensional (3D) subcellular structures from different, optimized viewpoints. To illustrate our method, we analyze the Z ring of E. coli. We obtain that the radial width of the Z ring in unconstricted cells is about 120 nm. This result suggests that the Z ring consists of an extremely sparse network of FtsZ filaments.

Timing of Z-ring localization in Escherichia coli

Bacterial cell division takes place in three phases: Z-ring formation at midcell, followed by divisome assembly and building of the septum per se. Using time-lapse microscopy of live bacteria and a high-precision cell edge detection method, we have previously found the true time for the onset of septation, τ(c), and the time between consecutive divisions, τ(g). Here, we combine the above method with measuring the dynamics of the FtsZ-GFP distribution in individual Escherichia coli cells to determine the Z-ring positioning time, τ(z). To analyze the FtsZ-GFP distribution along the cell, we used the integral fluorescence profile (IFP), which was obtained by integrating the fluorescence intensity across the cell width. We showed that the IFP may be approximated by an exponential peak and followed the peak evolution throughout the cell cycle, to find a quantitative criterion for the positioning of the Z-ring and hence the value of τ(z). We defined τ(z) as the transition from oscillatory to stable behavior of the mean IFP position. This criterion was corroborated by comparison of the experimental results to a theoretical model for the FtsZ dynamics, driven by Min oscillations. We found that τ(z) < τ(c) for all the cells that were analyzed. Moreover, our data suggested that τ(z) is independent of τ(c), τ(g) and the cell length at birth, L(0). These results are consistent with the current understanding of the Z-ring positioning and cell septation processes.

Timing the start of division in E. coli: a single-cell study

We monitor the shape dynamics of individual E. coli cells using time-lapse microscopy together with accurate image analysis. This allows measuring the dynamics of single-cell parameters throughout the cell cycle. In previous work, we have used this approach to characterize the main features of single-cell morphogenesis between successive divisions. Here, we focus on the behavior of the parameters that are related to cell division and study their variation over a population of 30 cells. In particular, we show that the single-cell data for the constriction width dynamics collapse onto a unique curve following appropriate rescaling of the corresponding variables. This suggests the presence of an underlying time scale that determines the rate at which the cell cycle advances in each individual cell. For the case of cell length dynamics a similar rescaling of variables emphasizes the presence of a breakpoint in the growth rate at the time when division starts, tau(c). We also find that the tau(c) of individual cells is correlated with their generation time, tau(g), and inversely correlated with the corresponding length at birth, L(0). Moreover, the extent of the T-period, tau(g) - tau(c), is apparently independent of tau(g). The relations between tau(c), tau(g) and L(0) indicate possible compensation mechanisms that maintain cell length variability at about 10%. Similar behavior was observed for both fast-growing cells in a rich medium (LB) and for slower growth in a minimal medium (M9-glucose). To reveal the molecular mechanisms that lead to the observed organization of the cell cycle, we should further extend our approach to monitor the formation of the divisome.

Mutual effects of MinD-membrane interaction: I. Changes in the membrane properties induced by MinD binding

In Escherichia coli and other bacteria, MinD, along with MinE and MinC, rapidly oscillates from one pole of the cell to the other controlling the correct placement of the division septum. MinD binds to the membrane through its amphipathic C-terminal alpha-helix. This binding, promoted by ATP-induced dimerization, may be further enhanced by a consequent attraction of acidic phospholipids and formation of a stable proteolipid domain. In the context of this hypothesis we studied changes in dynamics of a model membrane caused by MinD binding using membrane-embedded fluorescent probes as reporters. A remarkable increase in membrane viscosity and order upon MinD binding to acidic phospholipids was evident from the pyrene and DPH fluorescence changes. This viscosity increase is cooperative with regards to the concentration of MinD-ATP, but not of the ADP form, indicative of dimerization. Moreover, similar changes in the membrane dynamics were demonstrated in the native inverted cytoplasmic membranes of E. coli, with a different depth effect. The mobility of pyrene-labeled phosphatidylglycerol indicated formation of acidic phospholipid-enriched domains in a mixed acidic-zwitterionic membrane at specific MinD/phospholipid ratios. A comparison between MinD from E. coli and Neisseria gonorrhea is also presented.

Mutual effects of MinD-membrane interaction: II. Domain structure of the membrane enhances MinD binding

MinD, a well-conserved bacterial amphitropic protein involved in spatial regulation of cell division, has a typical feature of reversible binding to the membrane. MinD shows a clear preference for acidic phospholipids organized into lipid domains in bacterial membrane. We have shown that binding of MinD may change the dynamics of model and native membranes (see accompanying paper [1]). On the other hand, MinD dimerization and anchoring could be enhanced on pre-existing anionic phospholipid domains. We have tested MinD binding to model membranes in which acidic and zwitterionic phospholipids are either well-mixed or segregated to phase domains. The phase separation was achieved in binary mixtures of 1-Stearoyl-2-Oleoyl-sn-Glycero-3-[Phospho-rac-(1-glycerol] (SOPG) with 1,2-Distearoyl-sn-Glycero-3-Phosphocholine (DSPC) or 1,2-Distearoyl-sn-Glycero-3-[Phospho-rac-(1-glycerol)] (DSPG) and binding to these membranes was compared with that to a fluid mixture of SOPG with 1-Stearoyl-2-Oleoyl-sn-Glycero-3-Phosphocholine (SOPC). The results demonstrate that MinD binding to the membrane is enhanced by segregation of anionic phospholipids to fluid domains in a gel-phase environment and, moreover, the protein stabilizes such domains. This suggests that an uneven binding of MinD to the heterogeneous native membrane is possible, leading to formation of a lipid-specific distribution pattern of MinD and/or modulation of its temporal behavior.

Shape of nonseptated Escherichia coli is asymmetric

The shape of Escherichia coli is approximately that of a cylinder with hemispherical caps. Since its size is not much larger than optical resolution, it has been difficult to quantify deviations from this approximation. We show that one can bypass this limitation and obtain the cell shape with subpixel accuracy. The resulting contours are shown to deviate from the hemisphere-cylinder-hemisphere shape. In particular, the cell is weakly asymmetric. Its two caps are different from each other and the sides are slightly curved. Most cells have convex sides. We discuss our results in light of several mechanisms that are involved in determining the shape of cells.

Cell shape dynamics in Escherichia coli

Bacteria are the simplest living organisms. In particular, Escherichia coli has been extensively studied and it has become one of the standard model systems in microbiology. However, optical microscopy studies of single E. coli have been limited by its small size, approximately 1 x 3 microm, not much larger than the optical resolution, approximately 0.25 microm. As a result, not enough quantitative dynamical information on the life cycle of single E. coli is presently available. We suggest that, by careful analysis of images from phase contrast and fluorescence time-lapse microscopy, this limitation can be bypassed. For example, we show that applying this approach to monitoring morphogenesis in individual E. coli leads to a simple, quantitative description of this process. First, we find the time when the formation of the septum starts, tau(c). It occurs much earlier than the time when the constriction can be directly observed by phase contrast. Second, we find that the growth law of single cells is more likely bilinear/trilinear than exponential. This is further supported by the relations that hold between the corresponding growth rates. These methods could be further extended to study the dynamics of cell components, e.g., the nucleoid and the Z-ring.

The reactivation of DnaA(L366K) requires less acidic phospholipids supporting their role in the initiation of chromosome replication in Escherichia coli

DnaA(L366K), in concert with a wild-type DnaA (wtDnaA) protein, restores the growth of Escherichia coli cells arrested in the absence of adequate levels of cellular acidic phospholipids. In vitro and in vivo studies showed that DnaA(L366K) alone does not induce the initiation of replication, and wtDnaA must also be present. Hitherto the different behavior of wt and mutant DnaA were not understood. We now demonstrate that this mutant may be activated at significantly lower concentrations of acidic phospholipids than the wild-type protein, and this may explain the observed growth restoration in vivo.

Internal structure and dynamics of isolated Escherichia coli nucleoids assessed by fluorescence correlation spectroscopy

The morphology and dynamics of DNA in a bacterial nucleoid affects the kinetics of such major processes as DNA replication, gene expression. and chromosome segregation. In this work, we have applied fluorescence correlation spectroscopy to assess the structure and internal dynamics of isolated Escherichia coli nucleoids. We show that structural information can be extracted from the amplitude of fluorescence correlation spectroscopy correlation functions of randomly labeled nucleoids. Based on the developed formalism we estimate the characteristic size of nucleoid structural units for native, relaxed, and positively supercoiled nucleoids. The degree of supercoiling was varied using the intercalating agent chloroquine and evaluated from fluorescence microscopy images. The relaxation of superhelicity was accompanied by 15-fold decrease in the length of nucleoid units (from approximately 50 kbp to approximately 3 kbp).

Membrane-catalyzed nucleotide exchange on DnaA. Effect of surface molecular crowding

DnaA is the initiator protein for chromosomal replication in bacteria; its activity plays a central role in the timing of the primary initiations within the Escherichia coli cell cycle. A controlled, reversible conversion between the active ATP-DnaA and the inactive ADP forms modulates this activity. In a DNA-dependent manner, bound ATP is hydrolyzed to ADP. Acidic phospholipids with unsaturated fatty acids are capable of reactivating ADP-DnaA by promoting the release of the tightly bound ADP. The nucleotide dissociation kinetics, measured in the present study with the fluorescent derivative 3'-O-(N-methylantraniloyl)-5'-adenosine triphosphate, was dependent on the density of DnaA on the membrane in a cooperative manner: it increased 5-fold with decreased protein density. At all surface densities the nucleotide was completely released, presumably due to protein exchange on the membrane. Distinct temperature dependences and the effect of the crowding agent Ficoll suggest that two functional states of DnaA exist at high and low membrane occupancy, ascribed to local macromolecular crowding on the membrane surface. These novel phenomena are thought to play a major role in the mechanism regulating the initiation of chromosomal replication in bacteria.

Differences in membrane fluidity and fatty acid composition between phenotypic variants of Streptococcus pneumoniae

Phase variation in the colonial opacity of Streptococcus pneumoniae has been implicated as a factor in the pathogenesis of pneumococcal disease. This study examined the relationship between membrane characteristics and colony morphology in a few selected opaque-transparent couples of S. pneumoniae strains carrying different capsular types. Membrane fluidity was determined on the basis of intermolecular excimerization of pyrene and fluorescence polarization of 1,6-diphenyl 1,3,5-hexatriene (DPH). A significant decrease, 16 to 26% (P < or = 0.05), in the excimerization rate constant of the opaque variants compared with that of the transparent variants was observed, indicating higher microviscosity of the membrane of bacterial cells in the opaque variants. Liposomes prepared from phospholipids of the opaque phenotype showed an even greater decrease, 27 to 38% (P < or = 0.05), in the pyrene excimerization rate constant compared with that of liposomes prepared from phospholipids of bacteria with the transparent phenotype. These findings agree with the results obtained with DPH fluorescence anisotropy, which showed a 9 to 21% increase (P < or = 0.001) in the opaque variants compared with the transparent variants. Membrane fatty acid composition, determined by gas chromatography, revealed that the two variants carry the same types of fatty acids but in different proportions. The trend of modification points to the presence of a lower degree of unsaturated fatty acids in the opaque variants compared with their transparent counterparts. The data presented here show a distinct correlation between phase variation and membrane fluidity in S. pneumoniae. The changes in membrane fluidity most probably stem from the observed differences in fatty acid composition.

Compaction of the Escherichia coli nucleoid caused by Cyt1Aa

Compaction of the Escherichia coli nucleoid in the cell's centre was associated with the loss of colony-forming ability; these effects were caused by induction of Cyt1Aa, the cytotoxic 27 kDa protein from Bacillus thuringiensis subsp. israelensis. Cyt1Aa-affected compaction of the nucleoids was delayed but eventually more intense than compaction caused by chloramphenicol. The possibility that small, compact nucleoids in Cyt1Aa-expressing cells resulted in DNA replication run-out and segregation following cell division was ruled out by measuring relative nucleoid length. Treatments with membrane-perforating substances other than Cyt1Aa did not cause such compaction of the nucleoids, but rather the nucleoids overexpanded to occupy nearly all of the cell volume. These findings support the suggestion that, in addition to its perforating ability, Cyt1Aa causes specific disruption of nucleoid associations with the cytoplasmic membrane. In situ immunofluorescence labelling with Alexa did not demonstrate a great amount of Cyt1Aa associated with the membrane. Clear separation between Alexa-labelled Cyt1Aa and 4',6-diamidino-2-phenylindole (DAPI)-stained DNA indicates that the nucleoid does not bind Cyt1Aa. Around 2 h after induction, nucleoids in Cyt1Aa-expressing cells started to decompact and expanded to fill the whole cell volume, most likely due to partial cell lysis without massive peptidoglycan destruction.

Phosphatidylethanolamine and phosphatidylglycerol are segregated into different domains in bacterial membrane. A study with pyrene-labelled phospholipids

To detect and characterize membrane domains that have been proposed to exist in bacteria, two kinds of pyrene-labelled phospholipids, 2-pyrene-decanoyl-phosphatidylethanolamine (PY-PE) and 2-pyrene-decanoyl-phosphatidylglycerol (PY-PG) were inserted into Escherichia coli or Bacillus subtilis membrane. The excimerization rate coefficient, calculated from the excimer-to-monomer ratio dependencies on the probe concentration, was two times higher for PY-PE than for PY-PG at 37 degrees C. This was ascribed to different local concentrations rather than to differences in mobility. The extent of mixing between the two fluorescent phospholipids, estimated by formation of their heteroexcimer, was found very low both in E. coli and B. subtilis, in contrast to model membranes. In addition, these two pyrene derivatives exhibited different temperature phase transitions and different detergent extractability, indicating that the surroundings of these phospholipids in bacterial membrane differ in organization and order. Inhibition of protein synthesis, leading to condensation of nucleoid and presumably to dissipation of membrane domains, indeed resulted in increased formation of heteroexcimers, broadening of phase transitions and equal detergent extractability of both probes. It is proposed that in bacterial membranes these phospholipids are segregated into distinct domains that differ in composition, proteo-lipid interaction and degree of order; the proteo-lipid domain being enriched by PE.

Hyperstructures, genome analysis and I-cells

New concepts may prove necessary to profit from the avalanche of sequence data on the genome, transcriptome, proteome and interactome and to relate this information to cell physiology. Here, we focus on the concept of large activity-based structures, or hyperstructures, in which a variety of types of molecules are brought together to perform a function. We review the evidence for the existence of hyperstructures responsible for the initiation of DNA replication, the sequestration of newly replicated origins of replication, cell division and for metabolism. The processes responsible for hyperstructure formation include changes in enzyme affinities due to metabolite-induction, lipid-protein affinities, elevated local concentrations of proteins and their binding sites on DNA and RNA, and transertion. Experimental techniques exist that can be used to study hyperstructures and we review some of the ones less familiar to biologists. Finally, we speculate on how a variety of in silico approaches involving cellular automata and multi-agent systems could be combined to develop new concepts in the form of an Integrated cell (I-cell) which would undergo selection for growth and survival in a world of artificial microbiology.

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.

Coexistence of domains with distinct order and polarity in fluid bacterial membranes

In this study we sought the detection and characterization of bacterial membrane domains. Fluorescence generalized polarization (GP) spectra of laurdan-labeled Escherichia coli and temperature dependencies of both laurdan's GP and fluorescence anisotropy of 1,3-diphenyl-1,3,5-hexatriene (DPH) (rDPH) affirmed that at physiological temperatures, the E. coli membrane is in a liquid-crystalline phase. However, the strong excitation wavelength dependence of rlaurdan at 37 degrees C reflects membrane heterogeneity. Time-resolved fluorescence emission spectra, which display distinct biphasic redshift kinetics, verified the coexistence of two subpopulations of laurdan. In the initial phase, <50 ps, the redshift in the spectral mass center is much faster for laurdan excited at the blue edge (350 nm), whereas at longer time intervals, similar kinetics is observed upon excitation at either blue or red edge (400 nm). Excitation in the blue region selects laurdan molecules presumably located in a lipid domain in which fast intramolecular relaxation and low anisotropy characterize laurdan's emission. In the proteo-lipid domain, laurdan motion and conformation are restricted as exhibited by a slower relaxation rate, higher anisotropy and a lower GP value. Triple-Gaussian decomposition of laurdan emission spectra showed a sharp phase transition in the temperature dependence of individual components when excited in the blue but not in the red region. At least two kinds of domains of distinct polarity and order are suggested to coexist in the liquid-crystalline bacterial membrane: a lipid-enriched and a proteolipid domain. In bacteria with chloramphenicol (Cam)-inhibited protein synthesis, laurdan showed reduced polarity and restoration of an isoemissive point in the temperature-dependent spectra. These results suggest a decrease in membrane heterogeneity caused by Cam-induced domain dissipation.

Bacteriophage T4 development in Escherichia coli is growth rate dependent

Three independent parameters (eclipse and latent periods, and rate of ripening during the rise period) are essential and sufficient to describe bacteriophage development in its bacterial host. A general model to describe the classical "one-step growth" experiment [Rabinovitch et al. (1999a) J. Bacteriol.181, 1687-1683] allowed their calculations from experimental results obtained with T4 in Escherichia coli B/r under different growth conditions [Hadas et al. (1997) Microbiology143, 179-185]. It is found that all three parameters could be described by their dependence solely on the culture doubling time tau before infection. Their functional dependence on tau, derived by a best-fit analysis, was used to calculate burst size values. The latter agree well with the experimental results. The dependence of the derived parameters on growth conditions can be used to predict phage development under other experimental manipulations.

Hypothesis: membrane domains and hyperstructures control bacterial division

The mechanism responsible for creating the division site in the right place at the right time in bacteria is unknown. It has been attributed to the formation of proteolipid domains in the cytoplasmic membrane surrounding the nucleoids. We interpret the growing evidence for this hypothesis by invoking hyperstructures, which exist at a level of organization intermediate between macromolecules and genes. Non-equilibrium hyperstructures comprise the genes, mRNA proteins and lipids required for a particular function such as cell division, and assemble and disassemble according to the needs of the cell.

Quenching of cascade reaction between triplet and photochrome probes with nitroxide radicals. A novel labeling method in study of membranes and surface systems

We proposed a new method for the study of molecular dynamics and fluidity of the living and model biomembranes and surface systems. The method is based on the measurements of the sensitized photoisomerization kinetics of a photochrome probe. The cascade triplet cis-trans photoisomerization of the excited stilbene derivative sensitized with the excited triplet Erythrosin B has been studied in a model liposome membrane. The photoisomerization reaction is depressed with nitroxide radicals quenching the excited triplet state of the sensitizer. The enhanced fluorescence polarization of the stilbene probe incorporated into liposome membranes indicates that the stilbene molecules are squeezed in a relatively viscous media of the phospholipids. Calibration of the "triple" cascade system is based on a previously proposed method that allows the measurement of the product of the quenching rate constant and the sensitizer's triplet lifetime, as well as the quantitative detection of the nitroxide radicals in the vicinity of the membrane surface. The experiment was conducted using the constant-illumination fluorescence technique. Sensitivity of the method using a standard commercial spectrofluorimeter is about 10(-12) mol of fluorescence molecules per sample and can be improved using an advanced fluorescence technique. The minimal local concentration of nitroxide radicals or any other quenchers being detected is about 10(-5) M. This method enables the investigation of any chemical and biological surface processes of microscopic scale when the minimal volume is about 10(-3) microL or less.

Bacterial lysis by phage--a theoretical model

The similarity to materials corrosion is invoked to develop a model for phage-infected bacterial lysis based on the statistics of extremes. The importance of cell size, envelope thickness and lysozyme eclipse time on the final probability distribution of lysis is considered. Experiments are suggested to test the model.

Visualizing multiple constrictions in spheroidal Escherichia coli cells

An Escherichia coli cell grows by elongation and divides in a perpendicular plane. Alternating planes of successive divisions in three dimensions can only be ascertained when multiple constrictions exist simultaneously in large, spheroidal cells (with extended constriction process), if the division signals are enhanced. Large, spheroidal cells are obtained by a brief mecillinam treatment, and more frequent divisions are achieved by manipulating the rate of chromosome replication without affecting cell mass growth rate. Such a procedure has recently been performed by thymine-limitation of E. coli K12 strain CR34 (Zaritsky et al., Microbiology 145 (1999), 1052-1022). Enhancing the replication rate in cells with multi-forked replicating chromosomes (by addition of deoxyguanosine) shortens the intervals between successive terminations and thus triggers divisions more frequently. Monoclonal antibodies against FtsZ were used to visualize the rings of secondary constrictions, but apparent shortage of FtsZ to complete rings over wide cells allowed assembly of arcs only. The arcs observed were not parallel nor perpendicular; the tilted constriction planes are consistent with our 3-D 'nucleoid segregation'model for division under conditions which relieve the cylindrical constraint for nucleoid segregation by the bacillari peptidoglycan sacculus (Woldringh et al. , J. Bacteriol. 176 (1994) 6030-6038). The shortage in FtsZ may explain the longer time required to complete the division process in wide cells with long circumferences, observed during thymine step-up. Overexpression of fusion protein FtsZ-GFP on a multi-copy plasmid should circumvent the shortage.

Division-associated changes in membrane viscosity of Escherichia coli

Septum formation is initiated by the FtsZ ring assembly in the middle of rod-shape bacteria. The mechanism which determines the division site in the membrane and makes it recognizable by FtsZ is still unknown. We have recently demonstrated that the putative division membrane domains can be visualized by a fluorescent membrane probe (Fishov and Woldring, Mol. Microbiol., 1999) and that these domains can be dissipated by interrupting the process of coupled transcription and translation of proteins (Binenbaum et al., Mol. Microbiol., 1999). Here, we examined the membrane dynamics of Escherichia coli during division and after a reversible division arrest. Anisotropy of DPH fluorescence, used as an indicator of membrane dynamics (viscosity), correlated with the rate of division in synchronous cells. It decreased during filamentation caused by drugs or by temperature, but not in the ftsZ mutant and when DNA replication was blocked by nalidixic acid. Based on previous data, we incline to interpret these results as reflecting formation and dissipation of putative membrane domains marking the division sites; domains are formed by partitioning nucleoids and dissipate while used for constriction or after the nucleoids have been segregated too far in a filament.

Transcription- and translation-dependent changes in membrane dynamics in bacteria: testing the transertion model for domain formation

Cell cycle events have been proposed to be triggered by the formation of membrane domains in the process of coupled transcription, translation and insertion ('transertion') of nascent membrane and exported proteins. Disruption of domain structure should lead to changes in membrane dynamics. Membrane viscosity of Escherichia coli and Bacillus subtilis decreased after inhibition of protein synthesis by chloramphenicol or puromycin, or of RNA initiation by rifampicin, but not after inhibition of RNA elongation by streptolydigin or amino acid starvation of a stringent strain. The decrease caused by inhibitors of protein synthesis was prevented by streptolydigin if added simultaneously, but was not reversed if added later. The drug-induced decrease in membrane viscosity is energy dependent: it did not happen in KCN-treated cells. All treatments decreasing membrane viscosity also induced nucleoid compaction and fusion. Inhibition of macromolecular synthesis without membrane perturbation caused nucleoids to expand. Changes in membrane dynamics were also displayed during a nutritional shift-down transition that causes imbalance in macromolecular syntheses. The results are consistent with the transertion model, predicting dissipation of membrane domains by termination of protein synthesis or detachment of polysomes from DNA; domain structure is conserved if the transertion process is 'frozen'.

Visualization of membrane domains in Escherichia coli

Bacterial membrane and nucleoids were stained concurrently by the lipophilic styryl dye FM 4-64 [N-(3-triethylammoniumpropyl)-4-(6-(4-(diethylamino)phenyl) hexatrienyl)pyridinium dibromide] and 4',6-diamidino-2-phenylindole (DAPI), respectively, and studied using fluorescence microscopy imaging. Observation of plasmolysed cells indicated that FM 4-64 stained the inner membrane preferentially. In live Escherichia coli pbpB cells and filaments, prepared on wet agar slabs, an FM 4-64 staining pattern developed in the form of dark bands. In dividing cells, the bands occurred mainly at the constriction sites and, in filaments, between partitioning nucleoids. The FM 4-64 pattern of dark bands in filaments was abolished after inhibiting protein synthesis with chloramphenicol. It is proposed that the staining patterns reflect putative membrane domains formed by DNA-membrane interactions and have functional implications in cell division.

Varying division planes of secondary constrictions in spheroidal Escherichia coli cells

Planes of successive divisions in Escherichia coli have been proposed to be either parallel or perpendicular to each other, restricted to one or two dimensions. To test the hypothesis that divisions can occur in planes alternating in three dimensions, a method was developed to generate cells with secondary constrictions during growth in suspension. The method involves a combination of thymine limitation (to manipulate chromosome replication rate) and mecillinam treatment (to inhibit penicillin-binding protein 2). The former modifies timing of terminations, the latter results in spheroidal cells. Such cells displayed secondary constrictions after adding deoxyguanosine (accelerating replication rate), thus temporarily enhancing division signals. The successive constrictions were seen to develop in planes that were tilted relative to each other, and in positions related to those of the nucleoids, visualized by staining with DAPI (4',6-diamidino-2-phenylindole dihydrochloride hydrate). Visualizing cell envelopes with FM 4-64 by confocal scanning laser microscopy supported the conclusion that planes of successive divisions can alternate in three dimensions.

Bacteriophage T4 development depends on the physiology of its host Escherichia coli

Several parameters of phage T4 adsorption to and growth in Escherichia coli B/r were determined. All changed monotonously with the bacterial growth rate (mu), which was modified by nutritional conditions. Adsorption rate was faster at higher mu values, positively correlated to cell size, and increased by pretreatment with low penicillin (Pn) concentrations; it was directly proportional to total cellular surface area, indicating a constant density of T4 receptors on cell envelopes irrespective of growth conditions. Parameters of phage development and cell lysis were mu-dependent. The rate of phage release and burst size increased, while the eclipse and latent periods decreased with increasing mu. Differentiation between the contribution of several physiological parameters to the development of T4 was performed by manipulating the host cells. A competitive inhibitor of glucose uptake, methyl alpha-D-glucoside, was exploited to reduce the growth rate in the same effective carbon source. Synchronous cells were obtained by the "baby-machine' and large cells were obtained by pretreatment with low Pn concentrations. Lysis was delayed by superinfection, and DNA content and concentration were modified by growing a thy mutant in limiting thymine concentrations. The results indicate that burst size is not limited by cell size or DNA composition, nor directly by the rate of metabolism, but rather by the rates of synthesis and assembly of phage components and by lysis time. The rates of synthesis and assembly of phage components seem to depend on the content of the protein-synthesizing system and lysis time seems to depend on cellular dimensions.

Novel fluorescence-photochrome labeling method in the study of biomembrane dynamics

A novel photochrome-fluorescence method (PFLM) based on monitoring fluorescence parameters and kinetics of photochrome photoisomerization of para-substituted stilbenes (PSS) has been proposed. It was shown that PSS exhibits fluorescence characteristics which are similar to ones of typical membrane fluorescence probes such as diphenylhexatriene (DPH). A study of kinetics of PSS trans-cis and cis-trans photoisomerization makes it possible to estimate, under certain conditions, the rotational correlation time of the stilbene fragments in the excited state of PSS for the fixed angle 180 degrees. In viscous media this process is a rate-determining stage. Taken together, the both techniques, fluorescence and photochrome, make it possible to establish a detailed mechanism and measure quantitative parameters of stilbene probe (PSS) mobility in a membrane. The PFLM was applied to the study of E. coli membrane dynamics.

Division-inhibition capacity of penicillin in Escherichia coli is growth-rate dependent

Growing bacteria are sensitive to various beta-lactam derivatives due to their interference with peptidoglycan biosynthesis. At low concentrations, penicillin G (benzylpenicillin) blocks cell division without affecting mass growth rate. The MIC for division of Escherichia coli B/r (H266) was found to depend on the growth rate, which was modified by the nutritional conditions. Our hypothesis, that division sensitivity is proportional to the rate of peptidoglycan synthesis for septum formation, as well as to cell circumference, was thus confirmed.

On microbial states of growth

It is crucial to the reproducibility of results and their proper interpretation that the conditions under which experiments are carried out be defined with rigour and consistency. In this review we attempt to clarify the differences and interrelationships among steady, balanced and exponential states of culture growth. Basic thermodynamic concepts are used to introduce the idea of steady-state growth in open, biological systems. The classical, sometimes conflicting, definitions of steady-state and balanced growth are presented, and a consistent terminology is proposed. The conditions under which a culture in balanced growth is also in exponential growth and in steady-state growth are indicated. It is pointed out that steady-state growth always implies both balanced and exponential growth, and examples in which the converse does not hold are described. More complex situations are then characterized and the terminology extended accordingly. This leads to the notion of normal growth and growth that can be synchronous or otherwise unbalanced but still reproducible, and to the condition of approximate steady state manifested by growth in batch culture and by asymmetrically dividing cells, which is analysed in some detail.

Fluorescence polarization assay for endothelin-converting enzymes

The conversion of big endothelin to endothelin by alpha-chymotrypsin was determined by following its single Trp fluorescence polarization. This provides a novel, simple, fast, and sensitive identifying assay in the search for a native endothelin-converting enzyme.