Correlation of Physical and Biological Properties of Mouse Mammary Tumor Agent

Biophysical procedures have been used to determine the size and structure of the biologically active agent responsible for the transmission, through milk, of mouse mammary adenocarcinoma. Filtration of milk from RIII high-breast-cancer mice through gradocol membranes with decreasing pore sizes indicated that a minimum of activity passed through intermediate pore sizes (100 to 160 mµ). Filtrates through smaller pores were significantly active. Milk treated with small doses of deuteron irradiation produced more tumors than the control, unirradiated milk; larger doses indicated a particle size much less than 100 mµ. Free diffusion experiments indicated that the activity was associated with particles of two different sizes. Altogether the data denoted the presence of a large agent about 100 mµ in diameter and a small agent 20 to 30 mµ in diameter or possibly smaller. Furthermore, the presence in the milk of an inhibitor 40 to 60 mµ is indicated by the results of all three approaches. The complex nature of the milk agent disclosed by the physical measurements agrees with the picture of one of the structures revealed by electron microscopy as well as with seemingly conflicting measurements reported in the literature. The large agent defined by these indirect methods corresponds to the whole particle seen in the electron microscope and the small agent corresponds to its internal core or nucleoid. It is suggested that the nucleoid is essentially a nucleic acid which may, in the absence of the "inhibitor," retain its activity after being stripped of its outer membrane or sac.     

Image analysis-based measurement of DNA supercoiling changes in transformed and nontransformed human cell lines

An image analysis system was used to visualize and measure the changes in nucleoid diameter (nuclear matrix core plus extruded DNA loops) which occur when increasing concentrations of propidium iodide are used to titrate the DNA supercoiling response. Parallel core size measurements allow estimates of the changes in apparent DNA loop size. Unlike sedimentation assays, DNA loop size estimates are not influenced by particle mass, require no prior cell labeling, and can be performed on a per cell basis. This technique was used to examine changes in DNA loop characteristics which may occur when cells are transformed or undergo changes in their proliferative state. SV40-transformation of human diploid fibroblast lines resulted in a significant increase in both the nucleoid core and average DNA loop size. Lymphoblast cell lines also had larger nucleoid dimensions than normal lymphocytes. The response of several established human tumor cell lines indicated slightly increased loop but not core sizes as compared to normal human diploid fibroblasts. Changes in proliferative state also resulted in changes in DNA loop characteristics as measured in this assay. Both quiescent fibroblasts and unstimulated lymphocytes appeared to have smaller or more condensed DNA loop structures than their proliferating counterparts. These results demonstrate the utility of this assay in detecting changes in DNA loop structure which occur in association with changes in the proliferative activity of cells in culture.     

Centrifugal,biochemical, and electron microscopic analysis of cytoplasmic particulates in liver homogenates

A combined centrifugal, biochemical, and electron microscopic study of the cytoplasmic particulates present in 0.88 M sucrose homogenates of rat liver has been carried out. Size distribution analyses of particles containing pentose nucleic acid (PNA) and exhibiting several types of enzymatic activity revealed three major size groups within the range of particle radius between 10 and 500 mmicro. A different array of biochemical properties was associated with each size group. The largest particles, with an average radius (assuming spherical shape) in the region of 220 to 260 mmicro, contained all of the succinic dehydrogenase activity of the cytoplasmic extract, 29 per cent of the diphosphopyridine nucleotide (DPN)-cytochrome c reductase activity, and minor amounts of PNA and acid phosphatase activity. Cytologically, this group of particles was identified with the mitochondria. All of the uricase activity, 58 per cent of the acid phosphatase activity, and 26 per cent of the PNA was apparently associated with a second size group of particles (average radius 120 mmicro) which were tentatively identified by electron microscopy with vesicular structures derived from the ergastoplasm of the intact cell. The third particle group demonstrated by centrifugation exhibited a major size distribution peak at 25 mmicro and a second smaller peak at 55 mmicro. Over 50 per cent of the total cytoplasmic PNA and DPN-cytochrome c reductase activity was associated with particles in this size group. Electron microscopy revealed a morphologically heterogeneous population of particles within this size range.     

Oligomer formation of staphylococcal α-toxin analyzed by electron microscopy and image processing

Abstract The 12S oligomeric form of Staphylococcus aureus α-toxin has been studied with electron microscopy after incubation of the toxin with membrane preparations or liposomes. The target material originated from human platelet. Different electron microscopic preparation techniques were used including negative staining, freeze-fracture and vitrification in liquid ethane. Analysis of micrographs with image processing methods revealed two groups of ring-like structures corresponding to α-toxin oligomers. One form measured 75 Å in diameter and had a high stain density in the central protein deficient part while the other was larger with a diameter of 100 Å and less stain accumulation in the center. The conditions under which the latter were formed suggest that this corresponds to an inactive loosely-bound form of the toxin. The high stain density in the smaller particle is consistent with the presence of a penetrating pore in this structure.     

Pancreatic microsomes; an integrated morphological and biochemical study

The pancreatic exocrine cell of the guinea pig has a voluminous endoplasmic reticulum distinguished by extensive association with small, dense particles, and by its orderly disposition in the basal region of the cell. In addition to the small, ( approximately 15 mmicro), dense particles attached to the limiting membrane of the endoplasmic reticulum, numerous particles of similar appearance are found freely scattered in the cytoplasmic matrix. The various cell structures of pancreatic exocrine cells can be satisfactorily identified in pancreatic homogenates. The microsome fraction consists primarily of spherical vesicles (80 to 300 mmicro), limited by a thin membrane (7 mmicro) which bears small ( approximately 15 mmicro) dense particles attached on its outer surface. The content of the microsomal vesicles is usually of high density. Pancreatic microsomes derive by extensive fragmentation mainly from the rough surfaced parts of the endoplasmic reticula of exocrine cells. A few damaged mitochondria and certain dense granules ( approximately 150 mmicro) originating probably from islet cells, contaminate the microsome fraction. Pancreatic microsomes contain RNA, protein, and a relatively small amount of phospholipide and hemochromogen. They do not have DPNH-cytochrome c reductase activity. In six experiments the RNA/protein N ratios were found grouped around two different means, namely 0.6 and 1.3. Pancreatic microsomes are more labile than liver microsomes but react in a similar way to RN-ase-(loss of the particulate component and RNA), and deoxycholate treatment (loss of the membranous component and of phospholipide, hemochromogen, and most of the protein). Postmicrosomal fractions consisting primarly of small ( approximately 15 mmicro), dense particles of ribonucleoprotein (RNA/protein N ratio = 1 to 2) were obtained by further centrifugation of the microsomal supernatant. The small nucleoprotein particles of these fractions are frequently found associated in chains or clusters.     

Marrow-derived stem cell motility in 3D synthetic scaffold is governed by geometry along with adhesivity and stiffness

Design of 3D scaffolds that can facilitate proper survival, proliferation, and differentiation of progenitor cells is a challenge for clinical applications involving large connective tissue defects. Cell migration within such scaffolds is a critical process governing tissue integration. Here, we examine effects of scaffold pore diameter, in concert with matrix stiffness and adhesivity, as independently tunable parameters that govern marrow‐derived stem cell motility. We adopted an “inverse opal” processing technique to create synthetic scaffolds by crosslinking poly(ethylene glycol) at different densities (controlling matrix elastic moduli or stiffness) and small doses of a heterobifunctional monomer (controlling matrix adhesivity) around templating beads of different radii. As pore diameter was varied from 7 to 17 µm (i.e., from significantly smaller than the spherical cell diameter to approximately cell diameter), it displayed a profound effect on migration of these stem cells—including the degree to which motility was sensitive to changes in matrix stiffness and adhesivity. Surprisingly, the highest probability for substantive cell movement through pores was observed for an intermediate pore diameter, rather than the largest pore diameter, which exceeded cell diameter. The relationships between migration speed, displacement, and total path length were found to depend strongly on pore diameter. We attribute this dependence to convolution of pore diameter and void chamber diameter, yielding different geometric environments experienced by the cells within. Bioeng. 2011; 108:1181–1193. © 2010 Wiley Periodicals, Inc.     

Spatial variability of sulfate reduction in a shallow aquifer

The distribution and metabolic activity of sulfate-reducing bacteria (SRB) in a shallow, suboxic aquifer were studied. A radioimaging technique was used to visualize and quantify the activity of sulfate reducers in sediments at a centimetre-level scale. The distribution of SRB metabolic activity was heterogeneous with areas showing little activity far outnumbering areas with high activity. Variation in sulfate-reducing activity was not statistically correlated with variation in depth, bacterial numbers, or the following sediment properties: sediment type (sand, peat or silt), grain size, permeability and hydraulic conductivity. Sulfate-reducing bacteria activity did vary significantly with sediment porosity (multivariate analysis, r = 0.48). We hypothesized that the small pore sizes associated with sediments with low porosity restricted the ability of SRB to grow to high numbers as well as their access to nutrients. To further explore the relationship between pore size and microbial metabolic activity, columns with varying pore diameters were constructed. Sulfate-reducing bacteria in the columns with the smallest pore diameters had the lowest rates of metabolism and SRB metabolic rates increased as the pore diameter increased. For the aquifer studied, sediment porosities and pore sizes were the main factor controlling SRB activity.     

The reaction of rat blood plasma to gamma radiation studied by the use of laser photon-correlation spectroscopy]

Using the method of laser photon correlation spectroscopy it was shown that the typical pattern of distribution of blood plasma (BP) particles according to their sizes, characteristic of intact animals, was asymmetric with regard to a mean value. Gamma irradiation with sublethal doses was shown to change essentially the pattern of BP distribution. For instance, in BP of exposed animals, the number of particles of much smaller sizes increased as compared to that in intact animals. The smaller BP particle size was not the result of the direct effect of radiation on the blood. The in vitro irradiation of BP with various doses results usually in a considerable growth of particle sizes.     

Determination of size distribution and encapsulation efficiency of liposome-encapsulated hemoglobin blood substitutes using asymmetric flow field-flow fractionation coupled with multi-angle static light scattering

In this study, we investigated the size distribution, encapsulation efficiency, and oxygen affinity of liposome-encapsulated tetrameric hemoglobin (LEHb) dispersions and correlated the data with the variation in extruder membrane pore size, ionic strength of the extrusion buffer, and hemoglobin (Hb) concentration. Asymmetric flow field-flow fractionation (AFFF) in series with multi-angle static light scattering (MASLS) was used to study the LEHb size distribution. We also introduced a novel method to measure the encapsulation efficiency using a differential interferometric refractive index (DIR) detector coupled to the AFFF-MASLS system. This technique was nondestructive toward the sample and easy to implement. LEHbs were prepared by extrusion using a lipid combination of dimyristoyl-phosphatidylcholine, cholesterol, and dimyristoyl-phosphatidylglycerol in a 10:9:1 molar ratio. Five initial Hb concentrations (50, 100, 150, 200, and 300 mg Hb per mL of buffer) extruded through five different membrane pore diameters (400, 200, 100, 80, and 50 nm) were studied. Phosphate buffered saline (PBS) and phosphate buffer (PB) both at pH 7.3 were used as extrusion buffers. Despite the variation, extrusion through 400-nm pore diameter membranes produced LEHbs smaller than the pore size, extrusion through 200-nm membranes produced LEHbs with diameters close to the pore diameter, and extrusion through 100-, 80-, and 50-nm membranes produced LEHbs larger than the pore sizes. We found that the choice of extrusion buffer had the greatest effect on the LEHb size distribution compared to either Hb concentration or extruder membrane pore size. Extrusion in PBS produced larger LEHbs and more monodisperse LEHb dispersions. However, LEHbs extruded in PB generally had higher Hb encapsulation efficiencies and lower methemoglobin (metHb) levels. The choice of extrusion buffer also affected how the encapsulation efficiency correlated with Hb concentration, extruder pore size, and the metHb level. The most optimum encapsulation efficiency and amount of Hb entrapped were achieved at the highest Hb concentration and the largest pore size for both extrusion buffers (62.38% and 187.14 mg Hb/mL of LEHb dispersion extruded in PBS, and 69.98% and 209.94 mg Hb/mL of LEHb dispersion extruded in PB). All LEHbs displayed good oxygen-carrying properties as indicated by their P(50) and cooperativity coefficients. LEHbs extruded in PB had an average P(50) of 23.04 mmHg and an average Hill number of 2.29, and those extruded in PBS had average values of 27.25 mmHg and 2.49. These oxygen-binding properties indicate that LEHbs possess strong potential as artificial blood substitutes. In addition, the metHb levels in PB-LEHb dispersions are significantly low even in the absence of antioxidants such as N-acetyl-L-cysteine.     

Thin sections. I. A study of section thickness and physical distortion produced during microtomy

Knowledge of the thickness of sections is important for proper interpretation of electron micrographs. Therefore, the thicknesses of sections of n-butyl methacrylate polymer were determined by ellipsometry, and correlated with the color shown in reflected light. The results are: gray, thinner than 60 mmicro; silver, 60 to 90 mmicro; gold, 90 to 150 mmicro; purple, 150 to 190 mmicro; blue, 190 to 240 mmicro; green, 240 to 280 mmicro; and yellow, 280 to 320 mmicro. These results agree well with optical theory and with previous published data for thin films. Sections, after cutting, are 30 to 40 per cent shorter than the face of the block from which they were cut. Only a small improvement results from allowing the sections to remain in the collecting trough at room temperature. Heating above room temperature, however, reduces this shortening, with a corresponding improvement in dimensions and spatial relationships in the sections. When the thickness of the section is considered in interpreting electron micrographs instead of considering the section to be two-dimensional, a more accurate interpretation is possible. The consideration of electron micrographs as arising from projections of many profiles from throughout the whole thickness of the section explains the apparent lack of continuity often observed in serial sections. It is believed that serial sections are actually continuous, but that the change in size of structure through the thickness of one section and the consideration of only the largest profile shown in the micrograph can account for the lack of continuity previously observed.     

Nucleoid partitioning and the division plane in Escherichia coli.

Escherichia coli nucleoids were visualized after the DNA of OsO4-fixed but hydrated cells was stained with the fluorochrome DAPI (4',6-diamidino-2-phenylindole dihydrochloride hydrate). In slowly growing cells, the nucleoids are rod shaped and seem to move along the major cell axis, whereas in rapidly growing, wider cells they consist of two- to four-lobed structures that often appear to advance along axes lying perpendicular or oblique to the major axis of the cell. To test the idea that the increase in cell diameter following nutritional shift-up is caused by the increased amount of DNA in the nucleoid, the cells were subjected to DNA synthesis inhibition. In the absence of DNA replication, the nucleoids continued to move in the growing filaments and were pulled apart into small domains along the length of the cell. When these cells were then transferred to a richer medium, their diameters increased, especially in the region enclosing the nucleoid. It thus appears that the nucleoid motive force does not depend on DNA synthesis and that cell diameter is determined not by the amount of DNA per chromosome but rather by the synthetic activity surrounding the nucleoid. Under the non-steady-state but balanced growth conditions induced by thymine limitation, nucleoids become separated into small lobules, often lying in asymmetric configurations along the cell periphery, and oblique and asymmetric division planes occur in more than half of the constricting cells. We suggest that such irregular DNA movement affects both the angle of the division plane and its position.     

Determination of the size distribution of liposomes by SEC fractionation,and PCS analysis and enzymatic assay of lipid content

In this study, small liposomes obtained by high-pressure homogenization were fractionated according to their particle sizes by size exclusion chromatography (SEC). The subfractions were analyzed by photon correlation spectroscopy (PCS) as well as enzymatic phosphatidylcholine (PC) assay for their particle sizes and lipid contents, respectively. For small egg PC-liposomes, a size range of 15 nm to 60 nm was found, with 80% of the vesicles being smaller than 30 nm in size. This is in contradiction to a mean size of 85±32 nm as indicated by PCS without fractionation. The PCS technique appears to underestimate very small particles below 30 nm if (few) bigger particles are present. The PCS particle size analysis of unfractionated hydrogenated egg PC/cholesterol-liposomes (2:1, mole/mole) by PCS did not yield any significant results. On fractionation, however, a particle size range of 40 nm to 120 nm was determined in a reproducible manner. Our results indicate that the combination of size exclusion fractionation with subsequent photon correlation spectroscopic particle size analysis and enzymatic PC assay can give both more detailed and more reliable insight into the particle size distribution of small liposomes than PCS alone. Published: May 15, 2002.     

Enhanced viscoelasticity of human cystic fibrotic sputum correlates with increasing microheterogeneity in particle transport

Current biochemical characterizations of cystic fibrosis (CF) sputum do not address the high degree of microheterogeneity in the rheological properties of the mucosal matrix and only provide bulk-average particle diffusion coefficients. The viscoelasticity of CF sputum greatly reduces the diffusion rates of colloidal particles, limiting the effectiveness of gene delivery to underlying lung cells. We determine diffusion coefficients of hundreds of individual amine-modified and carboxylated polystyrene particles (diameter 100-500 nm) embedded in human CF sputum with 5 nm and 33 ms of spatiotemporal resolution. High resolution multiple particle tracking is used to calculate the effective viscoelastic properties of CF sputum at the micron scale, which we relate to its macroscopic viscoelasticity. CF sputum microviscosity, as probed by 100- and 200-nm particles, is an order of magnitude lower than its macroviscosity, suggesting that nanoparticles dispersed in CF sputum are transported primarily through lower viscosity pores within a highly elastic matrix. Multiple particle tracking provides a non-destructive, highly sensitive method to quantify the high heterogeneity of the mucus pore network. The mean diffusion coefficient becomes dominated by relatively few but fast-moving particles as particle size is reduced from 500 to 100 nm. Neutrally charged particles with a diameter <200 nm undergo more rapid transport in CF sputum than charged particles. Treatment with recombinant human DNase (Pulmozyme) reduces macroviscoelastic properties of CF sputum by up to 50% and dramatically narrows the distribution of individual particle diffusion rates but surprisingly does not significantly alter the ensemble-average particle diffusion rate.     

The fine structure of the rod-bipolar cell synapse in the retina of the albino rat

The fine structure of the rod-bipolar synapse is described and illustrated. Each rod spherule possesses a large, single, oval or elongate mitochondrion approximately 0.5 x 2.0 microns. Surrounding the mitochondrion are elements of agranular endoplasmic reticulum. The bipolar dendrite projects into the lower pole of the spherule and usually terminates in two lobes separated by a cleft. The plasma membranes appear dense and thicker in the region of the synapse. In the rod spherule cytoplasm, contiguous with the plasma membrane is a dense, slightly concave arciform structure, the rod arciform density, extending from the base of the bipolar bifid process through the cleft to an equivalent point on the opposite side. Also within the spherule, and external (towards the sclera) to the rod arciform density, is a parallel, dense, thin lamella, the rod synaptic lamella. This is approximately 25 mmicro in thickness and 400 mmicro in width at its widest extent. This halfmoon-shaped plate straddles the cleft between the two lobes of the bipolar process. The lamella appears to consist of short regular rodlets or cylinders 5 to 7 mmicro in diameter, oriented with their long axes perpendicular to the plane of the lamella. Minute cytoplasmic vesicles found in the cytoplasm of both the rod spherule and the bipolar terminal are most abundant near the rod synaptic lamella.     

Structure of the filamentous phage pIV multimer by cryo-electron microscopy

The homo-multimeric pIV protein constitutes a channel required for the assembly and export of filamentous phage across the outer membrane of Escherichia coli. We present a 22 A-resolution three-dimensional reconstruction of detergent-solubilized pIV by cryo-electron microscopy associated with image analysis. The structure reveals a barrel-like complex, 13.5 nm in diameter and 24 nm in length, with D14 point-group symmetry, consisting of a dimer of unit multimers. Side views of each unit multimer exhibit three cylindrical domains named the N-ring, the M-ring and the C-ring. Gold labeling of pIV engineered to contain a single cysteine residue near the N or C terminus unambiguously identified the N-terminal region as the N-ring, and the C-terminal region was inferred to make up the C-ring. A large pore, ranging in inner diameter from 6.0 nm to 8.8 nm, runs through the middle of the multimer, but a central domain, the pore gate, blocks it. Moreover, the pore diameter at the N-ring is smaller than the phage particle. We therefore propose that the pIV multimer undergoes a large conformational change during phage transport, with reorganization of the central domain to open the pore, and widening at the N-ring in order to accommodate the 6.5 nm diameter phage particle.     

MOVEMENT OF EELWORMS.: IV. THE INFLUENCE OF WATER PERCOLATION

A study was made of the relationship between rate of flow of water through sands of different particle size and the downward velocity of eelworms of various lengths and activities. The relationship between rate of flow of water and velocity of eelworms is not linear. For rates of flow less than about 500 cm./hr. the velocity of the eelworms relative to the velocity of water is greater than for flow rates greater than about 500 cm./hr. For rates of flow greater than 500 cm./hr. an equation is derived which relates velocity of eelworms, rate of water flow, length of eelworms and pore diameter in the sand. The following facts were established: the velocity of the eelworms increases as pore diameter increases; the velocity approaches zero when the eelworms' length exceeds four times the pore diameter; the relationship between velocity of eelworm and velocity of water is independent of the eelworms' own activity; the equation of eelworm movement is valid for both continuous and discontinuous saturated flow. With flow rates less than about 500 cm./hr., the velocity of the eelworms increases as the eelworms' activity increases and eelworms appear to pass through smaller pores than at high flow rates. Dead or inactive eelworms do not progress very far in sand even at high flow rates and observations suggest that slight flexing movements of the eelworm body is essential for passage.     

Permeability of tarsal sensilla in the tick Amblyomma americanum L. (Acarina, Ixodidae)

Ticks were submerged in silver-protein solution, prior to fixation for electron microscopy, in order to trace the pathway of molecules in supposed tarsal chemoreceptors. Sensilla with radially arranged cuticular canals (100-200 A in diameter) leading to the centrally located dendrites show silver granules inside the canals and in the central lumen, thus directly making contact with the dendrites. Sensilla with large, plugged pores (1200 A) exhibit an accumulation of silver granules in the pore openings but no granules (about 50 A in diameter) were observed penetrating into the lumen. Apparently silver granules could diffuse in, but not through the material which suspends the pore plugs. It is suggested that this material corresponds to the 'pore tubules' in insect olfactory sensilla and that it may play an essential role in transmitting a chemical stimulus from the environment to the dendrites.     

Molecular simulation of size-selective gas adsorption in idealised carbon nanotubes

Molecular simulations were used to examine the adsorption of diatomic molecules (nitrogen and oxygen) and similarly sized gases (argon and methane) in pores with van der Waals diameters similar in size to the gas diameters. Idealised carbon nanotubes were used to model generic pores, to better understand the effect of pore diameter on guest adsorption in the absence of defects, specific adsorption sites, or variations in pore diameter that often complicate studies of gas adsorption in other porous materials. Molecular dynamics simulations of open nanotubes show that argon and methane are able to enter tubes whose diameters are slightly smaller than the gas diameters. Diatomic gases are able to enter tubes that are significantly smaller than their kinetic diameters with the molecular axis aligned parallel to the nanotube. The results indicate that size-selective adsorption of these gases is theoretically possible, although differences in pore diameters of only a few tenths of an Angstrom are required. Grand canonical Monte Carlo simulations of a 3.38 Å nanotube indicate significant uptake by argon and oxygen, but not nitrogen or methane. The adsorption of nitrogen and methane gradually increases as the nanotube diameter approaches 4.07 Å, and all gases fully saturate a 4.54 Å nanotube. Of the nanotubes studied, the largest adsorption enthalpy for any gas corresponds to the 4.54 Å nanotube, with significantly lower enthalpies seen in the 5.07 Å nanotube. These results suggest an ideal pore diameter for each gas based on the gas–pore van der Waals interaction energies. Trends in the ideal diameter correlate with the minimum tube diameter accessible to each gas.     

Habitat-mediated cannibalism and microhabitat restriction in the stream invertebrate <Emphasis Type="Italic">Gammarus pulex</Emphasis>

In cannibalistic species, small individuals often shift habitats to minimize risk of predation by larger conspecifics. The availability of diverse size-structured habitats may mediate the incidence of cannibalism by larger individuals on smaller individuals and increase fitness of smaller individuals. We tested these hypotheses in a series of laboratory studies with Gammarus pulex, a freshwater amphipod inhabiting substrates with varying interstitial pore space sizes. In the absence of larger, potentially cannibalistic individuals, small Gammarus actively used all pore space sizes offered. They used only substrates containing food and preferred food items that provided cover to food items that did not. In the presence of larger G. pulex, small individuals almost exclusively used smaller pore spaces from which larger individuals were excluded. Small individual survival was significantly lower in the presence of larger Gammarus than in controls without larger individuals regardless of substrate size, but availability of mixed pore sizes significantly increased survival. Food consumption and growth per individual were not affected by the presence of larger individuals or substrate composition. Our results suggest that the distribution and availability of complex and high-quality habitats may affect the occurrence and significance of cannibalism in size-structured populations. Handling editor: K. Martens     

Bacterial chromosome segregation: evidence for DNA gyrase involvement in decatenation

Nucleoids isolated from a temperature-sensitive gyrB mutant of E. coli, incubated at restrictive temperatures, exhibit increased sedimentation rates and an abnormal doublet or dumbbell-shaped morphology. Shifting cells from restrictive to permissive temperature prior to nucleoid isolation leads to decreases in the percentage of doublet nucleoids and in nucleoid sedimentation rates. When nucleoids isolated from mutant cells exposed to restrictive temperature are incubated with purified gyrase, the percentage of doublet nucleoids decreases as the total number of nucleoids increases. These results, together with the demonstrated ability of gyrase to decatenate small circular DNA molecules in vitro, suggest that gyrase participates in bacterial chromosome segregation through its decatenating activity.