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排序方式: 共有102条查询结果,搜索用时 15 毫秒
51.
The broad-host-range plasmid RK2 has been a model for studying DNA metabolism in bacteria for many years. It is used as a vector allowing genetic manipulations in numerous bacterial species. The RK2 genome encodes several genes providing the plasmid with diverse functions allowing for its stable maintenance in a variety of bacterial hosts. This review will focus on two processes indispensable for plasmid DNA maintenance. We will summarize recent understanding of the molecular mechanisms contributing to the RK2 DNA replication and partitioning. 相似文献
52.
Statistical models concerning partitioning of pectinase in polyethylene glycol 1000/Na2SO4 aqueous two-phase system were established with response surface methodology. Concentrations of polyethylene glycol 1000 and
Na2SO4 were selected as independent variables to evaluate their impact on parameters of partitioning in aqueous two-phase system—the
partition coefficient of pectinase, purification factor and pectinase yield. An experimental space where over 2.5-fold purification
was achieved, followed by over 90% yield of pectinase. The established models showed good prediction of partitioning parameters. 相似文献
53.
Ulf Sivars Karin Bergfeldt Lennart Piculell Folke Tjerneld 《Journal of chromatography. B, Analytical technologies in the biomedical and life sciences》1996,680(1-2):43-53
The study includes partitioning of proteins in aqueous two-phase systems consisting of the polymer dextran and the non-ionic surfactant C12E5 (pentaethylene glycol mono-n-dodecyl ether). In this system a micelle-enriched phase is in equilibrium with a polymer-enriched phase. Charges can be introduced into the micelles by the addition of charged surfactants. The charge of the mixed micelles is easily varied in sign and magnitude independently of pH, by the addition of different amounts of negatively charged surfactant, sodium dodecyl sulphate (SDS), or positively charged surfactant dodecyl trimethyl ammonium chloride (DoTAC). A series of water-soluble model proteins (BSA, β-lactoglobulin, myoglobin, cytochrome c and lysozyme), with different net charges at pH 7.1, have been partitioned in non-charged systems and in systems with charged mixed micelles or charged polymer (dextran sulphate). It is shown that partition coefficients for charged proteins in dextran-C12E5 systems can be strongly affected by addition of charged surfactants (SDS, DoTAC) or polymer (dextran sulphate) and that the effects are directly correlated to protein net charge. 相似文献
54.
Aggarwal V Kulothungan SR Balamurali MM Saranya SR Varadarajan R Ainavarapu SR 《The Journal of biological chemistry》2011,286(32):28056-28065
Protein folding and unfolding are complex phenomena, and it is accepted that multidomain proteins generally follow multiple pathways. Maltose-binding protein (MBP) is a large (a two-domain, 370-amino acid residue) bacterial periplasmic protein involved in maltose uptake. Despite the large size, it has been shown to exhibit an apparent two-state equilibrium unfolding in bulk experiments. Single-molecule studies can uncover rare events that are masked by averaging in bulk studies. Here, we use single-molecule force spectroscopy to study the mechanical unfolding pathways of MBP and its precursor protein (preMBP) in the presence and absence of ligands. Our results show that MBP exhibits kinetic partitioning on mechanical stretching and unfolds via two parallel pathways: one of them involves a mechanically stable intermediate (path I) whereas the other is devoid of it (path II). The apoMBP unfolds via path I in 62% of the mechanical unfolding events, and the remaining 38% follow path II. In the case of maltose-bound MBP, the protein unfolds via the intermediate in 79% of the cases, the remaining 21% via path II. Similarly, on binding to maltotriose, a ligand whose binding strength with the polyprotein is similar to that of maltose, the occurrence of the intermediate is comparable (82% via path I) with that of maltose. The precursor protein preMBP also shows a similar behavior upon mechanical unfolding. The percentages of molecules unfolding via path I are 53% in the apo form and 68% and 72% upon binding to maltose and maltotriose, respectively, for preMBP. These observations demonstrate that ligand binding can modulate the mechanical unfolding pathways of proteins by a kinetic partitioning mechanism. This could be a general mechanism in the unfolding of other large two-domain ligand-binding proteins of the bacterial periplasmic space. 相似文献
55.
Simon G. Walker Christopher J. Dale Andrew Lyddiatt 《Journal of chromatography. B, Analytical technologies in the biomedical and life sciences》1996,680(1-2):91-96
This study describes the application of aqueous two-phase partition using polyethylene glycol (PEG)-potassium phosphate systems for the direct recovery of proteins, and aggregates thereof, from mammalian brain tissue homogenates. Investigation of established methodologies for the purification of prion proteins (PrP) from bovine brain affected with transmissible spongiform encephalopathy (BSE) has identified an alternative purification regime based on aqueous two-phase partition. This circumvents energy-intensive and rate-limiting unit operations of ultracentrifugation conventionally used for isolation of PrP. Selectivity of various PEG-phosphate systems varied inversely with polymer molecular mass. The maximum protein recovery from bovine brain extracts was obtained with systems containing PEG 300. Manipulation of the aqueous environment, to back-extract protein product from the PEG-rich top phase into the phosphate-rich lower phase, enabled integration of ATPS with conventional hydrophobic interaction chromatography (HIC) which selectively removes obdurate contaminating proteins (i.e. ferritin). 相似文献
56.
Grasa P Martí JI Muiño-Blanco T Cebrián-Pérez JA 《Journal of chromatography. B, Analytical technologies in the biomedical and life sciences》2003,795(1):83-91
The surface of spermatozoa plays a critical role in many stages involved in fertilisation. The plasma membrane undergoes important alterations in the male and female reproductive tract, which result in the ability of spermatozoa to fertilise eggs. One of these membrane modifications is sperm capacitation, a process by which sperm interacts with the zona pellucida receptors leading to the acrosome reaction. It has been proposed that the freezing process induces capacitation-like changes to spermatozoa, and that this premature capacitation could explain the reduction in longevity and fertilising capacity of cryopreserved mammalian spermatozoa. Our research focused on the relationship between membrane alterations occurring throughout freezing-thawing and the processes of capacitation and acrosome reaction. We used centrifugal countercurrent distribution (CCCD) analysis to compare the partition behaviour of ram spermatozoa that was either subjected to cold-shock or frozen-thawed with capacitated and acrosome reacted samples. In addition, the effect of the induced acrosome reaction on membrane integrity of ram spermatozoa was studied using biochemical markers and electron microscopy scanning. The CCCD analysis revealed important similarities between the surface characteristics of capacitated and cold-shocked sperm as well as between acrosome-reacted and frozen-thawed sperm. Cold-shocked and capacitated sperm showed an increased cell affinity for the lower dextran-rich phase as well as a decreased heterogeneity. Likewise, the induction of the acrosome reaction resulted in a loss of viability and an important decrease in cell surface heterogeneity compared to the untreated-control sample. Similar surface changes were found when semen samples were frozen with either Fiser or milk-yolk extender. These results confirm those obtained for membrane integrity by fluorescence markers. Thus, the high cell viability value found in the control sample (74.5%) was greatly decreased after cold-shock (22.2%), cryopreservation (26.38% Fiser medium, 24.8% milk-yolk medium) and acrosome reaction (6.6%), although it was preserved after inducing capacitation (46.7%). The study using electron microscopy scanning revealed dramatic structural alterations provoked by the induction of the acrosome reaction. 相似文献
57.
E. L. Anderson 《Plant and Soil》1986,95(2):293-296
Summary Root growth and morphology were compared between seven week old maize plants grown in the greenhouse and in the field. The
plants were similar in shoot dry weight and the partioning of N and dry matter to roots were similar except for the field
grown plants in 1983. Field grown plants had greater root mass per length and greater calculated diameter than greenhouse
plants. Nitrogen fertilization decreased N and dry matter partitioning to the root system in all three environments. 相似文献
58.
Photosynthetic pathway and ontogeny affect water relations and the impact of CO2 on Bouteloua gracilis (C4) and Pascopyrum smithii (C3) 总被引:2,自引:0,他引:2
The eastern Colorado shortgrass steppe is dominated by the C4 grass, Bouteloua gracilis, but contains a mixture of C3 grasses as well, including Pascopyrum smithii. Although the ecology of this region has been extensively studied, there is little information on how increasing atmospheric
CO2 will affect it. This growth chamber study investigated gas exchange, water relations, growth, and biomass and carbohydrate
partitioning in B. gracilis and P. smithii grown under present ambient and elevated CO2 concentrations of 350 μl l−1and 700 μl l−1, respectively, and two deficit irrigation regimes. The experiment was conducted in soil-packed columns planted to either
species over a 2-month period under summer-like conditions and with no fertilizer additions. Our objective was to better understand
how these species and the functional groups they represent will respond in future CO2-enriched environments. Leaf CO2 assimilation (A
n), transpiration use efficiency (TUE, or A
n/transpiration), plant growth, and whole-plant water use efficiency (WUE, or plant biomass production/water evapotranspired)
of both species were greater at elevated CO2, although responses were more pronounced for P. smithii. Elevated CO2 enhanced photosynthesis, TUE, and growth in both species through higher soil water content (SWC) and leaf water potentials
(Ψ) and stimulation of photosynthesis. Consumptive water use was greater and TUE less for P. smithii than B. gracilis during early growth when soil water was more available. Declining SWC with time was associated with a steadily increased
sequestering of total non-structural carbohydrates (TNCs), storage carbohydrates (primarily fructans for P. smithii) and biomass in belowground organs of P. smithii, but not B. gracilis. The root:shoot ratio of P. smithii also increased at elevated CO2, while the root:shoot ratio of B. gracilis was unresponsive to CO2. These partitioning responses may be the consequence of different ontogenetic strategies of a cool-season and warm-season
grass entering a warm, dry summer period; the cool-season P. smithii responds by sequestering TNCs belowground in preparation for summer dormancy, while resource partitioning of the warm-season
B. gracilis remains unaltered. One consequence of greater partitioning of resources into P. smithii belowground organs in the present study was maintenance of higher Ψ and A
n rates. This, along with differences in photosynthetic pathway, may have accounted for the greater responsiveness of P. smithii to CO2 enrichment compared to B. gracilis.
Received: 21 July 1997 / Accepted: 16 December 1997 相似文献
59.
Gte Johansson 《Journal of chromatography. B, Analytical technologies in the biomedical and life sciences》1996,680(1-2):123-130
New aqueous liquid-liquid two-phase systems based on bovine serum albumin and sodium thiocyanate in combination with either poly(vinyl alcohol) or poly(ethylene glycol) were investigated. Phase diagrams are presented. Lactate dehydrogenase and some mitochondrial enzymes were partitioned in the systems. All the phase components used influenced, either positively or negatively, the activity of lactate dehydrogenase. The enzymes showed a strong preference for the albumin phase. Possible scientific and biotechnological uses are discussed. 相似文献
60.
Alkali metal salts of tetraphenylboron dissociate in aqueous solution to yield the hydrophobic anion, BPh4?, which is strongly adsorbed at the surfaces of lipid bilayer membranes. Upon application of a transmembrane voltage pulse these anions cross the membrane without appreciable desorption, thereby exhibiting a transient electrical conductance. The relaxation time of this transient is governed by the height of the central potential barrier which the anions must surmount in crossing the membrane. Because of discrete charge effects, the barrier height and hence the observed relaxation time increase markedly with increasing surface density of adsorbed BPh4?. Since adsorbed BPh4? are in partition equilibrium with the same species dissolved in the aqueous phase, measurement of the relaxation time for BPh4? membrane conductance can be used to assay the aqueous-phase concentration of the hydrophobic anion. In this way we have observed the precipitation of KBPh4 in water, obtaining a solubility concentration product of 1.0·10?7 mol2·dm?6 for the precipitation reaction at 25°C. This result is larger by a factor of two than the most directly comparable published values from other sources. In additional experiments we have reduced the polarity of the aqueous phases bathing the membrane by adding varying amount of ethylene glycol to the water. Using the same conductance relaxation assay, we have determined that partitioning of BPh4? into the membrane/solution interfaces is lessened as the polarity of the bathing solutions is reduced. This result is attributed to a lowering of the chemical potential of the BPh4? in the less polar medium. 相似文献