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At least 919 records · Page 51Linked to original sources
PubMed1997

Ammonium repression of the nitrite-nitrate (nasAB) assimilatory operon of Azotobacter vinelandii is enhanced in mutants expressing the nifO gene at high levels.

A number of Tn5 mutants were isolated which were unable to fix nitrogen and showed enhanced ammonium repression of the nitrate/nitrite assimilation genes. They also had reduced nitrate reductase activity under fully inducing conditions. Insertions were localized within the nifB gene, and inability to fix nitrogen was shown to be due to disruption of the nifB gene. However, enhanced ammonium repression proved to be the result of constitutive expression of the downstream nifO gene from an 'out' promoter present in Tn5. Our results suggest that molybdenum metabolism might function as a regulatory factor that acts through the nitrate reductase.

Azotobacter vinelandii↗
PubMed1983

A rapid two-dimensional fractionation of DNA restriction fragments.

Genomic DNA that has been digested with a restriction endonuclease and fractionated by electrophoresis on an agarose gel can be recovered on glass-fiber filters by a new blotting scheme. The DNA fragments in each fraction are then digested with a second restriction nuclease and then separated on a slab gel, resulting in a two-dimensional display of the restriction fragments. This rapid fingerprinting technique is useful in the analysis of complex genomes and in the isolation and cloning of particular sequences.

Base Sequence↗
PubMed1989

A modified method for the production of 36ClO3- for use in plant nitrate transport studies.

A modified method for the production and purification of 36ClO3- in high yield is described. This procedure, involving the electrolytic oxidation of 36Cl- in a cell with simple electrode design and purification of the electrolysis products (36Cl-, 36ClO3-, and 36ClO4-) by aqueous column chromatography, allows for the recovery of about 80% of the initial radiolabel as 36ClO3-. The method is rapid and suitable for the production of this radiolabeled anion for use as a tracer analog for nitrate in plant membrane transport experiments.

Biological Transport↗
PubMed1991

Characterization of a H+/NO3- symport associated with plasma membrane vesicles of maize roots using 36ClO3- as a radiotracer analog.

The mechanism of NO3- transport was examined in isolated plasma membrane vesicles from maize (Zea mays L., hybrid B73 X LH 51) roots using 36ClO3- as a radiotracer analog for NO3-. When an acid-exterior delta pH was imposed across the vesicle membrane, uptake of 36ClO3- was stimulated and the time course of radiolabel uptake displayed an overshoot phenomenon characteristic of the coupling of one solute gradient ot the movement of another solute. Evidence supporting delta pH as the driving force for 36ClO3- uptake included a dependence of the overshoot peak and initial rate of 36ClO3- uptake on the magnitude of the imposed delta pH, the occurrence of delta pH-driven 36ClO3- uptake in the presence of KSCN/valinomycin, and the ability of an imposed delta pH to drive 36ClO3- uptake when radiolabel was equilibrated across the membrane. When delta pH-driven 36ClO3- transport was examined in the presence of NO3-, radiolabel uptake was inhibited in a competitive manner. This was consistent with the carrier having the capacity to use either ClO3- or NO3- and supports the use of this radiotracer as an analog for NO3- in transport studies. When delta pH-driven 36ClO3- uptake was examined as a function of 36ClO3- concentration and delta pH, saturation kinetics were observed and the magnitude of the imposed delta pH affected the Km but not the Vmax for 36ClO3- uptake. This suggested an ordered binding mechanism where 36ClO3- would bind to the protonated form of the carrier prior to translocation. Radiolabeled 36ClO3- uptake was inhibited by treatment of the vesicles with phenylglyoxal, suggesting the involvement of arginine moieties in the process of transport. Taken together, these results support the presence of a H+/NO3- symport carrier at the plasma membrane which could be involved in mediating energy-dependent NO3- uptake into plant cells.

Binding, Competitive↗
PubMed1991

A neutron diffraction study of the influence of ions on phospholipid membrane interactions.

Neutron diffraction is used to examine the effects of Ca2+ and ClO4- ions on interactions and some structural features of dipalmitoylphosphatidylcholine membranes in both solid and fluid lamellar phases. The results are described within the framework of Derjaguin-Landau-Verwey-Overbeek (DLVO) theory with reference to electrostatic, van der Waals, and hydration components of disjoining pressure. The Hamaker constants are evaluated under equilibrium conditions. Addition of 100 mM CaCl2 to the aqueous phase substantially increases the lamellar repeat spacing (d), which is interpreted in terms of adsorption of Ca2+ ions to bilayers followed by electrostatic repulsion between membranes. The rise of NaClO4 concentration in the presence of 100 mM CaCl2 leads to gradual decrease in d, evidently resulted from the diminution of Ca(2+)-induced positive surface potential by both electrostatic screening and binding of ClO4- ions. In the absence of CaCl2, elevation of NaClO4 concentration to 100-300 mM drastically enhances the repeat spacing and then dramatically decreases d at about 1 M NaClO4. Estimation of the hydration coefficients showed that the pronounced decrease of the repeat spacing at high NaClO4 concentrations was resulted mainly from the (partial) disruption of the structure of intermembrane bound water by chaotropic ClO4- ions and subsequent decrease in hydration repulsive pressure. Moreover, in the case of solid membranes (20 degrees C) high concentrations of ClO4- induced formation of interdigitated phase paralleled with marked reduction in bilayer thickness and corresponding increase in the effective cross-sectional area per lipid molecule.

1,2-Dipalmitoylphosphatidylcholine↗
PubMed1991

Effect of encapsulated Ca2+ on the surface properties of curved phosphatidylcholine bilayers.

In this paper, the influence of Ca2+ on the adsorption properties of 1,8-anilinonaphthalenesulfonate (ANS) and analogous probes to sonicated vesicles of phosphatidylcholine was studied by means of spectrofluorometry. The fluorescence of ANS added to the vesicle dispersion increases with the Ca2+ concentration in the inner media but remains constant when Ca2+ concentration is changed in the outside solution. However, the fluorescence decreases when large anions such as ClO4- are present in the external solution. Ca2+ inside large liposomes promotes a similar behaviour to that found in sonicated vesicles when they are osmotically contracted in hypertonic media. The results can be interpreted in terms of Ca2+ adsorption on the inner interface and a cooperative interaction between the monolayers.

Adsorption↗
PubMed1975

Fluorescence polarization studies on the conformational transition of bovine plasma albumin in acidic solutions.

The acid-induced isomerization (the N-F transition) and expansion of bovine plasma albumin were studied by measuring fluorescence polarization and lifetime of the excited state of tryptophyl fluorophors. Most of the changes (decreases) in the reciprocal of fluorescence polarization and lifetime of the excited state correlated exactly with the N-F1 transition and/or the initial part of the N-F transition. These findings suggest that though the N-F transition is the cooperative pH-dependent conformational transition, the N-F transition clearly involves an intermediate step, such as the N-F1 and F1-F2 transitions. Rotational relaxation times for the N- and F-forms obtained by Perrin plot of tryptophyl fluorescence polarization were approximately 75 and 120-180 ns, respectively. The unexpected short rotational relaxation time of 75 ns of the N-form might be due to the rotational freedom of the tryptophyl side chain itself and/or of small flexible loci where tryptophyl fluorophors attach.

Animals↗
PubMed1980

The kinetics of iron release from human transferrin by EDTA. Effect of salts and detergents.

The kinetics of iron release from diferric human transferrin by EDTA at pH 7.4 and 37 degrees C has been studied. Equations have been derived which give the expected absorbance-time curves when the two sites are kinetically homogeneous or heterogeneous and these have been compared to the experimental results. In the absence of added salts or detergents,the two sites are kinetically similar but not identical with respect to iron release. However, the two sites are kinetically homogeneous in 0.02 M laurylpyridinium chloride and 2 M LiCl. In NaClO4 (greater than 0.50 M) there are two parallel pathways for iron release; 80% of the total iron being released heterogeneously and 20% apparently homogeneously. For the simple electrolyte ions studied, the rate of iron release from diferric-transferrin increases in accordance with the lyotropic series: SO4 2- less than HCO-2 less than Cl- less than ClO4- and Na+ less than Li+. Sodium dodecyl sulphate (0.02 M) and LiCl (2.0 M) increase the rate of ion release by 0.1 M EDTA by 32- and 27-fold, respectively. The results show that conformational states are attainable from which iron can be released either homogeneously or heterogeneously and from which iron release is facilitated. It is proposed that the structure of the transferrin binding site on the cell membrane and/or the presence of another molecule may promote these conformers in vivo.

Bicarbonates↗
PubMed1988

Inhibitors of the sulfation of proteins, glycoproteins, and proteoglycans.

Two categories of compounds, substrates of sulfation and sulfate analogs, were tested for the ability to inhibit sulfation of macromolecules secreted by HepG2 cells. Several compounds which most effectively inhibited sulfation without toxic effects on cells were tested for their relative inhibition of sulfation of tyrosine residues (using the fourth component of complement as a model substrate), of N-linked oligosaccharides (alpha 2HS-glycoprotein as substrate), and of proteoglycans. Inhibitors decreased the sulfation of all three classes of substrate, but not always equally. Use of inhibitors from both categories in combination yielded synergistic effects, with more effective inhibition of sulfation and low toxicity. Such combinations of inhibitors should provide a valuable tool for probing the significance of the sulfation of macromolecules.

Blood Proteins↗
PubMed1988

Sulfation of tyrosine residues does not influence secretion of alpha 2-antiplasmin or C4.

Sulfation of tyrosine residues is a biosynthetic modification of many secretory proteins. The function of this modification is not known, but it has been proposed that tyrosine sulfate residues may act as sorting signals to direct proteins along the secretory pathway. We tested this hypothesis by examining the effect of sulfation inhibitors on the kinetics of secretion of proteins by HepG2 cells. The inhibitors induced no change in the rate of secretion of alpha 2-antiplasmin and C4 (fourth component of complement), both of which contain tyrosine sulfate residues. Sulfation of tyrosine residues does not contribute to secretion of these proteins.

Cell Line↗
PubMed1989

Glycosaminoglycan variants in the C2 muscle cell line.

Using a replica technique, we have isolated and characterized five genetic variants of the C2 mouse muscle cell line that are defective in incorporation of radiolabeled sulfate into glycosaminoglycans (GAGs). The variants incorporate free sulfate into GAGs at 5-20% of wild-type levels. None of the variants is defective in sulfate transport across the cell membrane, and in no case could the deficit in incorporation of sulfate be reversed by addition of an artificial initiator of GAG biosynthesis, p-nitrophenyl beta-D-xyloside. Analysis of the incorporation of [3H]glucosamine into GAGs by the variants revealed three different patterns: one variant incorporated [3H]glucosamine at the wild-type level; one, S27, at a severely reduced level; and three at intermediate levels. Four of the five variants showed marked deficits in their ability to differentiate and fuse. The remaining variant, S27, formed multinucleated myotubes and expressed acetylcholine receptor with a normal time course. Differentiation of the first four variants could not be restored by addition of exogenous GAGs or extracellular matrix. Because of the important roles that GAGs and proteoglycans are thought to play in the differentiation of muscle, these genetic variants should serve as useful tools in functional analyses of these molecules.

Animals↗