177,785 research outputs found
Oxygen control of nif gene expression in Klebsiella pneumoniae depends on NifL reduction at the cytoplasmic membrane by electrons derived from the reduced quinone pool
No full textIn Klebsiella pneumoniae , the flavoprotein, NifL regulates NifA mediated transcriptional activation of the N-2-fixation (nif) genes in response to molecular O-2 and ammonium. We investigated the influence of membrane-bound oxidoreductases on nif -regulation by biochemical analysis of purified NifL and by monitoring NifA-mediated expression of nifH'-'lacZ reporter fusions in different mutant backgrounds. NifL-bound FAD-cofactor was reduced by NADH only in the presence of a redox-mediator or inside-out vesicles derived from anaerobically grown K. pneumoniae cells, indicating that in vivo NifL is reduced by electrons derived from membrane-bound oxidoreductases of the anaerobic respiratory chain. This mechanism is further supported by three lines of evidence: First, K. pneumoniae strains carrying null mutations of fdnG or nuoCD showed significantly reduced nif- induction under derepressing conditions, indicating that NifL inhibition of NifA was not relieved in the absence of formate dehydrogenase-N or NADH:ubiquinone oxidoreductase. The same effect was observed in a heterologous Escherichia coli system carrying a ndh null allele (coding for NADH dehydrogenaseII). Second, studying nif -induction in K. pneumoniae revealed that during anaerobic growth in glycerol, under nitrogen-limitation, the presence of the terminal electron acceptor nitrate resulted in a significant decrease of nif-induction. The final line of evidence is that reduced quinone derivatives, dimethylnaphthoquinol and menadiol, are able to transfer electrons to the FAD-moiety of purified NifL. On the basis of these data, we postulate that under anaerobic and nitrogen-limited conditions, NifL inhibition of NifA activity is relieved by reduction of the FAD-cofactor by electrons derived from the reduced quinone pool, generated by anaerobic respiration, that favours membrane association of NifL. We further hypothesize that the quinol/quinone ratio is important for providing the signal to NifL
Pharmacological studies on the role of reactive oxygen species in IgE-dependent histamine secretion from human basophils
No full textFnr is required for NifL-dependent oxygen control of nif gene expression in Klebsiella pneumoniae
No full textIn Klebsiella pneumoniae, NifA-dependent transcription of nitrogen fixation (nif) genes is inhibited by NifL, in response to molecular oxygen and combined nitrogen. We recently showed that K. pneumoniae NifL is a flavoprotein, which apparently senses oxygen through a redox-sensitive, conformational change. We have now studied the oxygen regulation of NifL activity in Escherichia coli and K. pneumoniae strains by monitoring its inhibition of NifA-mediated expression of K. pneumoniae phi (nifH'-'lacZ) fusions in different genetic backgrounds. Strains of both organisms carrying fnr null mutations failed to release NifL, inhibition of NifA transcriptional activity under oxygen limitation: nif induction was similar to the induction under aerobic conditions. When the transcriptional regulator Fnr was synthesized from a plasmid, it was able to complement, i.e., to relieve NifL inhibition in the fnr mutant backgrounds. Hence, Fnr appears to be involved, directly or indirectly, in NifL-dependent oxygen regulation of nif gene expression in K. pneumoniae. The data indicate that in the absence of Fnr, NifL, apparently does not receive the signal for anaerobiosis. We therefore hypothesize that in the absence of oxygen, Fnr, as the primary oxygen sensor, activates transcription of a gene or genes whose product or products function to relieve NifL, inhibition by reducing the flavin adenine dinucleotide cofactor under oxygen-limiting conditions
Cloning, sequencing and characterization of Fnr from Klebsiella pneumoniae
No full textThe transcription factor Fnr (fumarate nitrate reductase regulator) globally regulates gene expression in response to oxygen deprivation in Escherichia coli. We report here the cloning and sequencing of the fnr gene from the facultative anaerobic bacterium Klebsiella pneumoniae M5al, another member of the enteric bacteria. The deduced amino acid sequence of K. pneumoniae fnr showed very high similarity (98% amino acid identity) to the Fnr protein from E. coli and contained the four essential cysteine residues which are presumed to build the oxygen-sensing [4Fe4S](+2) center. Transfer of the K. pneumoniae gene to a fnr mutant of E. coli complemented the mutation and permitted synthesis of nitrate reductase and fumarate reductase during anaerobic growth. A gene fusion between K. pneumoniae fnr and glutathione S-transferase was constructed and expressed in E. coli under anaerobic conditions in order to make the protein available in preparative amounts. The overproduced protein was purified by glutathione-Sepharose 4B affinity chromatography in the absence of oxygen, and biochemically characterized
Appropriate Similarity Measures for Author Cocitation Analysis
Get PDFWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Contrast-enhanced MR angiography of the arterial and portovenous system of the liver with varying concentrations of contrast medium
No full textPurpose: To evaluate the diagnostic value of a higher concentrated contrast medium (gadobutrol) for contrast-enhanced MRA of the hepatic arteries and portovenous system. Materials and Methods: The examinations were performed on a 1.5-Tesla whole body imaging system (Magnetom Symphony Quantum, Siemens) with a 30-mTesla/m gradient field strength using a phased-array body coil. A 3D FLASH sequence (TR/TE/FA 3.88 ms/1.44 ms/25degrees) was used imaging the hepatic arteries and portovenous system after determination of the circulation time. The study included 50 patients, with 25 patients (group 1) injected with 0.2 mmol Gd-GTPA/kg body weight and 25 patients (group 2) injected with 0.1 mmol gadobutrol/kg body weight. The Signal-to-noise ratio (SNR) and the contrast-to-noise ratio (CNR) were calculated for both groups. The image quality was graded by three radiologists on a 5-point scale. Results: The highest SNR was measured in group 1, with no statistically significant differences of the SNR in the abdominal aortal, coeliac trunk and common hepatic artery. CNR was also similar in both groups. Likewise, portal, superior mesenteric and splenic veins showed no statistically significant differences. All cases were found to have a good image quality. Conclusion: For MRA of the hepatic arteries and the portal veins, the higher concentrated Gd-DTPA contrast medium gadobutrol can be used at half the dosage recommended for the standard Gd-DTPA contrast medium
Characterization of GlnK(1) from Methanosarcina mazei strain Go1: Complementation of an Escherichia coli gInK mutant strain by GInK(1) (vol 184, pg 1040, 2002)
No full textValue of F-18FDG hybrid camera PET and MRI in early takayasu aortitis
No full textTakayasu aortitis (TA) is a chronic inflammatory and fibrotic vasculitis of large- and medium-sized arteries. Early stages of the disease show a panarteritis and inflammatory wall thickening of the aorta and its branches, whereas advanced (fibrotic) stages comprise stenosis, aneurismatic transformation and occlusion. Magnetic resonance imaging visualises early-stage disease with high accuracy and is considered to be the method of choice in the diagnosis of TA. The aim of this article is the detailed comparison of FDG-PET performed with a hybrid camera and MR imaging in five patients with early TA. Five patients (median age 60 years) were enrolled during an ongoing prospective study on [18F]2'-deoxy-2-fluoro-D-glucose (FDG) hybrid camera PET in patients with fever of unknown origin (FUO). These patients underwent MR imaging after establishing the diagnosis of TA. Abnormal FDG uptake in the wall of the aorta was noted in all patients. The bracheocephalic artery and the common carotid arteries were visualized in 3 cases. Increased uptake of the subclavian artery was found in 3 patients and in 4 patients pathological uptake was noted in the ilio-femoral vessels. Of 34 vascular regions studied, 26 (76%) showed elevated FDG uptake. On transversal MR images vessel wall thickening and contrast enhancement of the thoracic aorta was found in 4 patients (ascending aorta/ aortic arch: n=2; descending aorta: n=3; abdominal aorta: n=1). Additionally, vessel wall pathologies of the subclavian and the common carotid arteries could be shown in 1 patient and in another patient in the ilio-femoral arteries. No abnormalities were found using contrast-enhanced MR angiography. Of 28 vascular regions studied, 9 (32%) showed vasculitis on MRI. The FDG-PET is a suitable whole-body screening method in the primary diagnosis of early TA, especially in those cases with early disease that present with uncharacteristic symptoms such as FUO. Both MRI and MRA remain indispensable in the exact determination of the pathomorphological changes and in the documentation of complications such as stenosis, aneurismatic transformation and occlusion
Experimental evaluation of vessel diameter from 0.3 to 8 mm in CE MR angiography.
No full textPurpose: To evaluate the detection rate of vascular stenosis in contrast-enhanced 3D MR angiography using a flow phantom. Material and methods: The examinations were performed on a 1.5 T whole body imaging system (Magnetom Symphony/Quantum) with 30 mT/m gradient field strength using a body-phased-array coil. Different 3D sequences (TR/TE/FA < 5 ms/< 2 ms/25 degrees) with slice thicknesses ranging from 0.67 to 1.25 mm were applied. A gelantine-filled plastic cylinder with PVC tubes of 8 mm diameter was used as a vascular phantom. The tubes had concentric and excentric stenoses (50-90%) of different lengths. For the detection of different vessel diameters another phantom with 0.3 - 8 mm silicon tubes was used. Both systems were flushed with a solution of Gd-DTPA (0.15 mmol/l) and saline at flow rates from 50 to 200 cm/s. The phantoms were positioned 0 degrees, 45 degrees, and 90 degrees towards the z-axis. Results: The degree of stenosis was under- and overestimated in less than 10 %. The sequence with the highest spatial resolution provided the best results. Detection and evaluation of tubes greater than or equal to 2 mm proved to be reliable. Conclusion: Contrast-enhanced 3D MR angiography provides an almost exact evaluation of the degree of stenosis in the phantom study. Evaluation of vessel diameters < 2 mm is not possible
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