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21.
Selective Utilization of Pyrimidine Deoxyribonucleosides for Deoxyribonucleic Acid Synthesis in Pneumococcus 总被引:1,自引:1,他引:0 下载免费PDF全文
When pyrimidine deoxyribonucleosides are supplied to growing cultures of Diplococcus pneumoniae, they are selectively used for incorporation into deoxyribonucleic acid (DNA). Differently labeled molecules of deoxyuridine, thymidine, and deoxycytidine were used to study the precursor pathways of this organism. Each of these preformed pyrimidine deoxynucleosides is incorporated intact (i.e., without cleavage of the glycosidic bond) and is predominantly recoverable as DNA thymidine. During the utilization of deoxycytidine and deoxyuridine by pneumococci, large proportions of the available precursor are converted to free thymidine, which is secreted back into the growth medium. The biochemical pathways for selective incorporation into DNA and the regulation of concentrations of intracellular thymidine compounds by excretion of free thymidine are discussed. 相似文献
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The effects of aminophylline (10-500 microM) on isometric twitch and tetanic forces were studied in vitro on frog semitendinosus muscle. Two hypotheses were tested: 1) that micromolar concentrations of aminophylline enhanced contractility of isolated skeletal muscle and 2) that the potentiating effect of aminophylline was dependent on the presence of extracellular calcium ions. Muscles were removed, placed in aerated Ringer solution at 20 degrees C, attached to a force transducer, and stimulated directly. Muscles in normal Ringer and aminophylline Ringer were compared throughout the frequency-force relationship from twitches to maximum tetanic force. Aminophylline increased twitch force significantly at concentrations as low as 25 microM. Over a range of stimulation frequencies, but especially at 10 and 20 Hz, aminophylline increased tetanic force. The potentiating effect of aminophylline (100 microM) was reduced or eliminated in calcium-free Ringer containing 10 mM magnesium. We conclude that aminophylline, at therapeutic concentrations, enhances muscle contractility, and the enhancement is dependent on the presence of extracellular calcium. These findings support the concept that aminophylline is effective in improving respiration in humans with airway obstruction by enhancing diaphragmatic contractility. 相似文献
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A strain ofLactobacillus plantarum caTC2R isolated from a meat source was resistant to chloramphenicol (30 g/ml). Resistance was mediated through an inducible chloramphenicol acetyltransferase. Plasmid analysis of this strain showed three plasmids, of which the 8.5-kb plasmid apparently encodes the gene for chloramphenicol resistance. This plasmid was lost at high frequency (25%) when theLactobacillus was subcultured at a higher than optimal temperature (40°C). 相似文献
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Barry K Derham J Clive Ellory Anthony J Bron John J Harding 《European journal of biochemistry》2003,270(12):2605-2611
Alpha-crystallin, a molecular chaperone and lens structural protein protects soluble enzymes against heat-induced aggregation and inactivation by a variety of molecules. In this study we investigated the chaperone function of alpha-crystallin in a more physiological system in which alpha-crystallin was incorporated into red cell 'ghosts'. Its ability to protect the intrinsic membrane protein Na/K-ATPase from external stresses was studied. Red cell ghosts were created by lysing the red cells and removing cytoplasmic contents by size-exclusion chromatography. The resulting ghost cells retain Na/K-ATPase activity. alpha-Crystallin was incorporated in the cells on resealing and the activity of Na/K-ATPase assessed by ouabain-sensitive 86Rb uptake. Incubation with fructose, hydrogen peroxide and methylglyoxal (compounds that have been implicated in diabetes and cataract formation) were used to test inactivation of the Na/K pump. Intracellular alpha-crystallin protected against the decrease in ouabain sensitive 86Rb uptake, and therefore against inactivation induced by all external modifiers, in a dose-dependent manner. 相似文献
27.
Thomas Wachtler Christian Wehrhahn Barry B. Lee 《Journal of computational neuroscience》1996,3(1):73-82
We developed a physiologically plausible model of the first steps of spatial visual information processing in the fovea of the human retina. With the predictions of this model we could support the hypothesis that, for moderate contrasts ( 40%), hyperacuity is mediated by the magnocellular (MC-) pathway. Despite the lower sampling density in the MC pathway, as compared to the parvocellular (PC-) pathway, the information that is transferred by the MC ganglion cells is sufficient to achieve thresholds comparable to those of human subjects in psychophysical tasks. This is a result of the much higher signal-to-noise ratio of the MC pathway cell signals. The PC pathway cells do not transfer enough information for hyperacuity thresholds. 相似文献
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Christian Chervaux Nathalie Sauvonnet Annick Le Clainche Brendan Kenny A. Lesley Hunt Jenny K. Broome-Smith I. Barry Holland 《Molecular & general genetics : MGG》1995,249(2):237-245
An in frame gene fusion containing the coding region for mature -lactamase and the 3-end of hylA encoding the haemolysin secretion signal, was constructed under the control of a lac promoter. The resulting 53 kDa hybrid protein was specifically secreted to the external medium in the presence of the haemolysin translocator proteins, HlyB and HlyD. The specific activity of the -lactamase portion of the secreted protein (measured by the hydrolysis of penicillin G), approximately 1 U/g protein, was close to that of authentic, purified TEM--lactamase. This is an important example of a hybrid protein that is enzymatically active, and secreted via the haemolysin pathway. Previous studies have indicated that haemolysin is secreted directly into the medium, bypassing the periplasm, to which -lactamase is normally targeted. This study indicated, therefore, that normal folding of an active -lactamase, can occur, at least when fused to the HlyA C-terminus, without the necessity of entering the periplasm. Despite the secretion of approximately 5 g/ml levels of the active -lactamase fusion into the medium, there was maximally only a 50% detectable increase in the LD50 for resistance to ampicillin at the individual cell level. This result suggests that, normally, resistance to ampicillin requires a high concentration of the enzyme close to killing targets, i.e. in the periplasm, in order to achieve significant levels of protection.These authors made an equal contribution to this work 相似文献
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The nucleotide sequence of a cDNA prepared from poly(A)+ RNA from Lycopersicon esculentum fruit codes for a protein, M
r 20812, with features representative of the protein core of arabinogalactan proteins. The deduced amino acid sequence resembles that of peptides of arabinogalactan proteins isolated from carrot and rose and is most similar to the sequence of tryptic peptides from Lolium multiflorum (Gleeson et al., Biochem J 264 (1989) 857–862). The similar sequences include a number of Ala-Pro repeats, a feature considered distinctive of arabinogalactan proteins. The amino acid composition is similar to that of the peptide core of the Lolium multiflorum arabinogalactan protein; alanine, serine and proline account for 57% of the polypeptide. The mRNA corresponding to the cDNA sequence was detected in roots, leaves and fruit. The levels of mRNA are reduced in older leaves, in fruit that have commenced ripening and in leaves and fruit that have been wounded. 相似文献