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Summary The stability and gene expression of plasmid pBR322 in a chemostat with complex non-selective medium at different dilution rates were studied. It was observed that pBR322 was eventually lost from the population after a long lag period. The rate of plasmid loss decreases with decreasing dilution rate. This result is different from those obtained with cells grown in defined medium, where plasmid loss was observed to decrease with increasing dilution rate. In addition, it was observed that the -lactamase activity per ml per optical density of cell culture, independent of the dilution rates, increases with time and reaches a maximum around 4.5 units after 100 hrs of continuous culture. 相似文献
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Dynamics of plasmid maintenance in a CSTR upon square-wave perturbations in the dilution rate 总被引:1,自引:0,他引:1
The effects of forced oscillations in the dilution rate on a population of Escherichia coli K12 harboring the plasmid pBR322 in a chemostat with a nonselective medium were studied. In the constant dilution rate control experiments, the percentage of plasmid-containing cells decreased after a long lag time. Eventually, the culture approached a population consisting of 100% plasmid-free ells. However, under forced perturbations of the dilution rate, the culture maintained a mixed population of plasmid-free and plasmid-carrying cells for a longer period of ime. An unstructured model was developed to describe the above observations. Our results indicate that transient conditions created by dilution-rate perturbations provide a favorable environment for the plasmid-carrying population. In addition, experiments with different cycling frequencies suggest that adaptation by the culture to these transient conditions will reduce or totally eliminate such an advantage. 相似文献
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I De Dios J J Calvo J I San Roman M A Plaza M A Lopez 《Archives internationales de physiologie et de biochimie》1989,97(1):37-43
The effect of Dobutamine (a beta 1-adrenergic agonist) and Terbutaline (a beta 2-adrenergic agonist) on exocrine pancreatic secretion was studied in anaesthetized rabbits, simultaneously controlling pancreatic blood flow and blood pressure. The secretion of fluid and ions (bicarbonate, sodium and potassium) was unaffected by the infusion of Dobutamine (8 micrograms.kg-1.min-1) or Terbutaline (10 micrograms.kg-1.min-1). Neither were pancreatic blood flow or mean blood pressure altered. Dobutamine or Terbutaline depress the function of the acinar cells, amylase secretion being more affected by the action of Terbutaline. The results show that beta 1 and beta 2-adrenergic stimulation has no effect on the ductular cells but does decrease the secretion by the acinar cells. 相似文献
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A number of proteases have been immobilized on alumina in a two-step procedure: the first step converted them into semisynthetic phosphoproteins which, in the second step, spontaneously bonded to alumina through their phosphate function. The immobilized enzymes thus obtained showed the physical properties typical of the inorganic carrier and a high activity on low molecular weight substrates. 相似文献
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Patulin and griseofulvin production by twelve strains ofPenicillium griseofulvum Dierckx, eleven of which were isolated from pistachio (Pistacia vera) nuts and the other was supplied by the Spanish Collection of Type Culture, was investigated. Six strains of the eleven isolated had ability to produce patulin and griseofulvin in Yes medium. All the strains studied had no ability to produce patulin in Wickerham medium. Griseofulvin production was significant in both media but higher in Wickerham. These metabolites were separated and determined in the chloroform extracts of cultures by high performance liquid chromatography with ultraviolet detection. The best conditions were: acetonitrile — water (45∶55) as mobile phase, a flow rate of 2.0 mL/min and a μ Bondapack C18 column. 相似文献
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CO2 exchange between the atmosphere and soil algal crusts of the Trachypogon savannas of the Orinoco Llanos has been analyzed
using an open gas exchange system. These savannas encompass a wide range of physiognomic types, from herbaceous communities
to savanna woodlands. A maximum CO2 flux of 0.207 mg m-2 s-1 was measured in the crusts of the Guanipa savannas, while in the other examined crusts (0.035–0.105 mg m-2 s-1) the flux was similar to values reported for terrestrial algae. The CO2 flux data were statistically fitted to the photosynthetically active radiation by a logarithmic relationship, and the photosynthetic
efficiencies of the crusts were compared. The activation energy calculated for the CO2 fixation indicates that limitations by diffusion and photochemical processes were excluded in the Guanipa crusts (above 12
kcal mole-1), whereas they were evident in the other crust studied. An optimum CO2 incorporation as a function of the crust water potential was established and carbon gain strategies were proposed on the
basis of the results and characteristics of the habitats. 相似文献