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991.
Rospars Jean-Pierre Lánský Petr Tuckwell Henry C. Vermeulen Arthur 《Journal of computational neuroscience》1996,3(1):51-72
The coding of odor intensity by an olfactory receptor neuron model was studied under steady-state stimulation. Our model neuron is an elongated cylinder consisting of the following three components: a sensory dendritic region bearing odorant receptors, a passive region consisting of proximal dendrite and cell body, and an axon. First, analytical solutions are given for the three main physiological responses: (1) odorant-dependent conductance change at the sensory dendrite based on the Michaelis-Menten model, (2) generation and spreading of the receptor potential based on a new solution of the cable equation, and (3) firing frequency based on a Lapicque model. Second, the magnitudes of these responses are analyzed as a function of odorant concentration. Their dependence on chemical, electrical, and geometrical parameters is examined. The only evident gain in magnitude results from the activation-to-conductance conversion. An optimal encoder neuron is presented that suggests that increasing the length of the sensory dendrite beyond about 0.3 space constant does not increase the magnitude of the receptor potential. Third, the sensivities of the responses are examined as functions of (1) the concentration at half-maximum response, (2) the lower and upper concentrations actually discriminated, and (3) the width of the dynamic range. The overall gain in sensitivity results entirely from the conductance-to-voltage conversion. The maximum conductance at the sensory dendrite appears to be the main tuning constant of the neuron because it determines the shift toward low concentrations and the increase in dynamic range. The dynamic range of the model cannot exceed 5.7 log units, for a sensitivity increase at low odor concentration is compensated by a sensitivity decrease at high odor concentration. 相似文献
992.
993.
Gudrun C. Hartmann Elena Santamaria Victor M. Fernández R. K. Thauer 《Journal of biological inorganic chemistry》1996,1(5):446-450
H2–forming N 5,N 10 –methylenetetrahydromethanopterin dehydrogenase is a novel type of hydrogenase that contains neither nickel nor iron-sulfur clusters. Evidence has been presented that the reaction mechanism catalyzed by the enzyme is very similar to that of the formation of carbocations and H2 from alkanes under superacidic conditions. We present here further results in support of this mechanism. It was found that the purified enzyme per se did not catalyze the conversion of para H2 to ortho H2, a reaction catalyzed by all other hydrogenases known to date. However, it catalyzed the conversion in the presence of the substrate N 5,N 10 –methenyltetrahydromethanopterin (CH≡H4MPT+), indicating that for heterolytic cleavage of H2 the enzyme-CH≡H4MPT+ complex is required. In D2O, the formation of HD and D2 from H2 rather than a para–ortho H2 conversion was observed, indicating that after heterolytic cleavage of H2 the dissociation of the proton from the enzyme-substrate complex is fast relative to the re-formation of free H2. 相似文献
994.
Miguel Arroyo José María Sánchez-Montero José Vicente Sinisterra 《Biotechnology Techniques》1996,10(4):263-266
Summary We describe a qualitative method to predict the pre-equilibration aw, system value in which, covalent immobilized lipase B from Candida antarctica to sepharose and silica, displayed best synthetic activity. The methodology is based in the analysis of the water adsorption isotherms of the biocatalyst in air and in the organic solvent. The biocatalyst is active at pre-equilibration aw values higher than the divergence point between both isotherms. In addition, native and immobilized lipase display highest activity if the biocatalyst is pre-equilibrated at aw=P point. For preparative purposes, the validity of the method was proved in the esterification of racemic 2-(4-isobutyl phenyl) propionic acid with 1-propanol in isooctane at long reaction time. 相似文献
995.
996.
R. Pérez-Vicente M. I. Burón J. A. González-Reyes J. Cárdenas M. Pineda 《Protoplasma》1995,186(1-2):93-98
Summary Utilization of xanthine as the sole nitrogen source for growth byChlamydomonas reinhardtii cells involved the formation of a transient, intracellular pool of xanthine. Up to 20% of the total xanthine supplied to the medium was not assimilated after uptake but stored in the cells at concentrations that exceeded xanthine solubility in water. At the subcellular level, a massive accumulation of starch grains in the chloroplast and the appearance of many vacuoles in the cytoplasm distinguished xanthine-grown from ammonium-grown cells. Starch accumulation, but not development of vacuoles, was also observed in N-starved cells. Uptake experiments with radio-labelled xanthine showed that this accumulates only in the cytoplasm, most probably inside vacuoles. The electron-dense material observed in vacuoles of xanthine-grown cells suggests that the intracellular xanthine is in part solid xanthine. 相似文献
997.
Structural evolution of wheat chromosomes 4A, 5A,and 7B and its impact on recombination 总被引:3,自引:3,他引:0
K. M. Devos J. Dubcovsky J. Dvořák C. N. Chinoy M. D. Gale 《TAG. Theoretical and applied genetics. Theoretische und angewandte Genetik》1995,91(2):282-288
The construction of comparative genetic maps of chromosomes 4Am and 5Am of Triticum monococcum and chromosomes of homoeologous groups 4, 5 and 7 of T. aestivum has provided insight into the evolution of these chromosomes. The structures of chromosomes 4A, 5A and 7B of modern-day hexaploid bread wheat can be explained by a 4AL/5AL translocation that occurred at the diploid level and is present both in T. monococcum and T. aestivum. Three further rearrangements, a 4AL/7BS translocation, a pericentric inversion and a paracentric inversion, have taken place in the tetraploid progenitor of hexaploid wheat. These structural rearrangements and the evolution of chromosomes 4A, 5A and 7B of bread wheat are discussed. The presence of the 4AL/5AL translocation in several Triticeae genomes raises two questions — which state is the more primitive, and is the translocation of mono- or poly-phylogenetic origin? The rearrangements that have occurred in chromosome 4A resulted in segments of both arms having different positions relative to the telomere, compared to 4Am and to 4B and 4D. Comparisons of map length in these regions indicate that genetic length is a function of distance from the telomere, with the distal regions showing the highest recombination. 相似文献
998.
Summary Post-mitotic epidermal cells of barley leaves were found to contain, in addition to cortical microtubules (CMTs), distinct arrays of endoplasmic microtubules (EMTs). These encircle nuclei and continuously merge into the CMT arrays that underly the plasmalemma. Detailed three-dimensional reconstruction of both types of MTs during fungal infection showed that profound and very rapid MT rearrangements occurred especially in the case of incompatible (resistant) barley-powdery mildew genotype combination. The most early MT responses, followed by their subsequent complete disintegration, were recorded around nuclei. These events might be relevant for the induction of such nuclear processes as onset of DNA synthesis and nuclear chromatin condensation. Observed pattern of early infection events, as well as less prominent responses in the case of compatible (susceptible) barley-powdery mildew genotype combination, both findings suggest that rapid reorganization of the MT cytoskeleton could be involved in recognition of the fungus by host cells and in the initiation of resistance responses in barley leaves. We hypothesize that the integrity and dynamics of the MT cytoskeleton, especially of its perinuclear part, might participate in control mechanisms involved in activation of resistance genes.Abbreviations CMTs
cortical microtubules
- EMTs
endoplasmic microtubules
- MT
microtubules
- PI
propidium iodide
- SC
sensitive combination
- RC
resistant combination 相似文献
999.
M. Lorieux X. Perrier B. Goffinet C. Lanaud D. González de León 《TAG. Theoretical and applied genetics. Theoretische und angewandte Genetik》1995,90(1):81-89
In F2 populations, gametic and zygotic selection may affect the analysis of linkage in different ways. Therefore, specific likelihood equations have to be developed for each case, including dominant and codominant markers. The asymptotic bias of the classical estimates are derived for each case, in order to compare them with the standard errors of the suggested estimates. We discuss the utility and the efficiency of a previous model developed for dominant markers. We show that dominant markers provide very poor information in the case of segregation distortion and, therefore, should be used with circumspection. On the other hand, the estimation of recombination fractions between codominant markers is less affected by selection than is that for dominant markers. We also discuss the analysis of linkage between dominant and codominant markers. 相似文献
1000.
Summary Contents of arginine, ornithine, urea and ammonia in wine were monitored during aging process by two flor veil-forming yeast strains. The patterns of yeast growth were different for the two strains studied. Urea excretion and uptake were dependent on yeast strain, so that finished wines showed different concentrations of urea. The effect of limited aeration was also investigated for Saccharomyces cerevisiae var. capensis. The results indicated that aeration promoted yeast growth in veil, thus reducing urea concentration until its exhaustion in wine. 相似文献