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21.
We have begun a systematic search for potential tRNA genes in wheat mtDNA, and present here the sequences of regions of the wheat mitochondrial genome that encode genes for tRNAAsp (anticodon GUC), tRNAPro (UGG), tRNATyr (GUA), and two tRNAsSer (UGA and GCU). These genes are all solitary, not immediately adjacent to other tRNA or known protein coding genes. Each of the encoded tRNAs can assume a secondary structure that conforms to the standard cloverleaf model, and that displays none of the structural aberrations peculiar to some of the corresponding mitochondrial tRNAs from other eukaryotes. The wheat mitochondrial tRNA sequences are, on average, substantially more similar to their eubacterial and chloroplast counterparts than to their homologues in fungal and animal mitochondria. However, an analysis of regions 150 nucleotides upstream and 100 nucleotides downstream of the tRNA coding regions has revealed no obvious conserved sequences that resemble the promoter and terminator motifs that regulate the expression of eubacterial and some chloroplast tRNA genes. When restriction digests of wheat mtDNA are probed with 32P-labelled wheat mitochondrial tRNAs, <20 hybridizing bands are detected, whether enzymes with 4 bp or 6 bp recognition sites are used. This suggests that the wheat mitochondrial genome, despite its large size, may carry a relatively small number of tRNA genes.  相似文献   
22.
Neuroblastoma cells were used to determine the effect of sorbinil on myo-inositol metabolism in cells exposed to elevated levels of glucose in culture. Exposing cells to elevated levels of glucose led to an increase in levels of intracellular sorbitol. The increase in sorbitol levels was dependent on the extracellular glucose concentration. In contrast, the myo-inositol content of cells was decreased in the presence of increasing concentrations of extracellular glucose. Increasing the concentration of glucose in the culture medium caused a decrease in myo-inositol uptake and in the incorporation of extracellular myo-inositol into phospholipid. The effect of elevated glucose levels on myo-inositol metabolism and sorbitol accumulation was blocked by addition of 0.4 mM sorbinil. The ability of sorbinil to block the decrease in myo-inositol metabolism and sorbitol accumulation caused by 30 mM extracellular glucose was dependent on its concentration. Maximal effects were obtained with 0.4 mM sorbinil. However, there was some variation in the degree of effectiveness among batches of sorbinil. These results at the cellular level suggest that the intracellular accumulation of sorbitol is responsible for the alteration of myo-inositol metabolism observed in neuroblastoma cells exposed to elevated glucose concentrations.  相似文献   
23.
Distribution of the Glucose-1,6-Bisphosphate System in Brain and Retina   总被引:2,自引:2,他引:0  
The distribution of glucose-1,6-bisphosphate (G16P2) synthase was measured in more than 70 regions of mouse brain, and nine layers of monkey retina. Activities in gray areas varied as much as 10-fold, in a hierarchical manner, from highest in telencephalon, especially the limbic system, to lowest in cerebellum, medulla, and spinal cord. The synthase levels were significantly correlated among different regions with G16P2 itself, as well as with previously published levels of a brain specific IMP-dependent G16P2 phosphatase. In contrast, neither G16P2 nor either its synthase or phosphatase correlated positively with phosphoglucomutase, and in all regions the G16P2 levels greatly exceeded requirements for activation of this mutase. This strengthens the view that G16P2 has some function besides serving as coenzyme for phosphoglucomutase. However, attempts to correlate the "G16P2 system," as defined by the three coordinately related elements, synthase, phosphatase, and G16P2, with other enzymes of carbohydrate metabolism, or with regional data of Sokoloff et al. [J. Neurochem. 28, 897-916 (1977)] for glucose consumption, were unsuccessful. This leaves open the possibility that brain G16P2 might serve as a phosphate donor for specific nonmetabolic effector proteins.  相似文献   
24.
Neuroblastoma cells were used to analyze the effect of elevated glucose levels on myo-inositol metabolism and Na+/K+-pump activity. The activity of the Na+/K+ pump in neuroblastoma cells is almost totally sensitive to ouabain inhibition. Culturing neuroblastoma cells in 30 mM glucose caused a significant decrease in Na+/K+-pump activity, myo-inositol metabolism, and myo-inositol content, compared to cells grown in the presence of 30 mM fructose. Glucose supplementation also caused a large intracellular accumulation of sorbitol. The aldose reductase inhibitor sorbinil prevented the abnormalities in myo-inositol metabolism and partially restored Na+/K+-pump activity in neuroblastoma cells cultured in the presence of elevated glucose levels. These results suggest that the accumulation of sorbitol by neuroblastoma cells exposed to elevated concentrations of extracellular glucose causes a decrease in myo-inositol metabolism and these abnormalities are associated with a reduction in Na+/K+-pump activity.  相似文献   
25.
Summary Resource utilization by cutthroat trout (CT) and Dolly Varden charr (DV) was studied 8 years after experimental transfers from sympatry had established reproducing allopatric populations in two nearby fishless lakes. Allopatric DV significantly increased their utilization of shallow-dwelling zoobenthos, and increased their vertical distribution in comparison to that in sympatry. In contrast, allopatric CT showed little change in the proportions of major prey types utilized, and, if anything, restricted their vertical distribution in comparison to that in sympatry. The results can be explained by the hypothesis that the resource use of DV is strongly influenced by interspecific competition from CT, whereas CT largely remains unaffected by this interaction. An alternative hypothesis, that lake differences can explain the differences in resource use between sympatry and allopatry, was evaluated by comparing food resource availability and other biotic and abiotic characteristics of the three study lakes. None of these could account for the shift in resource use by DV between sympatry and allopatry, but lake differences may explain why allopatric CT showed a restricted habitat use in comparison with their sympatric donor stock. The results of this whole-lake transfer experiment are consistent with earlier reported field and laboratory studies, and suggest that the aggressive dominance of CT is the most important mechanism by which DV are displaced from littoral and near-surface habitats in sympathy with CT.  相似文献   
26.
Characterization of the functional organization of the photochemical apparatus in the light sensitive chlorophyll b-deficient oil yellow-yellow green (OY-YG) mutant of maize (Zea mays) is presented. Spectrophotometric and kinetic analysis revealed substantially lower amounts of the light harvesting complex of photosystem II (LHCII-peripheral) in high light-grown OY-YG thylakoids. However, accumulation of a tightly bound LHCII appears unaffected by the lesion. Changes in photosystem (PS) stoichiometry include lower amounts of PSII with characteristic fast kinetics (PSIIα) and a substantial accumulation of PSII centers with characteristic slow kinetics (PSIIβ) in the thylakoid membrane of the OY-YG mutant. Thus, PSIIβ is the dominant photosystem in the mutant chloroplasts. In contrast to wild type, roughly 80% of the mutant PSIIβ centers are functionally coupled to the plastoquinone pool and are probably localized in the appressed regions of the thylakoid membrane. These centers, designated PSIIβ-QB-reducing (QB being the secondary electron quinone acceptor of PSII), are clearly distinct from the typical PSIIβ-QB-nonreducing centers found in the stroma lamellae of wild-type chloroplasts. It is concluded that the observed changes in the stoichiometry of electron-transport complexes reflect the existence of a regulatory mechanism for the adjustment of photosystem stoichiometry in chloroplasts designed to correct any imbalance in light absorption by the two photosystems.  相似文献   
27.
Differential screening of a cDNA library from the PC12 rat pheochromocytoma cell line previously revealed a clone, clone 73, whose corresponding mRNA is induced by nerve growth factor (NGF). Induction parallels NGF-stimulated PC12 differentiation from a chromaffinlike phenotype to a sympathetic neuronlike phenotype. We report that DNA sequence analysis reveals that clone 73 mRNA encodes an intermediate filament (IF) protein whose predicted amino acid sequence is distinct from the known sequences of other members of the IF protein family. The sequence has highest homology with desmin and vimentin and includes the highly conserved central alpha-helical rod domain with the characteristic heptad repeat of hydrophobic residues, but has lower homology in the amino-terminal head and carboxyl-terminal tail domains. The head domain contains a large number of serine residues which are potential phosphorylation sites. The expression of clone 73 in vivo in the nervous system of the adult rat was investigated by in situ hybridization of clone 73 probes to tissue sections. The mRNA is expressed at high levels in ganglia of the peripheral nervous system, including the superior cervical ganglion (sympathetic), ciliary ganglion (parasympathetic), and dorsal root ganglion (sensory). In the central nervous system, motor nuclei of cranial nerves III, IV, V, VI, VII, X, and XII as well as ventral horn motor neurons and a restricted set of other central nervous system nuclei express the clone 73 mRNA. Tissues apart from those of the nervous system did not in general express the mRNA, with only very low levels detected in adrenal gland. We discuss the implications of these results for the mechanism of NGF-induced PC12 cell differentiation, the pathways of neuronal development in vivo, and the possible function of the clone 73 IF protein and its relationship to other IF proteins.  相似文献   
28.
29.
The limits of the cellular capacity to mediate an estrogen response.   总被引:8,自引:0,他引:8  
While steroid response is generally restricted by the availability of steroid receptors, the theoretical limits of the response are not known. We have constructed a series of cell lines that stably express the estrogen receptor (ER) at levels up to 5,000,000 ERs per cell and employed these cells to explore the limits of the estrogen response. Several reporter genes with estrogen response elements upstream of the herpes thymidine kinase promoter showed hyperbolic saturation kinetics with increasing ER. Maximum response was 10 times that seen in cell lines with receptor titers comparable to physiological levels. Half-maximal responses required 500,000 receptors per cell, and cells with 5,000,000 ERs showed greater than 90% maximum induction. Estradiol dose-response studies indicated that the receptors are limiting below 500,000 ERs per cell, but at higher ER titers there are spare receptors. In contrast to most reporters, the widely used reporter pA2-CAT, which has 200 base pairs of Xenopus vitellogenin DNA between the response element and the promoter, showed squelching at ER levels beyond 500,000 per cell. Cell lines that expressed ER above this level activated pA2-CAT with a distorted hormone dependence, where saturating ligand concentrations were inhibitory. All reporters displayed squelching when the ER was provided by transient transfection at a level that we judge is 20,000,000 per cell by extrapolation from the behavior of stable cell lines. These findings suggest that saturation of the cellular capacity to mediate an estrogen response and ER-dependent squelching occur at receptor titers well above those encountered in nature. If current models of steroid hormone action are correct, the findings also imply that estrogen response elements are occupied to very small extents under normal conditions.  相似文献   
30.
This study was designed to study the effects of stage of microspore development and culture medium on androgenic response in peanut (Arachis hypogaea L.). Anthers of various developmental stages were cultured for 7 days, then fixed and observed cytologically. Three sets of media, involving different basal media, growth regulators, sucrose levels and glutamine concentrations, were tested. In all experiments, the stage of development of the microspores at the time of culture was highly significant. The early uninucleate microspores stage was identified as producing the highest anther response rating. The effect of media was nonsignificant in all experiments. However, the stepwise modification of the media through the course of the study resulted in an almost 8 x increase in anther response rating. Numerically, the best media tested was N6 basal medium with 1 mg 1-1 NAA, 0.1 mg 1-1 BA, 5.5% sucrose, and 3.5 g 1-1 glutamine. While no haploids were obtained, four-nucleate cells were observed, indicating the potential in peanuts for an androgenic reponse.  相似文献   
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