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131.
N B Leontis  P Ghosh  P B Moore 《Biochemistry》1986,25(23):7386-7392
The imino proton nuclear magnetic resonance spectrum of Escherichia coli 5S ribonucleic acid (RNA) changes when the Mg2+ ion concentration drops below physiological levels. The transition between the physiological and low magnesium spectral forms of 5S RNA has a midpoint at approximately 0.3 mM Mg2+. Many of the most conspicuous changes observed in the downfield spectrum of 5S RNA as the magnesium concentration is reduced are due to adjustments in the structures of helices I and IV and the disappearance of resonances originating in helix V. The binding of ribosomal protein L25 to 5S RNA in the absence of magnesium stabilizes helix V structures.  相似文献   
132.
The distribution of calcium (Ca) in caps of vertically- and horizontally-oriented roots of Zea mays was monitored to determine its possible role in root graviresponsiveness. A modification of the antimonate precipitation procedure was used to localize Ca in situ. In vertically-oriented roots, the presumed graviperceptive (i.e., columella) cells were characterized by minimal and symmetric staining of the plasmalemma and mitochondria. No precipitate was present in plasmodesmata or cell walls. Within 5 min after horizontal reorientation, staining was associated with the portion of the cell wall adjacent to the distal end of the cell. This asymmetric staining persisted throughout the onset of gravicurvature. No staining of lateral cell walls of columella cells was observed at any stage of gravicurvature, suggesting that a lateral flow of Ca through the columella tissue of horizontally-oriented roots does not occur. The outermost peripheral cells of roots oriented horizontally and vertically secrete Ca through plasmodesmata-like structures in their cell walls. These results are discussed relative to proposed roles of root-cap Ca in root gravicurvature.  相似文献   
133.
Evidence is presented that the ferritin-inhibitable, Ia+ monocyte progenitor in murine marrow requires two signals for stimulation of clonal proliferation. Escherichia coli K235 lipopolysaccharide (LPS) at 0.1 ng/ml enhanced macrophage colony formation by 25 to 70% in murine marrow cultures stimulated with colony-stimulating factor (CSF-1). The progenitors which responded to LPS and CSF-1 represented a distinct subpopulation. Pretreatment of marrow cells with complement plus anti-Ia, anti-H2, anti-asialo GM1, and anti-Mac-1 antibodies specifically depleted the two-signal-requiring progenitors. In addition, the same progenitors were depleted by preincubation with hydroxyurea, indicating that these cells were in cell cycle when removed from the marrow. When compared with the quiescent progenitors, the Ia+, cycling cells were more sensitive to the antiproliferative effects of interferon alpha/beta but were more resistant to inhibition by E prostaglandins. Pretreatment with T cell-specific antibodies and complement specifically enhanced cloning of quiescent progenitors without affecting cloning of the Ia+, cycling subpopulation. Moreover, rat liver ferritin at 10(-8) to 10(-10) M specifically inhibited clonal proliferation of the Ia+ progenitors. Finally, the requirement for LPS as the additional stimulant could be replaced by the addition of haplotype-specific anti-Ia antibody to CSF-stimulated cultures. In contrast to LPS, anti-IA was competitive with inhibitory ferritin in clonal proliferation of the Ia+ progenitors. The significance of these observations in regulation of monocytopoiesis is discussed.  相似文献   
134.
Summary A method for the isolation of brush-border membranes of large intestinal epithelial cells was developed, which is based on the purification of intact brush-border caps by Percoll® density-gradient centrifugation followed by separation of the vesiculated brush-border membranes on sucrose gradients. The procedure has two major advantages in comparison to known methods: 1) its first step does not depend on the determination of marker enzymes and 2) the method is applicable to rats as well as rabbits without major modifications. Due to the lack of an accepted marker for the colonic brush-border membrane the validity of the isolation procedure was tested by its application to the small intestine. Rat small intestinal brush-border membranes were enriched 21-fold when compared to the homogenate. The method was used to evaluate alkaline phosphatase as a marker enzyme for the colonic brush-border membrane. The results suggest that alkaline phosphatase is not exclusively localized in the brush-border membrane since this enzyme was also associated with membranes having different physical properties.  相似文献   
135.
A murine monoclonal antibody raised against hamster cauda epididymal spermatozoa was shown to recognize an Mr 34,000 component of epididymal epithelium. Antigen was localized by immunocytochemistry on the surface and in the apical cytoplasm of principal cells in the proximal corpus epididymidis but not in the caput or initial segment regions. Spermatozoa from the corpus epididymidis expressed antigen on their post-acrosomal plasma membrane and annulus. Epididymal principal cells from the proximal corpus region when cultured in vitro bound antibody on their apical surface for at least 5 days. Spermatozoa from the caput epididymidis co-cultured with epithelium expressed antigen after incubation for 8 and 24 h. These results suggest that a surface change to epididymal spermatozoa during maturation in vivo may also be elicited during in-vitro culture.  相似文献   
136.
Among the earliest responses to mitogens that have been detected in normal quiescent cells are ionic changes: we have described rapid increases in the cytosolic free Ca2+ concentration ([Ca]i) and in the intracellular pH (pHi) in mitogen-stimulated thymocytes and fibroblasts (Hesketh, T. R., Moore, J. P., Morris, J. D. H., Taylor, M. V., Rogers, J., Smith, G. A., and Metcalfe, J. C. (1985) Nature 313, 482-484). Here we investigate the relationship between these ionic signals and the subsequent expression of the c-fos and c-myc proto-oncogenes in murine thymocytes. We show that the plant lectin concanavalin A (ConA), the phorbol ester 12-O-tetradecanoyl phorbol 13-acetate (TPA) and the Ca2+-ionophore A23187 each causes a rapid increase in both c-fos and c-myc mRNAs. The activation of both genes is completely dependent on the extracellular Ca2+ concentration ([Ca]o) for A23187 and independent of [Ca]o for TPA. Activation of c-myc, but not c-fos, by ConA is partially dependent on [Ca]o. The pHi increases generated by ConA or TPA are not necessary for expression of mRNA from either gene in response to these mitogens. Exogenous 8-bromo-cyclic AMP (but not 8-bromo-cyclic GMP) inhibits the c-myc responses to ConA and TPA. The data also show that neither early c-fos nor c-myc expression is sufficient to commit the cells to DNA synthesis.  相似文献   
137.
The effects of tryptophan administration on neurochemical estimates of synthesis [5-hydroxytryptophan (5-HTP) accumulation following administration of a decarboxylase inhibitor], storage [5-hydroxytryptamine (5-HT) concentrations], and metabolism [5-hydroxyindoleacetic acid (5-HIAA) concentrations] of 5-HT in selected regions of the hypothalamus were determined using HPLC coupled to an electrochemical detector. Tryptophan methyl ester HCl (30-300 mg/kg i.p.) produced a dose-dependent increase in the rate of 5-HTP accumulation throughout the hypothalamus but had no effect on the rate of accumulation of 3,4-dihydroxyphenylalanine. Peak 5-HTP levels were attained by 30 min following administration of tryptophan (100 mg/kg i.p.) and were maintained for an additional 60 min. Tryptophan also produced concomitant dose-dependent increases in 5-HT and 5-HIAA concentrations in these same regions without changes in the 5-HIAA/5-HT ratio. These results indicate that exogenous tryptophan administration selectively increases the synthesis, storage, and metabolism of 5-HT in the hypothalamus without altering the synthesis of catecholamines. Inhibition of 5-HT uptake with chlorimipramine or fluoxetine produced modest (10-40%) reductions in 5-HIAA concentrations throughout the hypothalamus, revealing that only a minor portion of 5-HIAA is derived from released and recaptured 5-HT, whereas the major portion of this metabolite reflects intraneuronal metabolism of unreleased 5-HT. In both chlorimipramine- and fluoxetine-treated rats, 5-HIAA concentrations were significantly increased by tryptophan administration, indicating that the increase in synthesis of 5-HT following precursor loading is accompanied by an increase in the intraneuronal metabolism of 5-HT.  相似文献   
138.
Summary The mechanism of action of retinoid in reversing keratinization in hamster trachea is yet unknown. The purpose of this study was to determine if cellular retinoic acid binding protein (CRABP) is present in tracheal epithelium following incubation in serum-free, vitamin A-deficient culture medium for 10 days, and if the effectiveness of a retinoid in reversing keratinization in organ culture is correlated with its ability to compete for CRABP sites. The cytosol prepared from tracheal cultures contained CRABP at a concentration of 2.61 pmoles per mg protein. Of the four retinoids with carboxyl end group selected for the study, two of the biological active retinoids competed for the CRABP sites. However, no correlation was observed between the biological activity of the inactive retinoids and their ability to associate with the CRABP sites. These results indicate that even though the action of retinoid may be mediated by retinoid binding protein, it cannot be used as a sole predicator of retinoid response in hamster trachea. This investigation was supported by Contract N01-CP-31012 and U. S. P. H. Grants CA30512 and CA32428, which were awarded by the Division of Cancer Etiology, National Cancer Institute, DHHS. Editor's Statement Tracheal organ cultures provide a useful model for the study of epithelial differentiation and carcinogenesis. Much attention has been given to the action of retinoids in this process. Mehta et al. demonstrate a lack of correlation between biological activity and specific cytosolic binding of members of this class of compounds, pointing out the need for a more complete biochemical understanding of the mechanism of action and active forms of retinoids in this and other systems in vivo and in vitro. David W. Barnes  相似文献   
139.
Primary roots of Zea mays seedlings germinated and grown in 0.1 mM chloramphenicol (CMP) were significantly less graviresponsive than primary roots of seedlings germinated and grown in distilled water. Elongation rates of roots treated with CMP were significantly greater than those grown in distilled water. Caps of control and CMP-treated roots possessed extensive columella tissues comprised of cells containing numerous sedimented amyloplasts. These results indicate that the reduced graviresponsiveness of CMP-treated roots is not due to reduced rates of elongation, the absence of the presumed gravireceptors (i.e., amyloplasts in columella cells), or reduced amounts of columella tissue. These results are consistent with CMP altering the production and/or transport of effectors that mediate gravitropism.  相似文献   
140.
A quantitative analysis of the relationship between pseudocopulatory behaviour and the ovarian cycle in the parthenogenetic lizard Cnemidophorus uniparens indicates (1) that this behaviour is frequently and regularly expressed by captive individuals, and (2) that the sexual role, either male-like or female-like, exhibited by an animal is correlated with its ovarian state. The expression of female-like behaviour patterns was associated with and primarily limited to the vitellogenic stage of the cycle. Male-like behaviour patterns occurred most frequently during post-ovulatory stages but was not limited to these stages. Neither behavioural role was ever expressed by non-reproductive individuals. Reproductive individuals often alternated in assuming the female-like and male-like roles during the progress of the ovarian cycle. These observations suggest that pseudosexual behaviour is hormonally activated in this species. However, it also appears that the prevailing social situation is an important factor determining which behavioural role is taken. This work strengthens the hypothesis that pseudosexual behaviour in all-female lizards occurs as the result of natural selection.  相似文献   
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