Effect of ammonium on the regulation of nitrate and nitrite transport systems in roots of intact barley (Hordeum vulgare L.) seedlings

The effect of NH ₄ ⁺ on the regulation of NO ₃ ^– and NO ₂ ^– transport systems in roots of intact barley (Hordeum vulgareL.) seedlings grown in NO ₃ ^– or NO ₂ ^– was studied. Ammonium partially inhibited induction of both transport systems. The inhibition was less severe in NO ₂ ^– -fed than in NO ₃ ^– -fed seedlings, presumably due to lower uptake of NH ₄ ⁺ in the presence of NO ₂ ^– . In seedlings pretreated with NH ₄ ⁺ subsequent induction was inhibited only when NH ₄ ⁺ was also present during induction, even though pretreated roots accumulated high levels of NH ₄ ⁺ . This indicates that inhibition may be regulated by NH ₄ ⁺ concentration in the cytoplasm rather than its total accumulation in roots. L-Methionine sulfoximine did not relieve the inhibition by NH ₄ ⁺ , suggesting that inhibition is caused by NH ₄ ⁺ itself rather than by its assimilation product(s). Ammonium inhibited subsequent expression of NO ₃ ^– transport activity similarly in roots grown in 0.1, 1.0, or 10 mM NO ₃ ^– for 24 h (steady-state phase) or 4 d (decline phase), indicating that it has a direct, rather than general feedback effect. Induction of the NO ₃ ^– transport system was about twice as sensitive to NH ₄ ⁺ as compared to the NO ₂ ^– transport system. This may relate to higher turnover rates of membraneassociated NO ₃ ^– -transport proteins.Abbreviations Mes 2(N-morpholino)ethanesulfonic acid - MSO L-methionine sulfoximine     

Plasma membrane protein composition and synthesis in soybean hypocotyl segments undergoing auxin-induced elongation

Summary The types and amount of plasma membrane proteins synthesized during cell elongation in response to auxin (2,4-dichlorophenoxyacetic acid) treatment were investigated. Auxin-treated and control soybean (Glycine max L.) hypocotyl segments were incubated with [³⁵S]methionine for various times, ranging from 0.5 to 18 h, prior to isolation of plasma membrane by aqueous two-phase partitioning. Protein accumulated in the plasma membrane after auxin treatment. Despite this accumulation, the protein incorporation rate, estimated by the amount of label in the plasma membrane following a 0.5 h [³⁵S]methionine pulse, was unaffected by auxin treatment at both 0.5 and 18 h of treatment. Protein apparently accumulated by a mechanism distinct from enhanced incorporation. The plasma membrane proteins synthesized by elongating segments differed from controls at 18 h, as evidenced by the pattern of fluorographs following a 0.5 h radiolabelling. However, auxin treatment did not alter the 2-D gel pattern of the polypeptides detectable by silver stain.Abbreviations 2,4-D 2,4-dichlorophenoxyacetic acid - IEF isoelectric focusing - PM plasma membrane - SDS-PAGE sodium dodecyl sulfate-polyacrylamide gel electrophoresis     

Effect of pH and calcium on short-term NO3- fluxes in roots of barley seedlings

The effect of pH and Ca2+ on net NO3- uptake, influx, and efflux by intact roots of barley (Hordeum vulgare L.) seedlings was studied. Seedlings were induced with NO3- or NO2-. Net NO3- uptake and efflux, respectively, were determined by following its depletion from, and accumulation in, the external solution. Since roots of both uninduced and NO2(-)-induced seedlings contain little internal NO3- initial net uptake rates are equivalent to influx (M. Aslam, R.L. Travis, R.C. Huffaker [1994] Plant Physiol 106: 1293-1301). NO3-, uptake (influx) by these roots was little affected at acidic pH. In contrast, in NO3(-)-induced roots, which accumulate NO3-, net uptake rates decreased in response to acidic pH. Under these conditions, NO3- efflux was stimulated and was a function of root NO3- concentration. Conversely, at basic pH, NO3- uptake by NO3- and NO2(-)-induced and uninduced roots decreased, apparently because of the inhibition of influx. Calcium had little effect on NO3- uptake (influx) by NO2(-)-induced roots at either pH 3 or 6. However, in NO3(-)-induced roots, lack of Ca2+ at pH 3 significantly decreased net NO3- uptake and stimulated efflux. The results indicate that at acidic pH the decrease in net NO3- uptake is due to the stimulation of efflux, whereas at basic pH, it is due to the inhibition of influx.     

Caprine Arthritis-Encephalitis Virus Is Distinct from Visna and Progressive Pneumonia Viruses as Measured by Genome Sequence Homology

Competitive inhibition of hybridization between ¹²⁵I-labeled caprine arthritis-encephalitis viral RNA and homologous cDNA by heterologous viral RNA shows that the caprine retrovirus shares <20% genome sequence homology with visna and progressive pneumonia viruses. These viruses, however, are indistinguishable in immunodiffusion reactions involving the major structural protein (p28).     

An electron microscope comparison of plasma membrane vesicles from meristematic and mature soybean root tissue

Plasma membrane vesicles were isolated from homogenates of meristematic and mature soybean root tissue by differential sucrose gradient centrifugation. Vesicles were positively identified by the phosphotungstic acid-chromic acid procedure (PACP). The two preparations were comparable in size class distribution, mitochondrial contamination, and per cent plasma membrane vesicles present. Purity levels were estimated to be greater than 75%. The specificity of PACP was observed for a variety of cell types from both regions. Some variability in PACP staining was offset by careful modulation of the stain protocol and was found to be independent of developmental stage in subcellular fractions. Patchy or discontinuous staining, observed in both intact tissue and in subcellular fractions from both regions, was found to be a function of stain time.     

1,25-Dihydroxyvitamin D3 receptor in bovine thymus gland

1,25-Dihydroxyvitamin D₃ [1,25-(OH)₂D₃] receptor was characterized after partial purification of thymus cytosol by ammonium sulfate fractionation. The 1,25-(OH)₂D₃ receptor sediments at 3.7S in 5–20% sucrose gradients. The binding of 1,25-(OH)₂D₃ in thymic cytosol was a saturable process with high affinity (Kd = 0.12?0.48 nM) at 4°C. Competition for 1,25-(OH)₂[³H]D₃ receptor by nonradioactive analogs demonstrated the affinities of these analogs to be in order; 1,25-(OH)₂D₃ = 1,24R,25-(OH)₃D₃ = 1,25S,26-(OH)₃D₃ = 1,25R,26-(OH)₃D₃ > 1,25-(OH)₂D₃-26,23 lactone > 25-OHD₃ > 23R,25-(OH)₂D₃ > 24R,25-(OH)₂D₃ > 23S,25-(OH)₂D₃ ? 25-OHD₃-26,23 lactone. The receptor bound to DNA cellulose columns in low salt buffer and eluted as a single peak at 0.21 M KCl. These findings provide evidence that the thymus possesses a 1,25-(OH)₂D₃ receptor with properties indistinguishable from 1,25-(OH)₂D₃ receptors in other tissues.     

Proteolytic inactivation of alpha-1-anti-chymotrypsin. Sites of cleavage and generation of chemotactic activity.

The effect of several microbial and mammalian proteinases on the inhibitory activity of human plasma alpha-1-anti-chymotrypsin (alpha-1-Achy) has been tested. Most of these enzymes caused rapid inactivation of the inhibitor by cleavage at single sites within the reactive-site loop between P5 Lys and P3' Leu, with additional cleavages also being detected in some cases near the NH2 terminus of the native protein. In contrast, two of the enzymes tested failed to inactivate alpha-1-Achy, although they could cause removal of peptides near the NH2 terminus. Studies of neutrophil chemotaxis revealed that native or NH2-terminally truncated alpha-1-Achy was not stimulatory. However, testing of two enzymatically inactivated forms of the inhibitor (alpha-1-Achy), cleaved at widely different positions within the reactive-site loop, indicated that they had become potent chemoattractants at concentrations within the nanomolar range. Competition studies using proteolytically inactivated alpha-1-proteinase inhibitor suggested that the chemotactic activity of the two inactivated serpins was probably mediated by the same mechanism. The physiological relevance of this chemotactic activity is underscored by the results of plasma elimination studies which indicate that alpha-1-Achy is eliminated at approximately the same rate as native alpha-1-Achy, thus prolonging chemotactic stimuli within the tissues.     

Cellular mechanisms of the antitumor activity of recombinant IL-6 in mice.

The systemic administration of human rIL-6 to mice resulted in the regression of established, 3-day pulmonary micrometastases from two weakly immunogenic tumors, but not from a nonimmunogenic tumor, in the absence of observable toxicity. Although IL-6 alone failed to have a significant therapeutic impact on advanced, 10-day pulmonary macrometastases from weakly immunogenic tumors, substantial cure rates of mice could be achieved when this cytokine was combined with cyclophosphamide. Histologic analysis of the lungs of mice receiving IL-6 revealed infiltration with lymphoid cells during the regression of pulmonary nodules from a weakly immunogenic tumor. IL-6-mediated tumor regression could be abrogated after selective in vivo depletion of either CD4 or CD8 T cell subsets by the systemic administration of specific mAb. In vivo generation of tumor-specific CTL, but not of lymphokine-activated killer cells, was detected in the lungs of IL-6-treated mice during regression of pulmonary metastases. Collectively, these findings demonstrate a role for IL-6 in the treatment of established solid tumors that have the capacity to elicit T cell responses in the host. Differences in host cellular mechanisms involved in tumor regression mediated by immunotherapy using IL-6 vs IL-2 are discussed.     

PHENOTYPIC PLASTICITY IN THE SAILFIN MOLLY,POECILIA LATIPINNA (PISCES: POECILIIDAE). I. FIELD EXPERIMENTS

Sailfin mollies (Poecilia latipinna) display marked interdemic variation in body size. We employed “common-garden” experiments in field enclosures to explore the potential role of environmental factors in determining the interdemic phenotypic variation in growth rate, age at maturity, and size at maturity. The largest single, consistent source of variation for all traits was family identity within populations. Environmental effects acted predominantly through family x environment interactions. There was little evidence for any intrinsic variation among populations once family heterogeneity had been accounted for. In general, when statistically significant differences existed, fish raised in a saltwater pond grew faster than their broodmates raised in a freshwater pond. Both males and females tended to mature at a smaller size and later in the freshwater pond than in the saltwater pond. The effects of the environmental conditions differed among the three years in which we performed these studies. In only one year was there a substantial difference between fish raised under the two environmental conditions. These results indicate that direct environmental effects are not strong enough to account for the differences in body size among natural populations and that intrinsic differences among natural populations are due to different frequency distributions of genotypes that are present in all populations.