Remobilization of labelled nitrogen from vegetative tissues to pods of mungbean

Summary Remobilization of¹⁵N from vegetative tissue of mungbean (Vigna radiata (L.) Wilczek) into pods was measured during the reproductive phase of growth. Plant tissue was labelled with¹⁵N during vegetative development. Experiments were conducted in the field at two sites. At one site the soil provided cowpeas with most of their N but at the other site N fixation provided most of the N. Remobilized N from vegetative tissue to pods occurred soon after they began to develop. The quantity of the labelled N ultimately remobilized to the pods amounted to 50% for one cultivar (Tx33) at the high soil N site and 70% at the low N site. For the other cultivar (Tx13) the values were 25% and 30%, respectively. The two cultivars performed very differently with respect to partitioning of N into pods and the rate of N fixation. Even though more N was accumulated in the shoots of the high N fixing cultivar (Tx13) less total N was contained in the pods.     

Reconstitution of the membrane-bound, ubiquinone-dependent pyruvate oxidase respiratory chain of Escherichia coli with the cytochrome d terminal oxidase

Pyruvate oxidase is a flavoprotein dehydrogenase located on the inner surface of the Escherichia coli cytoplasmic membrane and coupled to the E. coli aerobic respiratory chain. In this paper, the role of quinones in the pyruvate oxidase system is investigated, and a minimal respiratory chain is described consisting of only two pure proteins plus ubiquinone 8 incorporated in phospholipid vesicles. The enzymes used in this reconstitution are the flavoprotein and the recently purified E. coli cytochrome d terminal oxidase. The catalytic velocity of the reconstituted liposome system is about 30% of that observed when the flavoprotein is reconstituted with E. coli membranes. It is also shown that electron transport from pyruvate to oxygen in the liposome system generates a transmembrane potential of at least 180 mV (negative inside), which is sensitive to the uncouplers carbonyl cyanide p-(tri-chloromethoxy)phenylhydrazone and valinomycin. A trans-membrane potential is also generated by the oxidation of ubiquinol 1 by the terminal oxidase in the absence of the flavoprotein. It is concluded that (1) the flavoprotein can directly reduce ubiquinone 8 within the phospholipid bilayer, (2) menaquinone 8 will not effectively substitute for ubiquinone 8 in this electron-transfer chain, and (3) the cytochrome d terminal oxidase functions as a ubiquinol 8 oxidase and serves as a "coupling site" in the E. coli aerobic respiratory chain. These investigations suggest a relatively simple organization for the E. coli respiratory chain.     

Genetic studies of the lac repressor. XII. Amino acid replacements in the DNA binding domain of the Escherichia coli lac repressor

Out of the first 62 residues of the lac repressor, 38 positions have been substituted by at least one amino acid exchange. The total number of replacements in this region is 131. Data from several studies are considered.     

Model for microneurovascular muscle transplantation in the dog

Previous studies have suggested that successful transplantation of skeletal muscle to replace previously lost function depends on the mass of the transplanted tissue. In the present experiment, the possibility that careful microneurovascular surgical technique substantially improves the chances of successful transplantation of large-sized muscle was tested using dog gracilis muscle averaging 75 gm in weight. Gracilis muscles were completely excised ipsilaterally and were implanted into their original location (orthotopic) by reattaching tendons of insertion and origin. In addition, neurorrhaphies of nerve stumps were performed along with repair of the vascular pedicle using microsurgery techniques. After approximately 1 year, orthotopic transplants weighed about 70 percent of contralateral sham-operated gracilis muscles. Although average tension output of transplants declined to about 60 percent of control values, three of the most successfully transplanted muscles produced between 73 and 88 percent of control force. A significant increase in the number of slow-twitch-oxidative fibers was correlated with a slight but significant reduction in the maximal velocity of shortening of transplanted muscles. The ability of transplants to resist fatigue when repetitively stimulated was similar to the endurance capacity of control muscles. These results suggest that microneurovascular surgery may enhance the more complete restoration of function of transplanted skeletal muscles of relatively large size.     

Direct evidence for Ca++-induced lateral phase separation in black membranes of lipid mixtures by the analysis of gramicidin A single-channels

Single-channel conductance fluctuations are analysed for gramicidin A incorporated into binary-mixed black lipid membranes of charged phosphatidic acid and neutral lecithin in different molar ratios. At very low Ca⁺⁺ concentrations in the electrolyte (i.e. in the presence of EDTA) homogeneous lipid mixtures are identified through their conductance and life time probability distributions for integral gramicidin pores. As for the pure lipid components, the conductance histograms each show a single maximum with regular width and for all channels a single mean lifetime is found.For Ca⁺⁺-levels (10^-6–10^-5 M) that are close to the critical demixing concentration (10^-4 M) unusually broad conductance distributions and reduced lifetimes are found provided the PC content, x, of the membrane is close to the critical mixture (x _crit0.5). We interpret this as a first example of the coupling of a membrane function (the transport of ions) to a lipid matrix with locally fluctuating composition close to a critical demixing point.For the conductance histogram of gramicidin A in an equimolar mixture of PA and PC shows two well-separated maxima. A correlation analysis between conductance and lifetime of the single pores shows that the two channel populations also differ significantly in their mean channel lifetime, ^*. This finding is interpreted as being direct evidence for Ca⁺⁺-induced lateral phase separation in black lipid membranes, as has been postulated recently.Abbreviations used HEPES N-2-hydroxyethyl-piperazine-N-2-ethane-sulfonic acid - EDTA ethylenediaminetetraacetic acid     

The Kinetics of Selective Biological Transport: V. Further Data on the Erythrocyte-Monosaccharide Transport System

Measurements of the rate of loss of sugar from human erythrocytes into sugar-free solutions were made as a function of sugar concentrations. The half-saturation concentration of this process was found to be different from those half-saturation concentrations previously measured by other methods. These data, together with a number of similar data from former publications, are summarized in tabular form and their use in assessing postulated transport mechanisms is illustrated by consideration and rejection of a mechanism in which the transport process is assumed to be the result of a protein conformational change.     

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