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.     

Book reviews

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Book reviews

     

Use of phosphorus oxychloride in synthesizing nucleotides and oligonucleotides

Procedures are described for phosphorylating protected nucleotides, oligonucleotides and phosphoramidate oligonucleotide derivatives at the 3′-hydroxyl group. The conditions (phosphorylation with phosphorus oxychloride and pyridine in dioxane followed by hydrolysis with aqueous pyridine) are sufficiently mild that base labile (trifluoroacetylamino; β-cyanoethyl phosphotriester) and acid labile (O-monomethoxytrityl; phosphoramidate) functions are retained intact. Application of the technique is illustrated by the synthesis of dpT, dTp, d(CF₃CONH)Tp, dTp_NTp, and dTp_NTp_NTp. In addition, the utilization of phosphorus oxychloride in joining thymidine derivatives and dinucleoside phosphotriester blocks via phosphodiester links is described.