The Cellular Transport of Magnesium in Rat Liver

The bidirectional transport of Mg in rat liver was studied using slices labeled with ²⁸Mg in a closed two-compartment system under steady-state conditions. The influx (K_bs) and efflux (K_sb) transfer coefficients governing transport between the extracellular phase and a rapidly exchanging cell fraction were 0.074 and 0.019 per min, respectively. An increased extracellular concentration of Mg⁺⁺ caused a 30% decrease in K_bs and a 31% increase in K_sb. A decreased extracellular Mg⁺⁺ had an opposite effect. At 0°C, both transfer coefficients were reduced by 65%. Increased pH and NaCN increased transport, whereas Ca⁺⁺ reduced transport. Reduced pH, altered Na⁺:K⁺ ratio, Sr⁺⁺, glucose deletion, iodoacetate, ethanol, and lactate had no significant influence. Dinitrophenol reduced K_sb but had no effect on K_bs. These data support the thesis that the intracellular concentration of Mg is in part regulated by a reciprocal change in the influx transfer coefficient and a parallel change in the efflux transfer coefficient in response to altered extracellular concentrations of Mg⁺⁺. The qualitative and quantitative similarities of Mg and Ca transport in this system suggest that Mg and Ca share a common transport mechanism which is primarily dependent upon the binding of these divalent cations to macromolecular ligands within the cell membrane or within the cell.     

Exo-β-glucanases in yeast

1. A number of yeast species were examined for the presence of β-glucanases. Extracts obtained by cell disruption of Saccharomyces cerevisiae, Fabospora fragilis and Hansenula anomala hydrolysed laminarin and pustulan with the production of glucose. Enzymic activities were also detected in the culture fluids of F. fragilis and H. anomala grown aerobically in buffered mineral medium with glucose as the carbon source. 2. F. fragilis and H. anomala possessed approximately sevenfold higher β-(1→3)-glucanase activity than S. cerevisiae. 3. Intracellular exo-β-glucanase from baker's yeast was purified 344-fold from the dialysed cell extract. 4. Exo-β-glucanase from F. fragilis was purified 114-fold from the dialysed culture fluid and 423-fold from the dialysed intracellular extract. The purified extracellular and intracellular enzymes had similar properties and essentially the same specific activity, 79 enzyme units/mg. of protein. 5. Extracellular exo-β-glucanase of H. anomala was purified 600-fold. 6. The optimum pH of the enzymes from F. fragilis, S. cerevisiae and H. anomala was 5·5 in each case. Chromatographic evidence indicated that the three enzymes remove glucosyl units sequentially from laminarin as well as pustulan. 7. The ratio of activities towards laminarin and pustulan remained constant during purification of the exo-β-glucanase obtained from the three species, suggesting a single enzyme. Additional evidence for its unienzymic nature are: (i) the two activities were destroyed at exactly the same rate on heating of the purified enzyme from F. fragilis at three different temperatures; (ii) the competitive inhibitor glucono-δ-lactone gave the same value of K_i when tested with either substrate; (iii) quantitative application of the `mixed-substrate' method with the purified enzyme of S. cerevisiae gave data that were in excellent agreement with those calculated on the assumption of a single enzyme. 8. The purified exo-β-glucanases of the different species of yeast had different kinetic constants. The ratios of maximal velocities and K_m values with laminarin and pustulan differed markedly. Comparison of V_max. and K_m values suggests that the rapid release of spores from asci in F. fragilis might be explained in terms of an enzyme with higher maximal velocity and higher affinity to the ascus wall than that present in baker's yeast. 9. The estimated molecular weights for exo-β-glucanases from F. fragilis, S. cerevisiae and H. anomala were 22000, 40000 and 30000 respectively.     

Mechanism of repression of methionine biosynthesis in Escherichia coli. II. The effect of metJ mutations on the free amino acid pool

Summary Ethionine-resistant mutants (metJ mutants) were isolated and characterized as constitutive in the biosynthesis of methionine. Such mutations resulted in marked differences or alterations in the free amino acid pool. In some strains the levels of threonine and histidine were elevated by as much as 13 and 22 times that of the wild type level. The possibility that structural modifications of methionyl-tRNA were giving rise to constitutive methionine biosynthesis and the apparent aberrations in the free amino acid pool, was in large part ruled out by a comparison of the mobilities of wild type and mutant methionyl-tRNA on benzoylated DEAE-cellulose columns. The results obtained are consistent with the view that the product of the metJ locus is a repressor protein which is directly involved in the repression of the methionine genes.     

Essential oil analysis of the myricaceae of the eastern United States

The essential oil content of several members of the Myricaceae were examined for chemotaxonomic purposes. The analysis of the essential oils corroborates the suggestion that the Myricaceae should be divided into three genera. The study also suggests that Myrica pusilla and M. macfarlanei are not valid species. The analyses were carried out on the oil obtained from steam distillation of the foliage. Specific oil constituents were identified by GLC and IR.     

First reported chloroplast genome sequence of <Emphasis Type="Italic">Punica granatum</Emphasis> (cultivar Helow) from Jabal Al-Akhdar,Oman: phylogenetic comparative assortment with <Emphasis Type="Italic">Lagerstroemia</Emphasis>

Pomegranate (Punica granatum L.) is one of the oldest known edible fruits. It has grown in popularity and is a profitable fruit crop due to its attractive features including a bright red appearance and its biological activities. Scientific exploration of the genetics and evolution of these beneficial traits has been hampered by limited genomic information. In this study, we sequenced the complete chloroplast (cp) genome of the native P. granatum (cultivar Helow) cultivated in the mountains of Jabal Al-Akhdar, Oman. The results revealed a P. granatum cp genome length of 158,630 bp, characterized by a relatively conserved structure containing 2 inverted repeat regions of 25,466 bp, an 18,686 bp small single copy regions, and an 89,015 bp large single copy region. The 86 protein-coding genes included 37 transfer RNA genes and 8 ribosomal RNA genes. Comparison of the P. granatum whole cp genome with seven Lagerstroemia species revealed an overall high degree of sequence similarity with divergence among intergenic spacers. The location, distribution, and divergence of repeat sequences and shared genes of the Punica and Lagerstroemia species were highly similar. Analyses of nucleotide substitution, insertion/deletions, and highly variable regions in these cp genomes identified potential plastid markers for taxonomic and phylogenetic studies in Myrtales. A phylogenetic study of the cp genomes and 76 shared coding regions generated similar cladograms. The complete cp genome of P. granatum will aid in taxonomical studies of the family Lythraceae.