Distribution of leaf assimilates in the stem of Picea abies L.

Summary Autoradiographic and microautoradiographic studies of 2-year-old Picea abies plants show that in summer leaf assimilates from the second-year shoot are translocated basipetally. Leaf assimilates are first transported to the stem via leaf trace phloem, then to the base of the stem in the sieve cells of the latest increment of secondary phloem. On the way down leaf assimilates move radially from sieve cells into cells of the phloem parenchyma, the vascular cambium, the rays, the inner periderm and certain cells of pith and cortex, including the epithelial cells surrounding the resin ducts. Other cells of pith and cortex remain nearly free of label, despite the long translocation time (20 h). With the exception of the vascular cambial cells, the stem cells that gain leaf assimilates by radial distribution coincide with those that contain chlorophyll and starch.     

Cationic interactions with Na+-H+ exchange and passive Na+ flux in cardiac sarcolemmal vesicles

Na⁺-H⁺ exchange and passive Na⁺ flux were investigated in cardiac sarcolemmal vesicles as a function of changing the ionic composition of the reaction media. The inclusion of EGTA in the reaction medium resulted in a potent stumulation of Na⁺ uptake by Na⁺-H⁺ exchange. It was found that millimolar concentrations of Mg²⁺ and Li⁺ were capable of inhibiting Na⁺-H⁺ exchange by 80%. One mechanism by which these ions may inhibit intravesicular Na⁺ accumulation by Na⁺-H⁺ exchange is via an increase in Na⁺ efflux. An examination of Na⁺ efflux kinetics from vesicles pre-loaded with Na⁺ revealed that Na⁺, Ca²⁺, Mg²⁺ and Li⁺ could stimulate Na⁺ efflux. Na⁺-H⁺ exchange was potently inhibited by an organic divalent cation, dimenthonium, which screens membrane surface charge. This would suggest that Na⁺-H⁺ exchange occurs in the diffuse double layer region of cardiac sarcolemma and this phenomenon is distinctly different from other Na⁺ transport processes. The results in this study indicate that in addition to a stimulation of Na⁺ efflux, the inhibitory effects of Mg²⁺, Ca²⁺ and Li⁺ on Na⁺-H⁺ exchange may also involve a charge dependent screening of Na⁺ interactions with the membrane.     

The duct system of the avian salt gland as a transporting epithelium: structure and morphometry in the duck Anas platyrhynchos

Summary The duct system of the nasal salt gland of the duck comprises central canals, secondary ducts and main ducts. The secondary and main ducts consist of a layer of columnar cells overlying a layer of small cuboidal cells. The columnar cells have complex intercellular spaces showing evidence of Na⁺ K⁺ -ATPase at the apical regions. Approximately 70% of surface area of the duct system is external to the gland. During adaptation to salt water the duct system increases in size as does the gland. Although the components of the gland of adapted ducks, including the duct system within the gland, increase in size compared with normal ducks, the percentage volume densities of the components remain similar in both categories of ducks, i.e. the duct system increases in size in proportion to the glandular tissue. The volume of the duct system external to the gland is six to seven times larger than the volume within the gland. Thus, if ductal modification of secreted fluid occurs, it will be most likely to take place in the ducts external to the gland.Total surface areas of the duct system were measured from serial sections of glands and ducts from one normal and one adapted duck. These were used to calculate possible flux rates of water and sodium across the duct epithelium, assuming the occurrence of either water reabsorption or sodium secretion. Although these flux rates are high it is shown that they are similar to calculated flux rates across the luminal surface of the secretory tubules.     

Testing the “methanochondrion concept”: are nucleotides transported across internal membranes in Methanobacterium thermoautotrophicum?

In order to test the Methanochondrion concept, uptake of adenine nucleotides in various membrane preparations of Methanobacterium thermoautotrophicum was studied. The uptake showed properties which are in general interpreted as indicative of a transport mechanism: (i) kinetics in the time range of minutes, (ii) temperature dependence, (iii) substrate specificity and (iv) failure to remove the substrate by extensive washing.However, nucleotide transport as an interpretation of this uptake can definitely be excluded. Not only an exchange mechanism of the mitochondrial type, but also a general exchange or an uniport mechanism was ruled out. In contrast, the nucleotide uptake was shown to be actually a tight and specific binding of ADP and ATP to binding sites at the interior side of the cell membrane. This was conclusively demonstrated in protoplasts obtained from M. thermoautotrophicum cells. In these protoplasts which do not contain internal membranes also nucleotide binding was observed, but only after disruption of the plasma membrane by osmotic lysis, which leads to the exposure of binding sites.     

Evidence for a branched respiratory system with two cytochrome oxidases in autotrophically grown Alcaligenes latus

The respiratory system of chemolithoautotrophically-grown Alcaligenes latus contains a, b, and c type cytochromes. Two cytochrome oxidases were identified by their carbon monoxide difference spectra and their differing sensitivities to cyanide and carbon monoxide. The oxidases were cytochrome o and an a-type cytochrome. Ubiquinone was present in A. latus membranes and could be reduced by H₂. The quinone analogue, 2-heptyl-4-hydroxy-quinoline-N-oxide (HQNO), was a strong inhibitor of the H₂ oxidase reaction, but did not prevent the reduction of either ubiquinone or the cytochromes.Abbreviations HQNO 2-heptyl-4-hydroxy-quinoline-N-oxide - TMPD N,N,N,N-tetramethyl-p-phenylenediamine     

Effect of aging on the rate of axonal transport of choline-phosphoglycerides

The anterograde axonal transport of choline-phosphoglycerides was studied in sciatic nerve motoneurons of adult (3-month-old) and aged (24-month-old) rats. After the spinal cord injection of [2-³H]glycerol, choline-phosphoglycerides; the major phospholipid class was transported along the nerve. The axonal transport rate was determined by plotting the distance covered by the front of transported radioactivity as a function of the time employed. In aged animals the rate of the choline-phosphoglyceride anterograde axonal transport was about 68% lower than that of adults; furthermore, the rate slowed down along the nerve in the proximal-distal direction. This alterated axonal transport mechanism might contribute to the degenerative processes observed in distal regions of peripheral nerve fibers of aged animals.     

Transport across the outer membrane of Escherichia coli K12 via the FhuA receptor is regulated by the TonB protein of the cytoplasmic membrane

Summary Point mutations in the “TonB box” offhuA were suppressed by point mutations in thetonB gene, suggesting both a functional and physical interaction between the FhuA receptor protein in the outer membrane and the TonB protein in the cytoplasmic membrane ofEscherichia coli K12. Mutations influA were classified into four types according to the extent by which they impaired mutant cells in their growth on ferrichrome as sole iron source and in their sensitivity to the antibiotic albomycin and to colicin M. ThetonB mutation with a glutamine to leucine replacement at position 165 was less efficient in restoring the FhuA functions than the glutamine to lysine exchange at the same position. The better the coupling between FhuA and TonB the poorer was the inhibition of phage T1 binding to FhuA by ferrichrome. A working model is proposed in which the TonB protein assumes different conformations in response to the energized state of the cytoplasmic membrane and thereby allosterically regulates the activity of the FhuA receptor. This model implies an intermembrane coupling between two proteins in adjacent membranes.     

Comparison of sequences from the malB regions of Salmonella typhimurium and Enterobacter aerogenes with Escherichia coli K12: A potential new regulatory site in the interoperonic region

Summary The malE and malK genes from Salmonella typhimurium, and the MalEFG operon and a portion of malK from Enterobacter aerogenes were cloned and sequenced. Plasmid-borne malE genes from both species and the malF and malG genes from E. aerogenes were expressed normally in Escherichia coli, and their products function in maltose transport. This shows that the malB products from the three species are interchangeable, at least in the combinations tested. The general genetic organization of the malB region is conserved. Potential binding sites and distances between them are highly conserved in the regulatory intervals. An unexpected conserved region was detected, which we call the U box, and which could be another target for a regulatory protein. This hypothesis is supported by the presence of the U box in the regulatory, region of the pulA-malX operon in Klebsiella pneumoniae. The intergenic region between malE and malF from S. typhimurium and E. aerogenes, contains inverted repeats similar to the palindromic units (PU or REP) found at the same location in E. coli. The predicted amino acid sequence of the encoded proteins showed 90% or more identity in every pairwise comparison of species.