The effect of vitamin A on limb regeneration in Rana temporaria

Previous experiments in which vitamin A has been administered to developing or regenerating limbs have shown that different limb axes are affected. In regenerating axolotl limbs, serial reduplications in the proximodistal axis are produced. In the developing chick limb bud, mirror-imaged reduplications in the anteroposterior axis are produced. Results reported here on Rana temporaria limb buds reveal that vitamin A causes both effects to occur. That is, limbs are both serially reduplicated in the proximodistal axis and mirror imaged in the anteroposterior axis. Time and concentration effects are explored and the significance of these results for our current understanding of axial organisation in limbs is discussed.     

Biosynthesis and transport of envelope proteins of Escherichia coli

Bacterial protein synthesis takes place in the cytoplasm, thus periplasmic and outer membrane proteins pass through the cytoplasmic membrane during their dispatch to the cell envelope. The exported proteins are synthesized as precursor that contains an extra amino-terminal sequence of amino-acids. This sequence, termed "signal sequence", is essential for transport of the envelope proteins through the inner membrane and is cleaved during the exportation process. Various hypotheses for the mechanism have been presented, and it is likely that no signal model will be suitable to the export of all cell envelope proteins. This review is focused on the relationship between the cytoplasmic membrane and the precursor form. The physiological state of the membrane - fluidity, membrane potential for instance - is the strategic requirement of exportation process. Precursors can be accumulated in whole cells with various treatments which alter the cytoplasmic membrane. This inhibition of processing is obtained by modification of unsaturated to saturated fatty acids ratio or with phenylethyl alcohol which perturbs the membrane fluidity, with uncoupler agents such as carbonyl cyanide m-chlorophenyl hydrazone which dissipate the proton motive force, or with hybrid proteins which get jamming in the membrane. However, little is known about the early steps of translocation process across the cytoplasmic membrane ; for instance, it is not clear yet whether energy is required for either or both of the first interaction membrane-precursor and the crossing through the membrane. Several studies have recently shown the presence of exportation sites and of proteins which might play a prominent role in the export process, but the mechanism of discrimination between outer membrane proteins and periplasmic proteins is unknown. Considerable work has been done by genetic or biochemical methods and we have now the first lights of the expert mechanism.     

Ion-specific diffusion rates through transmembrane protein channels. A molecular dynamics study

The migration of different alkali metal cations through a transmembrane model channel is simulated by means of the molecular dynamics technique. The parameters of the model are chosen in close relation to the gramicidin A channel. Coulomb- and van der Waals-type potentials between the ions and flexible carbonyl groups of the pore-forming molecule are used to describe the ion channel interaction. The diffusion properties of the ions are obtained from three-dimensional trajectory calculations. The diffusion rates for the different ions Li+, Na+, K+ and Rb+ are affected not only by the mass of the particles but also very strongly by their size. The latter effect is more pronounced for rigid channels, i.e., for binding vibrational frequencies of the CO groups with v greater than 400 cm-1. In this range the selectivity sequence for the diffusion rates is the inverse of that expected from normal rate theory but agrees with that found in experiments for gramicidin A.     

Interactions of lens proteins. Self-association and mixed-association studies of bovine alpha-crystallin and gamma-crystallin

Concentrated solutions of calf alpha-crystallin (up to 45 g/l) and gamma-crystallin (up to 67 g/l) were subjected to frontal exclusion chromatography at pH 7.3, ionic strength 0.17 and 20 degrees C. The experimental concentration dependence of the weight-average partition coefficient was compared with theoretical expressions, which include considerations of thermodynamic non-ideality effects, for the concentration dependence of a single solute and of a solute undergoing reversible self-association. Two types of association pattern were examined, discrete dimerization and indefinite self-association. The partition chromatography results are consistent with an indefinite self-association of gamma-crystallin, governed by an isodesmic association constant of 6.7 X 10(-3) l/g. alpha-Crystallin appears to self-associate either very weakly, with a maximal association constant of 0.9 X 10(-3) l/g, or not at all; the distinction depends on the assessment of the non-ideality coefficients. The consequences of excluded volume effects on these self-association equilibria at high total protein concentration are discussed. Mixtures of alpha-crystallin and gamma-crystallin were analyzed by frontal exclusion chromatography (up to 14 g/l) and sedimentation velocity (up to 115 g/l): no interaction was observed.     

Experimental study of synchronization phenomena under periodic light irradiation of a nonlinear chemical system

A photosensitive chemical oscillating reaction, i.e., the Briggs-Rauscher (B.R.) reaction, exhibiting a wealth of nonlinear behavior, when performed in a continuous-flow stirred-tank reactor, and subjected to periodic light irradiation, is studied as an experimental example of entrainment phenomena observable in biological systems. The adaptation patterns under periodic light irradiation are elucidated by means of the response of the system to continuous and single-pulse light irradiation. It is shown that self-oscillating states, excitable steady states and bistable systems can exhibit the same types of synchronization patterns when submitted to periodic external forces with appropriate amplitude and time scale conditions.     

Transport properties of rigid, symmetrical oligomeric structures composed of prolate, ellipsoidal subunits

We have calculated translational and rotational diffusion coefficients and intrinsic viscosities of oligomeric structures composed of n identical subunits having a prolate ellipsoidal shape with axial ratio p. Results are presented for p = 1-6 for a variety of structures with n = 1-6. We compare our results with those obtained by a different modeling procedure, proposed by other workers, in which the monomeric subunit is represented as a string of touching, colinear spheres. If n and an estimate of p are known, the structure of the oligomer can be. in most cases, unambiguously determined by comparison of the experimental oligomer-to-monomer ratios of a given property with the numerical results of this work. As examples of the applicability of our results, we examine the relationship between structure and properties for neurophysin. bovine serum albumin, hemoglobin and phycocyanin.     

Cyclic AMP dependent regulation of mitosis in human lymphoid cells

Intracellular levels of cyclic AMP (cAMP), cAMP-dependent phosphodiesterase activity, and adenylate cyclase activity are examined in an established line of human lymphoid cells synchronized by either excess thymidine or by colcemid treatment. cAMP levels and adenylate cyclase activities during the two G periods are high when compared with the values in M. cAMP-dependent phosphodiesterase activity, which is low during early G 2, is shown to increase during G 2 and reach a maximum activity during M. Agents such as dibutyryl cAMP, 1-methyl-3-isobutyl xanthine, noradrenaline, and isopropyl noradrenaline, which increase the levels of intracellular cAMP were examined to determine their effects on mitosis and on DNA synthesis. In thymidine-synchronized cells the onset of mitosis is prevented by increasing or maintaining high levels of cAMP during G 2. The specificity of inhibition of DNA synthesis or mitosis by dibutyryl cAMP is a function of the time, during the cell cycle, when the analogue is added. The elevation of cAMP by methyl xanthine results in a more general inhibition of nucleic acid synthesis and mitosis. Although both catecholamine hormones inhibit mitosis, isopropylnoradrenaline also inhibits DNA synthesis while noradrenaline treatment does not result in such inhibition.