Showing papers by "Université libre de Bruxelles published in 1970"

Multiplicity of the Amino Acid Permeases in Saccharomyces cerevisiae IV. Evidence for a General Amino Acid Permease

Abstract: Kinetic and genetic evidences are presented to show that, in addition to specific amino acid permeases, Saccharomyces cerevisiae has a general amino acid permease which catalyzes the transport of basic and neutral amino acids, but most probably not that of proline. The general amino acid permease appears to be constitutive, and its activity is inhibited when ammonium ions are added to the culture medium. A mutant which has lost the general amino acid permease activity was isolated. Its mutation, named gap (general amino acid permease), is not allelic to the aap (amino acid permease) mutation of Surdin et al., which has a quite different phenotype and cannot be considered as having selectively lost the general amino acid permease activity.

GROWTH AND DIFFERENTIATION OF CYTOPLASMIC MEMBRANES IN THE COURSE OF LIPOPROTEIN GRANULE SYNTHESIS IN THE HEPATIC CELL I. Elaboration of Elements of the Golgi Complex

Abstract: The synthesis of "very low" density lipoprotein in liver cells is characterized by the fact that the synthesized products, mostly triglycerides, are processed in the form of discrete, size-limited granules or globules, about 400 A in diameter. The present investigation has been made possible in part by the use of a fixative (OsO4 in bidistilled H2O at pH 6.0, in the absence of electrolytes) particularly effective in preserving cytoplasmic membranes and lipids, and giving them high stainability and differential contrast. Under these technical conditions, the lipoprotein granules retain their morphology and high density to electrons practically unaltered, and may serve as tracers in determining their route of transport from the sites of synthesis, starting at the rough-smooth ER junctions, to the lumen of Golgi concentrating vesicles. From the observations, it may be deduced that, along with lipoprotein granule synthesis and transport, there are also production and transfer of new membranes in the form of tubular extensions of smooth ER network which, by progressive fusion and coalescence, participate in the elaboration of fenestrated plates and solid Golgi sacs. In contradistinction to the entire process of liver lipoprotein granule synthesis, transport, and segregation, as reported in the present paper, appears to constitute a developmental sequence which includes the following communicating compartments, in consecutive order: cisternae of rough ER where proteins and possibly phospholipids are synthesized, smooth ER network where triglycerides are synthesized and transported in the form of dense granules, fusion of smooth ER tubular extensions into Golgi fenestrated plates, and further coalescence into solid Golgi sacs, ending in the segregation of the granules in appended concentrating vesicles, or detached "secretory vesicles." It seems that it is this progressive evolution in growth and configuration of membranes which is reflected in the so called polarity, from forming to mature faces, of the Golgi apparatus.

Membrane excitability and dissipative instabilities.

Abstract: Electrical excitation is interpreted in terms of a cooperative structural transition of membrane protomers coupled with the translocation of a permeant molecule in a non-equilibrium environment. Equations for flow of permeant and for membrane conformation are derived for the simple case of a single non-charged permeant. On the basis of a few simple physical assumptions, the theory predicts several important properties of electrically excitable membranes: the steepness of the relation between membrane conductance and potential, the presence of a negative conductance, and the occurrence of instabilities following rapid perturbations of membrane environment, giving rise to some simple cases of action potentials. Several experimental tests of the membrane with its changes of electrical properties are proposed. From a thermodynamic point of view, an electrically excitable membrane, in its resting state, lies beyond a dissipative instability and consequently is in a non-equilibrium state but with stable organization, a “dissipative structure” of Prigogine. Membrane excitation following a small perturbation of the environment would correspond to a jump from such an organization to another stable organization but close to thermodynamic equilibrium. It is shown how the cooperative molecular properties of the membrane are amplified by energy dissipation at the macroscopic level.

Temperature dependence of the linewidth in critical spin fluctuation

Abstract: Starting from the kinetic equations obeyed by the time-dependent spin-correlation functions, we have calculated the homogeneous function which appears in the dynamical scaling expression for the linewidth of critical fluctuations in ferromagnets and antiferromagnets. In agreement with recent experimental findings, this function shows a minimum for κq inverse correlation length/wavenumber 0. © 1970 The American Physical Society.

Interactions of cytochromec with phospholipid membranes : Binding of cytochromec to phospholipid liquid crystals.

Abstract: Cytochromec added during the formation of lecithin-cardiolipin liquid crystals in 0.015M KCl is readily bound. After successive washings with 0.15M KCl, only about 50% of this bound cytochromec is removed. The remaining cytochromec is resistant to further salt extraction, and the amount of this cytochromec that is bound varies with the concentration of added cytochromec to a maximum binding ratio of 1∶70, mole ratio cytochromec to phospholipid. This binding appears to be electrostatic; it is competitively inhibited by increasing the initial molarity of KCl from 0.015 to 0.10M. Binding of cytochromec is insignificant in the absence of cardiolipin, and is affected by varying the pH. Electron microscope studies of osmium tetroxide-stained thin sections show that the liquid crystals consist of vesicles, each of which contains a large number of concentric, alternating light and dense lines. The dense lines have been identified by other workers with the polar head groups of the phospholipids on the surface of a bilayer, and the light area represents the hydrophobic interior. The addition of cytochromec causes an average decrease in the number of lines per vesicle. It increases the center-to-center distance between two neighboring light or dense lines and the width of the dense lines. On the basis of this evidence and electrostatic binding, it is concluded that cytochromec is binding on the polar surfaces of the phospholipid bilayers comprising the liquid crystalline vesicles.

On linearized hydrodynamic modes in statistical physics

Abstract: We formulate the linearized generalized Boltzmann equation as an (asymmetric) eigenvalue problem. This problem has five eigenvalues which tend to zero when the uniformity parameter tends to zero: to second order in this parameter, they correspond to damped sound (two modes), diffusing shear flow (two modes), and diffusing entropy flow (one mode). The microscopic expressions deduced from these results for the transport coefficients agree with the correlation-function formulas. Moreover, the corresponding eigenfunctions are explicitly displayed to lowest order in the uniformity parameter: they are microscopic analogs, in terms ofone-particle distribution functions, of the well-known linearized hydrodynamic modes of macroscopic physics. All results are established to all orders in the interactions.

Relations physiologiques entre les phages tempérés λ et Φ80

Abstract: This work deals with the ability of phage Φ80 to provide defective mutants of λ with their missing functions. Functions Involved in Recombination. As shown by others, the Int mechanism of Φ80 cannot excise prophage λ. However, Φ80 efficiently excises recombinants from tandem dilysogens, using its Ter mechanism. Likewise, the nonspecific mechanism Red is interchangeable between Φ80 and λ. Maturation of λ DNA by Φ80. The “Ter recombinants” excised by Φ80 from tandem dilysogens are packaged into a Φ80 protein coat. This contrasts with the fact, already mentionned by Dove, that Φ80 is extremely inefficient for packaging phage λ superinfecting a λ-lysogen. The latter result is also found when the helper phage is a hybrid with the left arm of λ (Φ80hy4 or Φ80hy41 — see Fig. 1). However, the maturation of the superinfecting λ is much more efficient if the Φ80hy used as a helper has the att-N region of λ (like Φ80hy1). Conversely a λ with the att-N region of Φ80 (λhy6 — see Fig. 1) is packaged more efficiently by Φ80 or Φ80hy4 than by Φ80hy1. It is suggested that the maturation of λ chromosome superinfecting an immune cell requires a recombination with the helper phage. Vegetative Functions. Among the replicative functoons O and P, the latter only can be supplied by Φ80. That λN mutants are efficiently helped by Φ80 does not tell that Φ80 provides the defective λ with an active N product; the λ chromosomes are simply packaged into a Φ80 coat. This shows that Φ80 is unable to switch on the late genes of λ. That neither Φ80 nor any of the Φ80hy tested can provide an active N product is shown in a more direct way by their complete failure to help λN -r14; this phage carries a polar mutation which makes the expression of genes O and P entirely N-dependant. The maturation of a λN - by Φ80 contrasts with the fact that mutants affected in late genes (A, F or H) are not efficiently helped by Φ80. This suggests that the products coded by these genes are not interchangeable between Φ80 and λ, and that packaging of λ DNA into Φ80 coats is possible but inhibited when λ late proteins are present in the cell. Activation of the Late Genes. Among the im ϕ80 h λ + hybrids tested, only Φ80hy41 is able to switch on the late genes of a λN defective mutant. This hybrid differs from the other hybrids studied here, by the fact that it has the Q-S-R region of λ (see Fig. 1). The results are consistant with the view that the product of Q gene is sufficient for activating the late genes of a λ DNA. N would thus control the expression of late genes only indirectly by controlling the expression of gene Q (Couturier & Dambly have independantly reached the same conclusion, 1970). Furthermore the failure of Φ80 and of the Φ80hy1 and Φ80hy4 to activate the late genes of λ would imply that these phages are unable to provide an Q product active on the λ chromosome Reciprocally, λ switches on the late genes of prophage Φ80hy41, but not of prophages Φ80hy1 and Φ80hy4. This suggests that the initiation of late genes expression takes place at a main specific site located in the Q-S-R region of the λ chromosome. The expression of the late genes would thus be sequential, and proceed through the left arm only when steaky ends cohere. Similar conclusions were reached independantly by Toussaint (1969) and by Herskowitz and Signer (1970).

Self-fertility in maritime and zinc mine populations of armeria maritima (mill.) willd.

Abstract: There is evidence for considerable variability in breeding systems of plants. This variability can be seen in many ways: closely related species often differ from each other in their breeding systems; cultivated plants may differ from their wild ancestors; differences have been recorded between individuals within populations; selection for change in breeding systems has been successful on many occasions (Stebbins, 1957; Antonovics, 1968a). All this is powerful evidence for the idea that in plants the breeding system of a species is not a fixed and rigid characteristic but is capable of change under the influence of selective processes. If this is so we could expect to see evidence of this, particularly within a species which occupies a range of habitats, where particular environments or situations may evoke genetical modifications to the breeding system. Support for these arguments has been given by many people (e.g. Darlington and Mather, 1949). Recently Antonovics (1968a) has shown a high level of self-fertility in populations of two grass species inhabiting toxic waste heaps which he suggests has arisen as a result of selection for reduced gene flow. The present paper reports the situation to be found in Armeria maritima (Mill.) Willd., a species complex found in maritime areas and in inland zinc contaminated calamine areas of Europe.

Time sequence of ultrastructural changes in the stimulated dog thyroid.

Abstract: The kinetics of the effects of acute administration and of multiple injections of TSH on the ultrastructure of the thyroid of the dog were studied. The results are correlated with data obtained by light microscopy and by biochemical study of the tissue. The response to TSH is more delayed in the dog than in the rat; it is characterized by interfollicular and intrafollicular heterogeneity: only after 3 hours do all thyroid cells demonstrate signs of stimulation. The stimulation of each thyroid cell results in three overlapping stages: colloid phagocytosis, colloid digestion and morphological changes suggesting increased biosynthetic activity. The former effects are concomitant with the effects of TSH on the secretory process in vivo and the metabolic activation of thyroid slices in vitro; the latter effect parallels an increased RNA content of the tissue. Thyroids stimulated for 3 and 6 days by TSH exhibit marked hyperplasia and cellular hypertrophy. The colloid is scant and no colloid droplets are evident. After three days of TSH administration, numerous intracellular spaces, containing colloid are observed.

Convection regions and coronas of sun and stars

Abstract: Photospheric models were calculated for 90 stars with effective temperatures between 2500 K and 41600 K for five logg-values ranging from 1 to 5. Molecule formation was taken into account. In order to have an idea about possible instabilities in the different stellar layers some quantities, characteristic for convection and turbulence were calculated, such as the Rayleigh-, Reynolds-, Prandtl- and Peclet-numbers. It turned out that all the investigated stars contain unstable layers, including the hottest. Nevertheless, only stars with effective temperatures of 8300 K or less contain layers where the convective energy transport is important. For all stars the convective velocities were calculated and also the generated mechanical fluxes in the convection zones were tabulated.

Radiolysis of polymethylmethacrylate in the presence of ethylmercaptan. I. Main chain scission and volatile products formation

Abstract: Main chain scission yields Gs are, respectively, 0.88 and 2.6 for polymethylmethacrylate (PMM) irradiated at 77 and 313°K. G values for volatile products (mainly CH4, CO, CO2, HCOOCH3) were measured. The ratio of methyl containing volatiles to main chain scission yield is about one at both temperatures. In the presence of ethylmercaptan, very efficient protection is observed for main chain scission whereas the amount of volatile products is little or not affected. These results show that side chain scission and main chain breaking are closely related and that the former process precedes and induces the latter. A mechanism is proposed. Die G-Werte der Hauptkettenspaltung (Gs) von Polymethylmethacrylat (PMM), das bei 77 und 313°K bestrahlt wurde, betragen 0,88 bzw. 2,6. Es wurden die G-Werte fur fluchtige Produkte (hauptsachlich CH4, CO, CO2, HCOOCH3) bestimmt. Das Verhaltnis von Methylgruppen enthaltenden, fluchtigen Stoffen zum Ausmas der Hauptkettenspaltung ist fur beide Temperaturen etwa 1. Athylmercaptan ubt einen sehr wirksamen Schutz auf die Hauptkettenspaltung aus, wahrend die Menge der fluchtigen Produkte wenig oder gar nicht verandert wird. Diese Ergebnisse zeigen, das die Seitenkettenabspaltung und die Hauptkettenspaltung in enger Beziehung stehen und das der erstgenannte Prozes vorangeht und den letzteren induziert. Es wird ein Reaktionsmechanismus vorgeschlagen.