Analysis of the inducible MEL1 gene of Saccharomyces carlsbergensis and its secreted product, alpha-galactosidase (melibiase)

We have determined both the nucleotide sequence of the MEL1 gene of Saccharomyces carlsbergensis and the N-terminal amino acid (aa) sequence of its extracellular gene product, alpha-galactosidase (melibiase) (alpha-Gal). The predicted translation product of MEL1 is a pre-alpha-Gal protein containing an 18 aa N-terminal signal sequence for secretion. The purified enzyme is a dimer consisting of two 50-kDal polypeptides, each of which is glycosylated with no more than eight side chains. The 5'-flank of the MEL1 gene contains a region (UASm) having certain areas of sequence homology to similar sites found upstream of the structural genes GAL1, GAL7 and GAL10, which are also regulated by the action of the products of genes GAL4 and GAL80. There are three TATA boxes between UASm and the initiation codon of pre-alpha-Gal, as well as a typical yeast cleavage/polyadenylation sequence in the 3'-flank of the gene.     

明适应条件下鲤属鱼L-型外水平细胞反应的给光-瞬变成分

在明适应条件下鲤属鱼 L-型外水平细胞的反应显示明显的给光-瞬变成分(on-transient)、它与刺激波长有关——对蓝、绿光的反应比对红光的反应有更明显的瞬变成分,其光谱特性提示它与绿敏锥细胞的输入信号有关。与已报道的其它动物 L 型水平细胞的给光-瞬变成分不同,它的出现在一定范围内与网膜受照射的面积无关。绿色(502nm)和红色(706nm)闪光同时照射所引起反应的给光-瞬变成分比各自单独刺激时要显著得多,提示它也与绿敏锥细胞和红敏锥细胞输入的相互作用有关。     

Three classes of signalling molecules on B-cell membranes

The question of whether surface immunoglobulin and Ia molecules have a signalling function in helper T cell-dependent activation of B cells has been evaluated. Two sources of B cells have been used, one a purified population of hapten-binding B cells, the other a B-cell lymphoma, CH12, with known antigen specificity. Evidence is presented that both immunoglobulin and Ia molecules are receptors actively involved in the initial activation of resting B cells. Nevertheless, the requirements for ligand binding to either receptor can be bypassed under appropriate conditions, and the implications of this result for the function of these molecules is discussed. With respect to B-cell Ia, the authors present data that demonstrate two distinct functions of this molecule, one as a restricting element for T-cell activation, the second as a signalling receptor for B-cell excitation. On the CH12 surface, the I-A molecule fulfills the former function, but T-cell interactions with I-A fail to result in B-cell stimulation, suggesting that B-cell Ia may limit helper T cell-B cell interactions. We suggest that the binding of antigen surface immunoglobulin and binding of helper T-cell receptors to the appropriate Ia molecule(s) results in the activation of genes that encode for a third class of membrane B-cell receptors, those that bind B-cell stimulating factors.     

Temperature-induced modifications in the surface features of stamens of a tomato mutant: An SEM study

Summary Stamenless-2 (sl₂/sl₂) is a temperature-sensitive mutant of tomato (Lycopersicon esculentum) which exhibits altered stamen development under different temperatures (Sawhney 1983). By using scanning electron microscopy, this study was conducted to investigate the differentiation of surface features of mutant and normal stamens grown under different temperatures, with the view to further determine the role of temperature in gene expression in stamen development. Mutant stamens grown under intermediate temperatures (23 °C day/18 °C night) differed from the normal in hair production, the shape of epidermal cells and in the pattern of cuticular thickenings. Under low temperatures (18 °C day/15 °C night), all surface features of mutant stamens closely resembled the normal, whereas under high temperatures (28 °C day/23 °C night), the patterns and types of hairs, epidermal cells, stomata, and cuticular thickenings on mutant stamens were similar to that of a gynoecium. The staminal features of normal stamens were not affected by different temperatures. This study shows that the expression of the sl₂/sl₂ allele is influenced by temperature conditions to the extent that the pattern of cellular differentiation characteristic of either the stamens or the carpels can be induced in mutant stamens.     

Three-dimensional structure of a regular surface layer from Pseudomonas acidovorans

The regular surface layer of Pseudomonas acidovorans was investigated by computer processing of a series of tilted view electron micrographs, and a reconstruction of the three-dimensional structure was obtained. The pattern is tetragonal and consists of massive identical subunits, block-like in face-view, which interlock loosely in a simple cobblestone pattern. The square unit cell has a lattice constant of 11 nm. The surface layer pattern of P. acidovorans appears to be more dependent on the underlying membrane for maintaining its integrity than those so far studied in other bacteria.     

The versatility of proteolytic enzymes

The growing realization of their physiological importance has generated renewed interest in the study of proteolytic enzymes. Modern methods of protein chemistry and molecular biology have revealed new insights into the protein and gene structure of a variety of protein precursors and their processing by limited proteolysis. Examples are given in this review for transmembrane processes and the role of signal peptidases of both eukaryotic and prokaryotic origin, the processing of prohormones and precursors of growth factors, protein components of blood coagulation, fibrinolysis, and of the complement system, and a group of granulocyte proteases, including the mast cell serine proteases. The relationship of homologous domains found in many of these proteases and their zymogens to protein evolution is a recurrent theme of this discussion.     

Capacitance--voltage relationship in phospholipid bilayers containing gangliosides

Changes in the position of the minimum of the parabolic capacitance-voltage curve allow the measurement of the amount of ganglioside present in artificial bilayers made with phosphatidylcholine-ganglioside mixtures and asymmetrically shielded with Ca2+. The screening effect of the ionic solution must be considered. With ganglioside/phospholipid molar ratios of up to 15%, all glycolipids can be found at the membrane surfaces.     

Effect of lipid phase transition on the binding of anions to dimyristoylphosphatidylcholine liposomes

Temperature dependence of the electrophoretic mobility of multilamellar liposomes prepared from dimyristoylphosphatidylcholine was measured in the presence of salts with different anions in aqueous solutions. It was established that specific binding of anions to liposome surface induced a pronounced zeta potential (electrostatic potential at the hydrodynamic plane of shear). A combination of Langmuir, Gouy-Chapman, and Boltzmann equations was used to describe the dependence of the zeta potential on the concentration of anions. The values of binding constants (K) and maximum numbers of binding sites per unit area (σ_max) were determined by this method. The sequence for anion affinities to liposome surface was found to be as follows: trinitrophenol >ClO₄⁻ >I⁻ >SCN⁻ >Br⁻ >NO₃⁻ >Cl⁻ SO₄²⁻. A sharp increase in the negative zeta potential was detected at the temperature of phase transition of the lipid from the gel to liquid-crystalline state. It was found that the parameter K did not change at lipid phase transition and the shifts in zeta potential might be due to alterations of σ_max. The binding sites were considered as defects in the package of lipid molecules in membranes.     

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.     

Diffusible IAA and dominance phenomena in fruits of apple and tomato

The relationships between indole-3-acetic acid (IAA) diffusing out of the fruit and competition among fruits, and between fruits and shoot tips were investigated using apple ( Malus domestica Borkh. cv. Jonagold) and tomato ( Lycopersicon esculentum Mill.) plants. Dominant fruits always had more diffusible IAA than subordinated, inhibited fruits. Alterations in dominance – by fruit- or shoot tip removal – led to significant changes in diffusible IAA by the remaining fruits. This change could be detected one day after dominance modification.
It is suggested that diffusible IAA is involved in the correlative signal regulating dominance relationship between fruits, and between fruits and shoots in apple and tomato.