Evolution de la musculature desNéréidiens (Annélides polychètes) au cours de l'Epitoquie

Résumé Au cours de l'épitoquie des Nereidiens, les fibres musculaires longitudinales ne sont pas forméesde novo, à partir de cellules indifferenciées ou myoblastes, mais proviennent des fibres anciennes atoques. Celles-ci subissent une véritable dédifférenciation plus ou moins synchrone d'une redifférenciation. Les deux processus ne sont pas successifs mais simultanés, et une dédifférenciation complète est absente.Les premières cellules en évolution appartiennent à la couche musculaire externe; ensuite, les fibres des assises plus profondes se transforment à leur tour.Les transformations consistent en: 1) La dédifférenciation du bord interne ou coelomique de la fibre. Les structures contractiles disparaissent dans cette zone et de nombreuses particules de glycogène se différencient sans relation avec le reticulum endoplasmique ou les ribosomes. Aucun lysosome ou signe précurseur ne peuvent être observés avant la disparition des filaments contractiles et des éléments Z. 2) Le bord coelomique s'hypertrophie. Dans la région axiale de la fibre, de nombreuses mitochondries et particules et de glycogène remplacent le matériel contractile. Corrélativement, l'épaisseur des bandes A et I diminue. 3) La fibre hétéronéreidienne ou épitoque est constituée et présente deux parties: un cortex myoplasmique et une médulla sarcoplasmique, remplie de mitochondries et de glycogène. Le noyau renfermant un nucléole volumineux est situé dans une hernie sarcoplasmique latérale.

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Evolution of muscles inNereidae (Annelida polychaeta) during Epitoky. III. Dedifferentiation of the longitudinal fibres

Summary During epitoky inNereidae, the longitudinal muscle fibres are not formedde novo from undifferentiated cells or myoblasts, but arise from the old atokous fibres. These undergo a true dedifferentiation more or less synchronously with a redifferentiation. The two processes are not successive but simultaneous and there is no complete dedifferentiation.The first cells that develop are in the outside muscle layer; then the fibres of the inside layers are transformed in their turn.The transformations consist of: 1) Dedifferentiation of the edge of the inner or coelomic fibre. The contractile structures disappear in this part and numerous glycogen particles differentiate, unrelated to endoplasmic reticulum or ribosomes. No lysosomes or precursory markings are observed before the disappearance of contractile filaments and Z rods. 2) The coelomic edge becomes enlarged. In the axial region of the fibre, numerous mitochondria and and glycogen particles take the place of the contractile material. Consequently, the thickness of A and I bands decreases. 3) The heteronereid or epitokous fibre is formed and shows two parts: a myoplasmic cortex and a sarcoplasmic medulla, filled with mitochondria and glycogen. The nucleus with a voluminous nucleolus settles inside a lateral sarcoplasmic swelling.

     

Regioelectronic factors in metabolic hydroxylation of aliphatic carbon atoms

EHT calculations have been performed on model molecules acting as substrates for mammalian mono-oxygenases. C_α---H bonds are consistently found to have larger overlap populations compared with C_β---H and C_γ---H bonds. It is known on the other hand that metabolic hydroxylation of aliphatic carbon atoms shows a marked regioselectivity for α-carbons. The quantum-mechanical results sustain the view that C---H bonds of relatively high electronic density are preferred target sites for the cytochrome P-450-mediated oxygenation, and that the oxygen atom being activated is transformed into an electrophilic species capable of C---H bond insertion.     

Influence of dicarboxylic phosphatidylcholines on phosphatidylcholine liposomes as revealed by gel chromatography and electron microscopy

The effect of dicarboxylic phosphatidylcholines (glutarylphosphatidylcholine) on the structural changes of phosphatidylcholine liposomes is examined by using multilamellar liposomes prepared with egg phosphatidylcholine or dipalmitoylphosphatidylcholine and by varying the surface charge by addition of dicetyl phosphate. Investigations are performed by gel chromatography and electron microscopy. Glutarylphosphatidylcholine is in micellar form (rod-like micelles or globular micelles). The structures obtained depend on the fatty acid saturation of liposomes and on the charge of liposome (addition or not of dicetyl phosphate). With egg phosphatidylcholine/glutarylphosphatidylcholine dispersions, an aspect more similar to myelinic figures than liposomes is observed, while in the presence of dicetyl phosphate, liposomes similar to control egg phosphatidylcholine liposomes are obtained. Gel chromatography on Sepharose 4B and turbidity measurements prove that dicetyl phosphate increases the stability of egg phosphatidylcholine/glutarylphosphatidylcholine mixtures. On the other hand, in dipalmitoylphosphatidylcholine/glutarylphosphatidylcholine dispersions, incorporation of dicetyl phosphate destabilizes bilayer structure and the formation of mixed micelles occurs. Viscosity measurement shows, in the presence of dicetyl phosphate, an increased fluidity for dipalmitoylphosphatidylcholine/glutarylphosphatidylcholine dispersions, in agreement with the micellar organization. These data confirm that the disorganization of liposomal membranes by dicarboxylic phosphatidylcholine depends on the fatty acid composition of phosphatidylcholine and on the presence of dicetyl phosphate.     

Characterization of messenger RNA for aldolase B in adult and fetal human liver

High molecular weight cellular RNA was isolated from adult and fetal human liver tissue by a procedure of ethanol precipitation in concentrated guanidine-HCl solutions. About 5 mg of RNA were obtained from one gram of liver. RNA was fractionated by sucrose gradient ultracentrifugation. Aldolase B neosynthesized in a reticulocyte lysate cell-free system under the direction of total or fractionated RNA was purified by immunoaffinity microchromatography. Messenger RNA specifying synthesis of aldolase B exhibited a sedimentation coefficient of 16 S both in adult and fetal liver. This enzyme represented 1.3 % of the total neosynthesized proteins in adult liver, 0.1 % in the liver of a 6-month-old fetus and less than 0.01 % in the liver of a 4.5 month-old fetus.     

Mitochondrial DNA evolution in lagomorphs: Origin of systematic heteroplasmy and organization of diversity in European rabbits

Summary A characterization was conducted on mitochondrial DNA (mtDNA) molecules extracted separately from 107 European rabbits (Oryctolagus cuniculus) both wild and domestic, 13 European hares (Lepus capensis), and 1 eastern cottontail (Sylvilagus floridanus). Experimentally this study took into account restriction site polymorphism, overall length variation of the noncoding region, and numbers of repeated sequences. Nucleotide divergences indicate that the mtDNAs from the three species derived from a common ancestor some 6–8 million years (Myr) ago. Every animal appeared heteroplasmic for a set of molecules with various lengths of the noncoding region and variable numbers of repeated sequences that contribute to them. This systematic heteroplasmy, most probably generated by a rate of localized mtDNA rearrangements high enough to counterbalance the cellular segregation of rearranged molecules, is a shared derived character of leporids.The geographic distribution of mtDNA polymorphism among wild rabbit populations over the western European basin shows that two molecular lineages are represented, one in southern Spain, the second over northern Spain, France, and Tunisia. These two lineages derived from a common ancestor some 2 Myr ago. Their present geographical distribution may be correlated to the separation of rabbits into two stocks at the time of Mindel glaciation.Finally the distribution of mtDNA diversity exhibits a mosaic pattern both at inter- and intrapopulation levels.     

Biosynthesis and structure of membrane and secretory immunoglobulins

Summary Almost all of the body's extracellular immunoglobulin (Ig) is derived from Ig-secreting plasma cells of lymphoid tissues. The secreted material is a heterogeneous mixture of different classes and specificities. Lymphoid tissues also contain a large number of essentially non-secretory cells — B lymphocytes — which bear Ig firmly associated with their plasma membranes. Ig molecules thus exist in two functionally different forms, as membrane-bound antigen receptors on the surface of B lymphocytes on the one hand, and as humoral secreted Ig antibodies on the other. On B cells, membrane-bound heavy chains have an apparent mol. wt. slightly larger than that of secreted heavy chains from plasma cells. Membrane-bound but not secreted heavy chains bind detergents, thus suggesting the presence of a hydrophobic region in membrane-bound heavy chains, which is absent in secreted heavy chains. Most investigations have dealt with immunoglobulin M. The two types of IgM heavy chains differ at their carboxy termini. Recent investigations at the nucleic acid level demonstrate that membrane-associated µ chains contain a 41-residue hydrophobic tail adjacent to the last constant domain, whereas secretory µ chains contain a 20-residue hydrophilic tail. At the present time, evidence is accumulating that all membrane-bound Ig heavy chain classes may contain similar hydrophobic structures necessary for anchorage of the molecules into the lipid bilayer.     

Construction of hybrid plasmids containing the lysA gene of Escherichia coli: studies of expression in Escherichia coli and Saccharomyces cerevisiae

Summary The lysA gene of Escherichia coli has been cloned from a transducing phage on various plasmids, present in different copy numbers in bacterial cells. Synthesis of the product of this gene, diaminopimelate (DAP)-decarboxylase, and its regulation have been studied. Expression does not follow a simple gene dosage effect, maximal expression already being obtained with a six-copy plasmid. This result suggests that either a positive or an autogenous regulatory mechanism is involved. We also used one of the hybrid plasmids to look for expression of the bacterial lysA gene in Saccharomyces cerevisiae. The results indicate that the product of the E. coli gene is not actively translated in yeast.     

Iron-porphyrin catalysis of the oxidative dealkylation of para-nitro-anisole and 7-ethoxycoumarin by cumylhydroperoxide: a possible model for the corresponding cytochrome P 450-dependent reactions

A biphasic system containing an iron porphyrin, Fe (TPP) (C1)¹ or [Fe(TPP)]₂O, efficiently catalyzes the cumyl-or tertiobutyl-hydroperoxide-supported dealkylation of p-nitroanisole and 7-ethoxycoumarin to the corresponding phenol and formaldehyde. Stoichiometric amounts of iron porphyrin and hydroperoxide give a quantitative reaction. Catalytic amounts of iron porphyrin give reaction rates and yields which are proportional to substrate concentration. With increasing hydroperoxide concentrations, the rates level offto limit values and the yield rapidly decreases. The maximum rates obtained approach those of the reactions mediated by cytochrome P 450-dependent monooxygenases.