“Pleiotypic Response”

A hypothesis has been developed to relate stringent control in bacteria to a set of interactions involved in the regulation of growth of transformed and untransformed mammalian cells.     

Inhibition of DNA Synthesis but not of Poly-dAT Synthesis by the Arabinose Analogue of Cytidine <Emphasis Type="Italic">in vitro</Emphasis>

Kimball and Wilson¹ reported that the arabinose analogue of cytidine (ara-C) inhibited DNA polymerase in a crude extract prepared from Ehrlich ascites cells. Furth and Cohen² observed cytosine arabinoside triphosphate (ara-CTP) inhibited DNA polymerase in extracts from either calf thymus or bovine lymphosarcoma tissue, although these investigators³ had already found no effect of ara-CTP on DNA polymerase from Escherichia coli. The inhibition in both of these cases could be substantially reversed by dCTP; but incorporation of the arabinose nucleotide (ara-CMP) into DNA could not be unequivocally demonstrated. Graham and Whitmore⁴ reported the incorporation of ara-C into DNA in vivo and the inhibition of a DNA polymerase from L cells by ara-CTP. They found that ara-CMP was initially incorporated into small DNA strands but subsequently appeared in long strands. Momparler⁵ has presented evidence that, in vitro, ara-C incorporation was limited to the 3′-hydroxyl end of DNA chains. Such incorporation might be expected to block further chain elongation but this expectation was not supported by the evidence presented by Graham and Whitmore.     

Endopeptidase Activity of Phage λ-Endolysin

JACOB and Fuerst^1,2 demonstrated the presence of a bacteriolytic enzyme (λ-endolysin) in the induced cultures of lysogenic Escherichia coli K12 (λ). The enzyme was later identified as the product of gene R; of phage λ³ which is involved in bacterial lysis at the end of a latent period. The enzyme is apt to form spheroplast-like structures in E. coli² and one would therefore expect its substrate to be murein.     

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.