Binding of β-lactam antibiotics to the exocellular dd-carboxypeptidase–transpeptidase of Streptomyces R39

Benzylpenicillin and cephaloridine reacted with the exocellular dd-carboxypeptidase–transpeptidase from Streptomyces R39 to form equimolar and inactive antibiotic–enzyme complexes. At saturation, the molar ratio of chromogenic cephalosporin 87-312 to enzyme was 1.3:1, but this discrepancy might be due to a lack of accuracy in the measurement of the antibiotic. Spectrophotometric studies showed that binding of cephaloridine and cephalosporin 87-312 to the enzyme caused opening of their β-lactam rings. Benzylpenicillin and cephalosporin 87-312 competed for the same site on the free enzyme, suggesting that binding of benzylpenicillin also resulted in the opening of its β-lactam ring. In Tris–NaCl–MgCl₂ buffer at pH7.7 and 37°C, the rate constants for the dissociation of the antibiotic–enzyme complexes were 2.8×10⁻⁶, 1.5×10⁻⁶ and 0.63×10⁻⁶s⁻¹ (half-lives 70, 130 and 300h) for benzylpenicillin, cephalosporin 87-312 and cephaloridine respectively. During the process, the protein underwent reactivation. The enzyme that was regenerated from its complex with benzylpenicillin was as sensitive to fresh benzylpenicillin as the native enzyme. With [¹⁴C]benzylpenicillin, the released radioactive compound was neither benzylpenicillin nor benzylpenicilloic acid. The Streptomyces R39 enzyme thus behaved as a β-lactam-antibiotic-destroying enzyme but did not function as a β-lactamase. Incubation at 37°C in 0.01m-phosphate buffer, pH7.0, and in the same buffer supplemented with sodium dodecyl sulphate caused a more rapid reversion of the [¹⁴C]benzylpenicillin–enzyme complex. The rate constants were 1.6×10⁻⁵s⁻¹ and 0.8×10⁻⁴s⁻¹ respectively. Under these conditions, however, there was no concomitant reactivation of the enzyme and the released radioactive compound(s) appeared not to be the same as before. The Streptomyces R39 enzyme and the exocellular dd-carboxypeptidase–transpeptidase from Streptomyces R61 appeared to differ from each other with regard to the topography of their penicillin-binding site.     

A nuclease from Neurospora crassa specific for d(A-T) rich regions in double stranded DNA

A nuclease from N. crassa has been prepared to the hydroxylapatite stage of purification described by Rabin and Fraser (1). It degrades single stranded DNA in an essentially exonucleolytic process. It does not give any appreciable acid soluble material with double stranded DNA as substrate. This shows its high degree of specificity towards single stranded DNA.     

Effects of nitrogen,phosphorus and potassium fertilizers on the assimilation capacity ofBeta vulgaris chloroplasts (I)

Summary Aspects of non-cyclic photophosphorylation and NADP photoreduction,viz (a) the effects produced on these processes by the three fertilizer elements: nitrogen, phosphorus and potassium; (b) variations in the catalase activity of reaction mixtures following fertilizer application, and (c) correlations between photosynthesis as measured on leaf-tissue discs and the assimilation capacity of chloroplast suspension, were studied. The role of catalase in the non-cyclic photophosphorylation processes was also studied.While photophosphorylation is influenced chiefly by the level of available soil phosphorus, NADP reduction is affected by all three nutrients. In addition, there was a greater degree of significance, for diagnostic and application purposes, in the values obtained if these two activities were referred to the chloroplast count rather than to the chlorophyll content.Catalase activity, in addition to responding in a different way to the respective fertilizer treatments and, in particular to available soil nitrogen, was governed by the principal constituents of the reaction mixture and in a manner contrary to that of non-cyclic photophosphorylation as measured in terms of oxygen evolution.Experimental findings further showed that photosynthesis is correlated chiefly with NADP-reduction capacity.     

Lipotropha dorci n. sp. [Neogregarinida], parasite des larves deDorcus parallelipipedus L. [Coleopt. scarab.]

Résumé Des larves deDorcus parallelipipedus L. provenant de Saint-Hilaire-de-Brethmas (Gard) dans la région méditerranéenne de France et récoltées dans des troncs décomposés de chataigniers, se sont révélées intensément parasitées par des Grégarines appartenant à l'ordre desNeogregarinida. Nous étudions leur localisation dans le tissu adipeux, leur cycle, l'action pathogène sur l'h?te et discutons leur place systématique.

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Summary Lipotropha dorci n. sp. (Neogregarinida) is a parasite developing inDorcus parallelepipedus. The parasite is situated only in the fat body which can be destroyed completely by its activity. The life cycle ofLipotropha includes only one type of schizogony with 3–4 μ long merozo?ts, the spherical uninucleate gametocytes are gathered in pairs and form 23 μ long gametokysts. These give rise to 11 to 12 μ long sporocysts with a thick, non ornamented envelope, gathered round rests of cytoplasma. The size of these different stages enables to distinguishLipotropha dorci from the already described species:Lipotropha macrospora Keilin andLipotropha microspora Keilin.