Acceleration of Senescence induced in Wheat by Light

CELLS from patients with G-trisomy or E-trisomy and XXY cells from patients with XY/XXY mosaic Klinefelter's syndrome are more susceptible to transformation in vitro by SV40 than are cells from normal individuals^1–3. We have used triploid (69XXY) human cells to determine whether the presence of extra chromosomes per se increases susceptibility to transformation.     

Cytidine Deaminase and the Development of Resistance to Arabinosyl Cytosine

THE effectiveness of arabinosyl cytosine (cytarabine: 1-β-D-arabinofuranosylcytosine, ‘Cytosar’, abbreviated to ara-C) in the treatment of human leukaemia can be correlated with the rate of uptake of the molecule in vitro¹ and its subsequent phosphorylation. In man, this nucleoside is also rapidly deaminated to produce arabinosyl uridine which has no therapeutic activity²: both liver and serum contain pyrimidine nucleoside deaminase³. Consequently the half-life of unchanged ara-C in human sera is brief (about 15 min^?1) (ref. 2). These characteristics suggest a relationship between cytidine deaminase and the susceptibility of the leukaemic cell to ara-C and we have found that initial therapeutic responses in human leukaemia are correlated with lower intracellular concentrations of deaminase than those present in non-responders., Furthermore, in sequential treatment schedules, increasing concentrations of deaminase are associated with markedly reduced susceptibility of the tumour cell to ara-C.     

Demonstration of an “Excitation-Contraction Recoupling” Mechanism in Mammalian Ventricular Myocardium

A PROCESS called “excitation-contraction coupling” has been generally accepted to take place only in the direction of excitation to contraction. Through this mechanism a propagated action potential initiates an active state in skeletal or cardiac muscle and the muscle contracts. We propose that, in the mammalian ventricular myocardium at least, the process is not unidirectional and an important reverse coupling between the contractile system and the excitable plasma membrane has been overlooked. Through this feedback interaction the mode of contraction (that is, isotonic or isometric) not only determines the instantaneous electrical state of the plasma membrane, but also influences the mechanical events of the subsequent beats. Thus when Kaufmann et al.¹ recorded intracellular action potentials from cat papillary muscle, the time course of the repolarization was altered depending on the mode of contraction. Some kind of contraction-excitation feedback has also been suggested by Stauch² and Lab^3,4. They showed a difference in the shape of the monophasic action potential, as recorded by a suction electrode, when comparing isotonic and isovolumic contraction of the intact ventricle. But their experimental conditions did not allow satisfactory analysis of the phenomenon.     

Bicuculline and its Effect on γ-Amino Butyric Acid Transaminase in the Guinea-pig Brain Stem

IT has been claimed that the inhibitory effect of γ-amino butyric acid (GABA) is antagonized post-synaptically by the alkaloid bicuculline^1–3, although others⁴ have been unable to demonstrate this in feline cortical neurones. When applied through a microtap, GABA has a profound inhibitory effect on many cochlear nucleus⁵ and other brain stem neurones⁶ and the brain stem is also rich in GABA-transaminase (GABA-T) (ref. 7 and our unpublished results), the enzyme catalysing the initial step in the degradation of GABA. Although there is little doubt that GABA-T is largely a mitochondrial enzyme^8,9, there is considerable activity in purified nerve ending fractions. The data of Salganicoff and de Robertis¹⁰ indicate that two iso-enzymes of GABA-T exist, one in free mitochondria, the other in nerve ending mitochondria.     

Structure of the “Peroxy-Y Base” from Liver tRNA<Superscript>Phe</Superscript>

WE wish to report that reconstituted sperm whale myoglobin prepared by the method of Breslow¹ (except that pH 2 was found sufficient to remove all the haem) (I) crystallizes² in a different habit from those prepared by the method of Rossi-Fanelli et al.³ (II) using haemin of Sigma lot 77B-0220 and our own ⁵⁷Fe photoporphyrin preparation and the native myoglobin (III). Although all three form type A³ monoclinic prisms, the best developed plane is [001] for II and III, it is [100] for I. There seems to be great interest in reconstituted haemoproteins^4,5, so it is important that crystallization habit may be a sensitive test for subtle changes in protein structures.     

Effect of Maternal Age on DNA and RNA Content of <Emphasis Type="Italic">Drosophila</Emphasis> Eggs

THE influence of maternal age on the characteristics of offspring may appear quite early where egg size and the time of first larval hatching are concerned^1–4. We have therefore investigated the DNA and RNA content of eggs produced by ageing Drosophila melanogaster females (Nettlebed inbred strain). We used virgin eggs, which do not degenerate even several hours after laying⁵ and which therefore do not require immediate collection. We have measured DNA content by a fluorometric technique⁶ and RNA content by the orcinol method.     

Cooperative Non-enzymatic Base Recognition and the Stability of the G-U Wobble Pair

INVESTIGATIONS of paired complementary polynucleotides containing occasional mismatched bases have indicated that these bases are looped out of the double helical regions thereby making the helix unstable. But in a system containing mismatched G and U bases, no such definite conclusion could be drawn^1–4—stoichiometry and T_m values of such complexes can be interpreted in terms of the formation of a G-U wobble pair⁴. Recently the complexes formed by self-complementary block oligomers interrupted by mismatched bases, for example, A_nXU_n (X=G, C and U), have been studied in detail⁵·⁶. It seems that stacking interactions play a much more dominant role in the stabilization of double helices than was previously thought. Thus the extent of looping out and of non-Watson-Crick base pairing can hardly be assessed independently.     

Synthesis of Substance P

SUBSTANCE P has been synthesized by the solid-phase procedure of Merrifield^1,2 according to the sequence H-Arg-Pro-LysPro-Gln-Gln-Phe-Phe-Gly-Leu-Met-NH₂ reported in the previous letter.     

Role of the Galactose Binding Protein in Chemotaxis of <Emphasis Type="Italic">Escherichia coli</Emphasis> toward Galactose

The galactose binding protein is the part of the galactose chemoreceptor which recognizes the attractants galactose, glucose and a number of structurally related chemicals.     

Ribosomes,G-factor and Siomycin

G-factor interacts with the 50S ribo-somal subunit at a site which is distinct from the peptidyl transferase centre and which is inactivated by siomycin.