Inactivation of a newly transferred gene in recombination-deficient recipients of Escherichia coli

Summary The expression of a newly transferred lacZ ⁺ gene in lacZ recipients carrying various mutations in the recA and recB genes was studied by measuring the rates of induced synthesis of -galactosidase in zygotes formed after mating with either F or Hfr donors. The ability to synthesize -galactosidase decreases with time in both recA and recB zygotes when the lacZ ⁺ gene is transferred from an Hfr donor, but not when the lacZ gene is transferred from an F donor. There is no such inactivation of the newly transferred lacZ ⁺ gene in Rec⁺ zygotes. We conclude that the functioning of the transferred DNA is progressively inactivated in rec recipients unless the DNA is contained in an episome such as F.     

Map location of arginyl-tRNA synthetase mutations in Escherichia coli K-12

Summary Mutants of Escherichia coli K-12 previously isolated in the authors' laboratory have reduced arginyl-tRNA synthetase activity. The mutants fall into two classes. All mutants grow slowly on arginine-free medium. On arginine-supplemented medium some mutants grow at a normal rate (Class I) while others still grow slowly (Class II). Matings were performed to located a Class I and a Class II mutation on the E. coli chromosome map, and on the basis of our results we have assigned both to one locus, argS.     

Cholesterinkristalle in Kryostat-Gefrierschnitten von nervösem Gewebe als Ursache von Artefakten in Autoradiogrammen

Zusammenfassung Auf Kryostat- Gefrierschnitten von nativem und auch von Formaldehydoder Glutaraldehyd-fixiertem nervösem Gewebe entstehen besonders über der weißen Substanz Cholesterinkristalle in oft dichten Lagen, wenn die trockenen Schnitte bei Raumtemperatur aufbewahrt werden. Ein mehrstündiges Einwirken von gasförmigem Formaldehyd bei 80° C auf die Schnitte sofort nach der Entnahme aus dem Kryostaten verhindert die Kristallbildung. Diese Behandlung der Schnitte beeinflußt die im histologischen Präparat vorhandene Isotopenverteilung nicht und führt auch nicht zu chemographischen Effekten bei autoradiographischen Untersuchungen. Es wird berichtet, bei welchen autoradiographischen Techniken Artefakte durch Cholesterinkristalle zu erwarten sind und welche Auswirkungen diese Kristalle auf die Auswertung von Autoradiogrammen haben.

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Cholesterol-crystal deposition in frozen sections of nervous tissue as a cause of artefacts in autoradiograms

Summary In frozen sections of native or formaldehyde or glutaraldehyde fixed nervous tissue, layers of cholesterol crystals have been observed after storage at room temperature, most abundantly in the regions rich in white matter. The formation of the cholesterol depositions could be prevented by treatment of the sections with gaseous formaldehyde at 80° C for several hours, immediately after their removal of the cryostat. The distribution of the radioactive material in the sections was not affected by the formaldehyde treatment. Chemographic effects in the autoradiograms have not been observed. The techniques of autoradiography disturbed by the depositions of cholesterol crystals are reviewed.

     

Inhibition of Replication of an F′lac Episome in Hfr Cells of Escherichia coli

Hfr strains of Escherichia coli K-12 were found capable of accepting a F'lac episome during mating, with a frequency approximating that of F(-) strains. However, the F'lac episome was unable to replicate in the Hfr cells, and was diluted out during the growth of the culture. The lac(+) gene of the episome can be "rescued" by recombination into the host chromosome, as shown by the appearance of variegated recombinant colonies on a lactose-fermentation indicator medium. In recA Hfr strains, however, no lac(+) offspring were obtained in crosses with F'lac donors. The induced synthesis of beta-galactosidase in F'lac(+) x Hfr zygotes was studied. Rates of enzyme synthesis were approximately constant with respect to time as expected from unilinear inheritance of the F'lac episome. However, the rate of synthesis eventually increased, presumably due to integration of the lac(+) gene in some of the zygotes. In F'lac(+) x recA Hfr zygotes the rate of beta-galactosidase synthesis remained constant with respect to time, as expected.     

The effect of iron chelate fertilization of poplar upon CO2-uptake,leaf size,and content of leaf pigments and iron

Summary Potted poplars (strainsmarilandica, serotina andFlachslanden ofPopulus euramericana) which developed iron-deficiency symptoms (chlorosis of upper leaves, winter die-back of leader, flushing of lateral buds) were treated with a soil application of iron chelate to study the effect of iron nutrition upon CO₂-uptake, iron and pigment content of leaves, and leaf size of a tree species. Foliar content of each iron, chlorophyll, -carotene, lutein, and violaxanthin was significantly increased by the treatment. Chlorophyll b proved to be particularly sensitive to iron supply and the Q^a/b was also significantly altered.CO₂-uptake increased in fertilized and non-fertilized leaves with increasing light up to 40,000 Lux, but fertilized leaves assimilated more CO₂ than non-fertilized leaves, especially at light intensities from 5,000 Lux upwards. The assimilatory number was decreased by the iron application since larger amounts of chlorophyll were present in fertilized leaves. If CO₂-uptake was based upon an area unit basis the fertilizer effect became distinct even at 500 Lux. Thus CO₂-uptake is a quick, valuable measure of fertilizer responses.In severe cases, iron deficiency also affects leaf size and thus indirectly reduces photosynthetic activity. A chelate application during the growing season will not affect the size of leaves already formed but may considerably increase the size of leaves formed subsequent to the treatment.     

Effect of Ethanol on the Sodium and Potassium Conductances of the Squid Axon Membrane

The effects of ethanol on squid giant axons were studied by means of the sucrose-gap technique. The membrane action potential height is moderately reduced and the duration sometimes shortened by ethanol in sea water. Voltage clamp experiments showed that ethanol in sea water reduced the maximum membrane conductances for sodium (g'_Na) and potassium (g'_K). In experiments with multiple application of ethyl alcohol to the same spot of membrane, a reduction of g'_Na to 82 per cent and of g'_K to 80 per cent of their value in sea water was brought about by 3 per cent ethanol (by volume) while 6 per cent caused a decrease of g'_Na to 59 per cent and of g'_K to 69 per cent. Ethanol has no significant effect on the steady-state inactivation of g_Na (as a function of conditioning membrane potential) or on such kinetic parameters as τ_h or the time course of turning on gi g_Na and g_K. It is concluded that ethanol mainly reduces g_Na and g_K in the Hodgkin-Huxley terminology.     

Some Effects of Calcium Ions on the Action Potential of Single Nodes of Ranvier

Action potentials of single frog nerve fibers were recorded with the air-gap method in "low Ca" (0.26 mM) and "high Ca" (4.2 mM) solutions and compared to spikes in normal Ringer''s (1.05 mM Ca). On increasing (Ca)_o the action potentials became shorter, the "knee" during the falling phase as well as the threshold for abolition moved to internal potentials more positive, and the spike recovery during the relative refractory period was faster. Outward current pulses applied during an action potential affected its configuration more in low Ca than in high Ca. The onset of the delayed rectification (in the absence of Na) was found faster in high Ga. After-potentials during anelectrotonus declined more rapidly in high Ca than in low Ca. The results are compared primarily with the voltage-clamp analysis of Ca effects on squid axons and satisfactory qualitative agreement is reached.     

STUDIES ON THE FIXATION OF ARTIFICIAL AND BACTERIAL DNA PLASMS FOR THE ELECTRON MICROSCOPY OF THIN SECTIONS

The process of fixation of DNA-containing plasms is investigated by macroscopical and electron microscopical observations on solutions of DNA, nucleohistones, as well as on bacterial nuclei. The following treatments were found to produce a gelation of a solution of DNA or nucleohistones: (a) OsO₄ fixation at pH 6 in the presence of amino acids (tryptone) and Ca⁺⁺. (b) Exposure to aqueous solutions of uranyl acetate. (c) Exposure to aqueous solutions of indium chloride. Observed in the electron microscope, these gels show a fine fibrillar material. From experiments in which solutions of DNA or nucleohistones are mixed with bacteria and treated together, it is concluded that the behavior of the bacterial nucleoplasm is similar to that of the DNA solutions. The appearance of birefringence indicates that uranyl acetate and indium chloride produce an orientation of the molecules of a DNA solution during gelation. Bacterial chromosomes fixed by these agents also show a certain order, while those fixed by the OsO₄-amino acid-Ca⁺⁺ formula do not. Whether or not the order can be considered to be artificial is discussed, and a tentative conclusion is presented: (a) Uranyl acetate may induce artificial order. (b) Fixatives which do not gel DNA probably result in the grossest artifacts. (c) OsO₄ fixation at pH 6 in the presence of amino acids (tryptone) and Ca⁺⁺ may give the most accurate preservation of the in vivo disposition of DNA (RK⁺ fixation).