Degradation of Enteric Bacterial Deoxyribonucleic Acid by the Escherichia coli B Restriction Endonuclease

The deoxyribonucleic acid of five different genera of enteric microorganisms was shown to be degraded by the Escherichia coli B restriction endonuclease.     

Effects of Ribonucleosides on Thymidine Incorporation: Selective Reversal of the Inhibition of Deoxyribonucleic Acid Synthesis in Thymineless Auxotrophs of Escherichia coli

Addition of certain ribonucleosides to exponentially growing cultures of Escherichia coli increased the extent of thymidine incorporation. The prolonged uptake of thymidine was correlative with the ability of these ribonucleosides to prevent the degradation of thymidine. In addition to protecting thymidine, uridine reversed partially (70 to 80%) the inhibition of deoxyribonucleic acid (DNA) synthesis in thymineless auxotrophs by cytosine arabinoside, hydroxyurea, and nalidixic acid. This reversal was selective for auxotrophic strains since no reversal of inhibition by uridine was observed in any of the prototrophic strains examined. In the presence of uridine, the rapid assimilation of thymidine by prototrophic and auxotrophic strains was prevented and the rate of DNA synthesis became a function of the available exogenous thymidine. Under these conditions, prototrophic strains accumulated equivalent amounts of thymidine into the acid-soluble (pool) and acid-insoluble (DNA) cell fractions. In contrast, 95 to 98% of the thymidine taken up by auxotrophs was found in the acid-insoluble (DNA) cell fraction. The results suggest that different mechanisms for DNA synthesis exist in auxotrophs and prototrophs. Based on these observed differences, some possible mechanisms for the selective reversal of the inhibition of DNA synthesis in auxotrophs are discussed.     

Mechanism of Oncogenic Transformation by Rous Sarcoma Virus: I. Intracellular Inactivation of Cell-Transforming Ability of Rous Sarcoma Virus by 5-Bromodeoxyuridine and Light

Chick embryo fibroblasts brought into stationary phase of growth by maintenance in serum-free Eagle's MEM medium were infected with the Bryan strain of Rous sarcoma virus (B-RSV) and incubated for 18 hr in the presence of 5-bromo-deoxyuridine (BUdR). The cells were then allowed to resume growth and deoxyribonucleic acid (DNA) synthesis by addition of an enriched F12 medium containing serum and RSV antibody to prevent spread of viral infection. After 48 hr, the cultures were exposed for various periods to visible light, overlaid with solid culture medium, and observed for the appearance of foci of transformed cells. In cultures treated with BUdR at the time of infection, exposure to light resulted in a suppression of focus formation of from 50 to 90% in various experiments. Treatment with BUdR for 18 hr before infection or on the day after infection, followed by exposure to light, had no effect on focus formation. In cultures in which almost all cells were infected, treatment with BUdR followed by exposure to light did not result in cell death. This suggests that suppression of transformation is not due to selective killing of infected cells by this treatment but rather to the intracellular inactivation of the transforming ability of Rous sarcoma proviral DNA.     

Viremia and Virus Measurements of Rabbit Pox in CV-1 Cells

Rabbit pox virus (RPV) produced cytopathic effect (CPE) in five types of cells grown in tissue cultures. The CPE on CV-1 cells was characterized by cell fusion and lysis. The CV-1 line is a useful and sensitive cell culture for measuring concentrations of RPV in blood and tissues of infected rabbits. Viremia was detected between the 2nd and 4th days after parenteral infection. By the 6th and 7th days, the concentration of RPV in various tissues ranged from 10(5-3) to 10(8-5) TCID(50)/g. Cross-reactivity was demonstrated by the fluorescent-antibody technique between rabbit pox, vaccinia, and monkey pox viruses.     

Statistical Signs of Synaptic Interaction in Neurons

The influence of basic open-loop synaptic connections on the firing of simultaneously recorded neurons has been investigated with auto- and cross-correlation histograms, using experimental records and computer simulations. The basic connections examined were direct synaptic excitation, direct synaptic inhibition, and shared synaptic input. Each type of synaptic connection produces certain characteristic features in the cross-correlogram depending on the properties of the synapse and statistical features in the firing pattern of each neuron. Thus, empirically derived cross-correlation measures can be interpreted in terms of the underlying physiological mechanisms. Their potential uses and limitations in the detection and identification of synaptic connections between neurons whose extracellularly recorded spike trains are available are discussed.     

Putrescine-mediated Degradation of Ribonucleic Acid in Chloramphenicol-treated Escherichia coli

Escherichia coli treated with chloramphenicol (CM) accumulated ribonucleic acid (RNA) in the absence of protein synthesis. The accumulated RNA (CM-RNA) was largely ribosomal (23S and 16S) and soluble (4S). The stability of CM-RNA depended upon the incubation conditions following the removal of CM. Thus, conditions which allowed the complete recovery of cultures from CM inhibition resulted in only a 30% loss of CM-RNA. The addition of proflavine to recovering cultures, which prevented further RNA synthesis, also resulted in about 30 to 35% degradation of CM-RNA. However, when RNA synthesis was inhibited by starving the recovering cultures for the required amino acid, histidine, 55% of the CM-RNA was degraded. The decreased stability of CM-RNA in histidine-starved cultures appeared to be due specifically to the intracellular buildup of putrescine. Under the above conditions of incubation, that RNA which was stable sedimented in sucrose gradients as 23S, 16S, and 4S RNA. It is suggested that intracellular putrescine plays a role in the stability of ribosomal RNA accumulated during CM treatment.