Ribosome degradation in Escherichia coli induced by colicin E2

The mode of action of colicin E₂ on ribosomes in $\text{Escherichia coli}$ cells was investigated by zonal centrifugation analysis. Ribosome particles, both 50S and 30S, were degraded to smaller contents with the lapse of time by the action of colicin E₂. Gradual reduction of S values of each particles could not be observed and degradative intermediates of possible RNA-protein complex were detected only at the position between 30S and 4S in the zonal centrifugation profile, which indicated the destruction of ribosome in burst-out attitude. 50S ribosome fraction influenced by colicin E₂ contained both 23S and half-sized RNA. From these data, the mode of action of colicin E₂ on ribosomes in $\text{E. coli}$ was discussed.     

Control of mixed-substrate utilization in continuous cultures of Escherichia coli

The chemostat culture technique was used to study the control mechanisms which operate during utilization of mixtures of glucose and lactose and glucose and l-aspartic acid by populations of Escherichia coli B6. Constitutive mutants were rapidly selected during continuous culture on a mixture of glucose and lactose, and the beta-galactosidase level of the culture increased greatly. After mutant selection, the specific beta-galactosidase level of the culture was a decreasing function of growth rate. In cultures of both the inducible wild type and the constitutive mutant, glucose and lactose were simultaneously utilized at moderate growth rates, whereas only glucose was used in the inducible cultures at high growth rates. Catabolite repression was shown to be the primary mechanism of control of beta-galactosidase level and lactose utilization in continuous culture on mixed substrates. In batch culture, as in the chemostat, catabolite repression acting by itself on the lac enzymes was insufficient to prevent lactose utilization or cause diauxie. Interference with induction of the lac operon, as well as catabolite repression, was necessary to produce diauxic growth. Continuous cultures fed mixtures of glucose and l-aspartic acid utilized both substrates at moderate growth rates, even though the catabolic enzyme aspartase was linearly repressed with increasing growth rate. Although the repression of aspartase paralleled the catabolite repression of beta-galactosidase, l-aspartic acid could be utilized even at very low levels of the catabolic enzyme because of direct anabolic incorporation into protein.     

Protection of Escherichia coli from the lethal effect of colicins by high osmotic pressure

The sensitivity of Escherichia coli to the lethal effect of colicin E(2) was reduced by elevation of osmotic pressure of the incubation medium. Optimal protection of the cells from the lethal effect of colicin E(2) was achieved with 0.6 to 0.8 m NaCl or with 0.8 m sucrose containing 0.01 m MgSO(4). Under such conditions, the degradation of deoxyribonucleic acid caused by colicin E(2) was also suppressed markedly. It was concluded that a high concentration of sucrose with Mg(++) might prevent the action of the adsorbed colicin E(2). A similar protection was observed against the lethal effect of colicin K.     

Interactions of Schwann cells with neurites and with other Schwann cells involve the calcium-dependent adhesion molecule,N-cadherin

During embryogenesis, Schwann cells interact with axons and other Schwann cells, as they migrate, ensheath axons, and participate in organizing peripheral nervous tissues. The experiments reported here indicate that the calcium-dependent molecule, N-cadherin, mediates adhesion of Schwann cells to neurites and to other Schwann cells. Cell cultures from chick dorsal root ganglia and sciatic nerves were maintained in media containing either 2mM Ca⁺⁺ or 0.2 mM Ca⁺⁺, a concentration that inactivates calcium-dependent cadherins. When the leading lamellae of Schwann cells encountered migrating growth cones in medium with 2 mM Ca⁺⁺, they usually remained extended, and the growth cones often advanced onto the Schwann cell upper surface. In the low Ca⁺⁺ medium, the frequency of withdrawal of the Schwann cell lamella after contact with a growth cone was much greater, and withdrawal was the most common reaction to growth cone contact in medium with 2 mM Ca⁺⁺ and anti-N-cadherin. Similarly, when motile leading margins of two Schwann cells touched in normal Ca⁺⁺ medium, they often formed stable areas of contact. N-cadherin and vinculin were co-concentrated at these contact sites between Schwann cells. However, in low Ca⁺⁺ medium or in the presence of anti-N-cadherin, interacting Schwann cells usually pulled away from each other in a behavior reminiscent of contact inhibition between fibroblasts. In cultures of dissociated cells in normal media, Schwann cells frequently were aligned along neurites, and ultrastructural examination showed extensive close apposition between plasma membranes of neurites and Schwann cells. When dorsal root ganglia explants were cultured with normal Ca⁺⁺, Schwann cells migrated away from the explants in close association with extending neurites. All these interactions were disrupted in media with 0.2 mM Ca⁺⁺. Alignment of Schwann cells along neurites was infrequent, as were extended close apposition between axonal and Schwann cell plasma membranes. Finally, migration of Schwann cells from ganglionic explants was reduced by disruption of adhesive contact with neurites. The addition of antibodies against N-cadherin to medium with normal Ca⁺⁺ levels had similar effects as lowering the Ca⁺⁺ concentration, but antibodies against the neuronal adhesive molecule, L1, had no effects on interactions between Schwann cells and neurites.     

Mode of antitumor action of recombinant human tumor necrosis factor on the sarcoma Meth A transplanted in the mouse

The mode of antitumor action of rHu-TNF was elucidated in BALB/c mice bearing Meth A fibrosarcoma 7 days after transplantation with respect to time course, dose-response relationships and selectivity of the effects. The maximal cytotoxic effect on tumor cells revealed by inhibition of DNA synthesis and maximal lesional effect on tumor vasculature revealed by change in blood pool-size in the tissue were detected at 30 min and I h after administration of rHu-TNF, respectively. The dose-response relationship between cytotoxic and tumoricidal effects of rHu-TNF was irrespective of administration route. ED₅₀s of these antitumor effects afteri.v. administration of rHu-TNF were about 50 times as high as ED₅₀s afteri.t. administration. ED₅₀ ofi.t. given rHu-TNF for vascular effect was about 20 times as high as that for cytotoxicity while ED₅₀ ofi.v. rHu-TNF for vascular effect was only 2–3 times as high as that for cytotoxicity. The whole body autoradiographies with [¹²⁵I] HSA giveni.v. to see the blood influx into tumor tissue and [¹⁴C]thymidine given i.v. to see DNA synthesis in the whole body after administration of rHu-TNF revealed that the distribution of radioactivity was markedly changed in the tumor alone without any detectable change in other whole body tissues.In conclusion, thein vivo antitumor effect of rHu-TNF giveni.t. ori.v., appears to be exerted through the direct action on Meth A sarcoma rather than indirectly on tumor vasculature. Under present conditions, the effect of rHu-TNF in the whole body tissues seems rather selective on cells and vasculature of the tumor.