Suppressor-sensitive mutants of coliphage phi80

About fifty suppressor sensitive (sus) mutants of phage phi80 were isolated after hydroxylamine treatment, and these were classified into fourteen cistrons. In vitro and in vivo complementation experiments revealed that at least five cistrons were concerned with head formation and that at least six cistrons were concerned with tail formation in phi80. Some presumed "early" mutants were also found. Defective lysogens were isolated from a phi80-lysogenic nonpermissive strain using the colicin plate method (Gratia, 1966). In these strains deletions which affected the colicin B receptor gene extended for varying distances into prophage phi80. Marker rescue experiments were carried out with these deletion lysogens by infecting various sus mutants, and the gene order in prophage phi80 was determined. Clustering of all head genes and also of tail genes, as in phage lambda, were demonstrated by the results of the prophage deletion mapping as well as the two-factor crosses performed among the sus mutants. Moreover, the gross gene arrangement of phi80 was also similar to that of phage lambda: namely, the cluster of head genes was found to be located at one end of the vegetative map of phi80, being followed by that of tail genes, and presumed "early" genes are located at the other end of the map.

The ionic permeability changes during acetylcholine-induced responses of Aplysia ganglion cells

ACh-induced depolarization (D response) in D cells markedly decreases as the external Na(+) is reduced. However, when Na(+) is completely replaced with Mg(++), the D response remains unchanged. When Na(+) is replaced with Tris(hydroxymethyl)aminomethane, the D response completely disappears, except for a slight decrease in membrane resistance. ACh-induced hyperpolarization (H response) in H cells is markedly depressed as the external Cl(-) is reduced. Frequently, the reversal of the H response; i.e., depolarization, is observed during perfusion with Cl(-)-free media. In cells which show both D and H responses superimposed, it was possible to separate these responses from each other by perfusing the cells with either Na(+)-free or Cl(-)-free Ringer's solution. High [K(+)](0) often caused a marked hyperpolarization in either D or H cells. This is due to the primary effect of high [K(+)](0) on the presynaptic inhibitory fibers. The removal of this inhibitory afferent interference by applying Nembutal readily disclosed the predicted K(+) depolarization. In perfusates containing normal [Na(+)](0), the effects of Ca(++) and Mg(++) on the activities of postsynaptic membrane were minimal, supporting the current theory that the effects of these ions on the synaptic transmission are mainly presynaptic. The possible mechanism of the hyperpolarization produced by simultaneous perfusion with both high [K(+)](0) and ACh in certain H cells is explained quantitatively under the assumption that ACh induces exclusively an increase in Cl(-) permeability of the H membrane.