Ionic channels through the axon membrane (a review)

Ionic channels are discrete sites at which the passive movement of ions takes place during nervous excitation. Three types of channels are distinguished. 1. Leakage channels that are permanently open to various cations. 2. Na channels that open promptly on depolarization but slowly close again (inactivate) on sustained depolarization and that are predominantly permeable to Na⁺ ions. 3. K channels that on depolarization open after some delay but stay open and that are mainly passed by K⁺ ions. The selectivity sequence of the Na channels of the squid axon (or frog nerve) is as follows: Na⁺ ≈ Li⁺>(T1+)>NH⁺ ₄?K⁺> Rb⁺, Cs⁺; that of K channels is: (T1+)>K⁺>Rb⁺>NH⁺ ₄?Na⁺, Cs⁺, Na channels are selectively blocked by tetrodotoxin (TTX) or saxitoxin (STX), K channels by tetraethylammonium ions (TEA). Either channel type is reversibly blocked when one drug molecule binds to one site per channel, the equilibrium dissociation constant of these reactions being about 3×10^?9 MTTX (or STX) and 4×10^?4 M TEA, respectively. Because of their specificity and high affinity, TTX and STX are used to “titrate” the Na channels whose density appears to be of the order of 100/Μm². The “gates” of the channels operate as a function of potential and time but independent of the permeating ion species. Drugs (e.g. veratridine) and enzymes (e.g. pronase, applied intraaxonally) cause profound changes in the gating function of the Na channels without influencing their selectivity. This points to separate structures for gating and ion discrimination. The latter is thought to be, in part, brought about by a “selectivity filter” of which detailed structural ideas exist. Recent experiments suggest that the gates of the Na channels are controlled by charged particles moving within the membrane under the influence of the electrical field.     

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.