Repressive control of potassium transport in Escherichia coli

Turgor-controlled K+ fluxes and their pathways in Escherichia coli

Escherichia coli like most gram-negative bacteria with walls maintains a cytoplasmic osmolarity exceeding that of the medium; the resulting hydrostatic pressure (turgor pressure) pushes the cytoplasmic membrane against the peptidoglycan and creates a tension in the two envelopes. Potassium is the only cation which takes part in the regulation of cellular osmolarity. The adaptation of intracellular K+ concentration to external osmolarity involves K+ turgor-controlled fluxes. When the medium osmolarity is raised an osmodependent influx of K+ can be observed; this is carried out by the K+ transport system TrkA which can also taken up rubidium. A specific and unidirectional pathway allows K+ ions to flow out of the cell when the medium osmolarity is decreased; this pathway reveals two characteristics: it has no affinity for rubidium and it can be blocked by the blockers of eukaryotic K+ channels. Osmodependent fluxes are turned on immediately after the medium osmolarity is disturbed; in contrast, they are turned off gradually as the rate of K+ fluxes approach zero. The rate of K+ influx seems to depend on the level of internal osmolarity and not on the extent of the increase in medium osmolarity. The rate of the efflux is directly proportional to the decrease in medium osmolarity and is independent on the level of internal osmolarity.

Opening of potassium channels in Escherichia coli membranes by thiol reagents and recovery of potassium tightness

The retention of high potassium levels in Escherichia coli is not dependent on intact energy metabolism, since without the presence of a carbon source or in the presence of energy inhibitors significant K+ gradients can be maintained. In contrast, with 0.5 mM N-ethylmaleimide, K+ depletion is immediate and complete. As a final result, intracellular K+ is approximately three times more concentrated than the K+ in the medium. Increase of K+ in the medium is immediately followed by K+ uptake whereas in the unpoisoned state only an increase in the osmotic pressure of the medium would result in an increase of the K+ pool. The intracellular K+ undergoes continuous turnover in the poisoned cells whereas in intact cells turnover is strictly dependent on the presence of a metabolizable carbon source. After removal of the thiol reagent the cell recovers its capacity to concentrate potassium. The recovery process is inhibited by energy inhibitors or by incubation at low temperature but not by chloramphenicol. It is only slightly slowed down by carbon or sulfur starvation. The leak provoked by N-ethylmaleimide is similar in wild-type E. coli cells when a derepressed kdp uptake system working in the micromolar range of the K+ concentration is responsible for the intracellular pool of K+ and when, in a medium millimolar K+ concentration range, the trkA and trkD systems are predominant.

Kinetics of RNA synthesis in toad bladder epithelium: action of aldosterone during the latent period

Processing of RNA in the toad bladder was analyzed by polyacrylamide-gel electrophoresis to determine whether aldosterone causes any changes in the 1 hr before it potentiates transport of sodium ion. No change was found in the quantity or in the specific activity of bulk RNA labeled with uridine-5-(3)H. In vivo and in vitro with either uridine-5-(3)H or with methionine-(methyl)-(3)H as precursors, processing of RNA was extremely slow. Heterodisperse RNA was obvious after 30 min of continuous labeling, but labeling of the 40S precursor of ribosomal RNA was not apparent for 60 min. Labeling of mature 28S and 18S RNA first became apparent after 8 hr. approximately 7S RNA was the principal fastmigrating species labeled at 30 min, and 4S RNA was not heavily labeled until 1 hr. Aldosterone (5 x 10(-7) mole/liter) produced no changes. If care were not taken to inhibit metabolism of native bacteria colonizing the bladder, bacterial RNA of high specific activity predominated. We conclude that RNA metabolism in the toad bladder is extraordinarily slow, that a major acceleration of de novo synthesis in response to physiologic doses of aldosterone was not demonstrable, and that some reports to the contrary may have been influenced by artifacts from bacterial RNA metabolism. Earlier evidence for obligatory alterations in RNA metabolism during the latent period is not strong.

[The uptake of zinc into the interior of Chlorella]

The uptake of labelled Zn by asynchronous Chlorella fusca was measured under conditions of optimum and minimum energy supply (light and air, or dark and nitrogen, respectively). Part of the Zn equilibrates rapidly with the medium, and soon after the uptake it can be washed out quickly with non-labelled Zn carrier solution; it is probably bound in the free space (cell wall). This component of the uptake is also observed with cells kept in minimum conditions, with dead cells and with isolated cell wall material.Under optimum conditions Zn is taken up strongly for long periods and cannot be washed out completely. The velocity of uptake depends on the Zn content of the cells, and follows saturation kinetics. The uptake has no influence on the efflux of preabsorbed Rb. The retained Zn is thought to be inside the cells. It may enter by way of a pump. However, an energy-independent path of entry, with a differnt temperature coefficient, is also observed. All uptake processes are influenced by Ca.Part of the labelled Zn is slowly removed from the interior of the cell on treatment with a solution of carrier Zn or EDTA. This loss does not depend on energy supply and may occur via the second pathway. Net loss of labelled Zn is also observed after the energy supply is turned off, without change of solution, but the loss can be compensated by a simultaneous increase in the external concentration of labelled Zn. The question is discussed whether the energy-dependent Zn uptake is an "active" process.

Regulation of biotin transport in Saccharomyces cerevisiae

The metabolic control of biotin transport in Saccharomyces cerevisiae was investigated. Nonproliferating cells harvested from cultures grown in excess biotin (25 ng/ml) took up small amounts of biotin, whereas cells grown in biotin-sufficient medium (0.25 ng/ml) accumulated large amounts of the vitamin. Transport was inhibited maximally in cells grown in medium containing 9 ng (or more) of biotin per ml. When avidin was added to biotin-excess cultures, the cells developed the ability to take up large amounts of biotin. Boiled avidin was without effect, as was treatment of cells with avidin in buffer. Avidin did not relieve transport inhibition when added to biotin-excess cultures treated with cycloheximide, suggesting that protein synthesis was required for cells to develop the capacity to take up biotin after removal of extracellular vitamin by avidin. Cycloheximide did not inhibit the activity of the preformed transport system in biotin-sufficient cells. The presence of high intracellular free biotin pools did not inhibit the activity of the transport system. The characteristics of transport in biotin-excess cells (absence of temperature or pH dependence, no stimulation by glucose, absence of iodoacetate inhibition, independence of uptake on cell concentration, and nonsaturation kinetics) indicated that biotin entered these cells by diffusion. The results suggest that the synthesis of the biotin transport system in S. cerevisiae may be repressed during growth in medium containing high concentrations of biotin.