Magnesium Transport in Escherichia coli

Cobaltous ion, at concentrations as low as 10 µm, inhibits the energy-dependent transport of magnesium ion into cells of Escherichia coli. Co++ also substitutes for Mg++ in the medium in promoting the energy-dependent efflux of cellular Mg++. Cells accumulate Co++ in a reaction which is dependent upon metabolic energy. The maximum rate and extent of Co++ uptake are similar to those for Mg++, although the apparent affinity for Co++ is only about one-tenth that for Mg++. Mg++ at concentrations as low as 10 µm inhibits the uptake of Co++. Although intracellular Mg++ exchanges readily with external Mg++, Co++ within cells does not easily exchange with either Mg++ or Co++ in the medium. The physiological responses of E. coli to Co++ and Mg++ in the medium reflect the antagonistic effect of each ion on the uptake of the other. Exposure to a low level of Co++ in the medium, in the absence of Mg++, kills cells, but Mg++ protects against this lethal effect of Co++. Other conditions which prevent Co++ uptake, such as low temperature, or deprivation of an energy source, also reduce the lethality of exposure to Co++. A mutant selected for its relative resistance to Co++ in the medium had a decreased capacity to transport both Co++ and Mg++. It appears likely that Co++ is transported into cells of E. coli by the same system responsible for Mg++ transport. This transport, however, requires a much higher concentration of Co++ than is normally found in growth media, and the transport system described here may not be responsible for the uptake of Co++ present in very low concentrations during normal growth.