The energy-dependent exchange of intracellular Mg2+ with extracellular Mg2+ or Co2+ is inhibited by colicin E1 and, less strongly, by colicin K. Treatment with either colicin causes a net loss of intracellular Mg2+. This loss begins immediately in cells treated with colicin E1, but in colicin K-treated cells the onset of Mg2+ loss is delayed 1 to 10 min, depending upon the temperature and the multiplicity of colicin K. Both colicins differ from chemical inhibitors of energy-yielding metabolism; energy poisons block transport of Mg2+ and Co2+, but both colicins increase passive permeability to Mg2+ and Co2+. Inhibitors of energy-yielding metabolism (and of Mg2+ exchange) block the initiation of Mg2+ loss by either colicin, but do not stop colicin-promoted efflux once it has begun. Colicin E1 added before colicin K prevents the more rapid Mg2+ efflux characteristic of colicin K-treated cells. Quantitative comparisons of the effects of colicins E1 and K upon permeability to Mg2+ and Co2+ lead us to conclude that the two colicins are not identical in their mode of action
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