Cell volume change of Escherichia coli under stringent control. Apparent increase of K+ in rel- cells

With several pairs of rel+ and rel- strains of Escherichia coli, the effects of amino acid starvation on the intracellular concentration of K+ and the rate of uptake of 42K+ were investigated. In the early phase of the experiments, the intracellular concentration of K+ was estimated by the conventional method in which the cell volume per A660 value of the culture was assumed to be constant, being not influenced by the variation of growth condition and strain. Apparently, the K+ concentration of rel+ cells was kept almost constant, while that of rel- cells increased about 1.5-fold 2 h after the exposure to amino acid starvation. Unexpectedly, however, the above assumption was found not to be valid in the present study. The cell volume per A660 changed only slightly in CP78 (rel+) cells, while it increased markedly in CP79 (rel-) cells after the exposure to amino acid starvation. Reestimation of the K+ concentrations based on the estimated respective values of cell volumes per A660 revealed no significant difference between both strains. After all, the above apparent phenomenon was found to be due to the fact that the increase in cell volume of the rel+ cells was arrested upon amino acid starvation whereas that in the rel- cells was not. The 42K+ uptake by the rel+ cells was depressed upon amino acid starvation, whereas that by the rel- cells increased. Some regulatory mechanism was suggested to operate in both strains to keep their K+ concentrations constant. When intracellular concentration of a metabolite is to be determined, importance of measurement of cell volume under the respective conditions, without assuming the constancy of the cell volume per A660 of the culture, was pointed out.