Interaction of ethidium bromide with the transport system for monovalent cations in yeast

Ethidium was found to be taken up by yeast cells in a process that, at certain concentrations has the main following characteristics: a) a substrate is required; b) it presents cooperative kinetics, with n, according to the Hill equation approximately 3; c) ethidium can be concentrated more than 100-fold; d) the uptake is inhibited by Ca2+; e) the uptake of the dye is inhibited by monovalent cations with a selectivity pattern similar to that observed in their transport by yeast; f) ethidium inhibits the uptake of K+, and, at concentrations up to about 250 muM produces a competitive inhibition on the uptake of Rb+; and g) ethidium produces the same effects as K+ on respiration and the extrusion of H+. It is concluded that ethidium is taken up by yeast cells in a selective way by the same transport system normally employed for monovalent cation uptake.