Fluorocytosine causes uncoupled dissipation of the proton gradient and behaves as an imperfect substrate of the yeast cytosine permease

At‐pH 5–6 ATP‐depleted washed cell preparations of strain NC233‐10b[pII4‐9], in which the cytosine permease was overexpressed, absorbed cytosine, hypoxanthine or fluorocytosine stoichiometrically with, respectively, about 1, 1·4 and 5 proton equivalents. The cellular pH fell proportionately. The membrane depolarization caused by each compound was assayed in the presence of glucose with a voltage‐sensitive dye and increased in the same order. Fluorocytosine significantly lowered the growth yield that a ‘petite’ strain of the yeast formed at limiting glucose concentrations. At pH 5·6 with extracellular [K + ] below 1 m M , each of the three substrates was accumulated about 200‐fold from a dilute solution at the expense of the proton gradient. This concentration ratio corresponds to a solute gradient (Δμ s ) of 13 kJ mol −1 . Raising [K + ] 0 systematically lowered the substrate accumulation ratio and Δμ H . The mean ratio Δμ s /Δμ H was 0·82 all three substrates. It was concluded that whereas the behaviour of cytosine approximated to that expected for a symport of unit proton stoichiometry, the absorption of protons with fluorocytosine and, to a lesser extent, hypoxanthine, was only partly conserved as useful work. A possible mechanism of this novel phenomenon is outlined.