Use of plasmid vectors to show that the uracil and cytosine permeases of the yeast Saccharomyces cerevisiae are electrogenic proton symports

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Abstract
Strains of Saccharomyces cerevisiae bearing the structural gene for either the uracil permease or the cytosine permease on a multicopy plasmid adsorbed uracil or cytosine at several times the rate exhibited by the wild-type. The basal rate of proton absorption by the yeast increased by at least a factor of 3 in the presence of uracil and a factor of 2 with cytosine. Assays with a voltage-sensitive carbocyanine dye showed that substrate uptake depolarized the plasma membrane. Amplification of the uracil permease activity led to uracil being concentrated about 2000-fold in comparison with 25-fold in the wild-type system. Enhanced cytosine permease activity produced a similar but smaller effect.