Evidence for a Common Transport System for Cytosine, Adenine and Hypoxanthine in Saccharomyces cerevisiae and Candida albicans

Abstract

In Saccharomyces cerevisiae, the utilization of exogenous cytosine by pyrimidine‐requiring strains is prevented by the presence of adenine or hypoxanthine in the culture medium. Vice versa, the utilization of adenine or hypoxanthine by purine‐requiring strains is inhibited by cytosine. Neither uracil nor uridine behave as cytosine does under these conditions. Growth behaviour of purine‐requiring mutants as well as direct uptake measurements show that adenine and hypoxanthine uptake are strongly depressed in a mutant of S. cerevisiae lacking cytosine permease. This mutant is resistant to 5‐fluorocytosine. In Candida albicans, as well as in S. cerevisiae, the uptake of each of the substrates adenine, hypoxanthine, cytosine and 5‐fluorocytosine is competitively inhibited by the others and the apparent affinity constant (Km) for uptake of each substrate is equal to the inhibition constant (K1) of the same substance used as an inhibitor of the uptake of the others. It is concluded that adenine, hypoxanthine, cytosine and 5‐fluorocytosine share a common recognition site for uptake and that this site is not active in the mutant lacking cytosine permease. Copyright © 1973, Wiley Blackwell. All rights reserved