The Utilization of Exogenous Pyrimidines and the Recycling of Uridine‐5′‐Phosphate Derivatives in Saccharomyces cerevisiae, as Studied by Means of Mutants Affected in Pyrimidine Uptake and Metabolism

Mutants impaired in cytosine uptake ( cyt‐p ) or in cytosine deamination ( cyt‐da ) are unable to use cytosine as a source of pyrimidines or as a source of nitrogen. The cyt‐p mutation makes it possible to define a specific cytosine permease. A mutation ( ura‐p ) affecting exclusively uracil uptake delineates a specific uracil permease. The study of mutants lacking uridine‐kinase activity ( urid‐k ) has shown that, whereas uridine can no longer be used as a source of pyrimidines in the absence of this enzymic activity, uridine kinase is not an obligatory step in the formation of UMP from uracil. A mutation ( urid‐p ) affecting specifically uridine uptake delineates a uridine permease. The behaviour of mutants ( ups ) blocked in UMP formation from uracil indicates that they have lost an enzyme catalyzing this reaction in one step. A study of the mutants ups and urid‐k has shown that, when the utilization of uracil or uridine is decreased as a result of a mutation, the rate of uptake of uracil or uridine, respectively, is diminished by feedback inhibition of internal pyrimidines on the corresponding permeases, and, furthermore, that the enzymes coded or controlled by the ups and the urid‐k genes are not constituents of the transport systems. The excretion of uracil by several mutants under different conditions has suggested that endogenous UMP liberates uracil in two steps catalyzed by a UMP phosphatase and a uridine‐ribohydrolase respectively. A mutant lacking the uridine‐ribohydrolase activity ( urid‐rh ) has been isolated. The observations reported suggest that uracil produced endogenously from UMP is continuously recycled. The results of a chromatographic study of the fate of labelled pyrimidines added to the medium, and accumulated in the soluble pool, are in agreement with the interpretation of the various mutations.