Uptake of the radiolabelled antimycotic 5-fluorocytosine (5-FC) by Candida albicans and distribution of radioactivity in the cell fractions, expandable pool, internal pool, and RNA, as influenced by the three strong antagonists against the fungistatic activity of the drug, adenine, uridine and cytosine were studied. In the stationary growth phase of the yeast, a reciprocal correlation was found between the concentrations of the antagonists and the presence of radioactive compound(s) in the cells, particularly in RNA. The correlation was similar with radio-labelled cytosine which was studied comparatively, but, considering the proportions, the normal pyrimidine was preferred to its fluorinated analogue. In the logarithmic phase of yeast growth, two phases of transport were distinguished: a first phase of 5-FC uptake, when there was a transient accumulation of radioactivity in the internal pool, and a second one of incorporation into RNA. The addition of cytosine resulted in a marked impairment of 5-FC uptake in the expandable pool, preventing any significant passage to the internal pool and RNA, whereas with uridine only the incorporation into RNA was inhibited. With adenine, the picture was similar to that observed with cytosine, suggesting a hitherto unknown interference of purines with pyrimidine uptake in yeasts. Confirming the findings of Giégé and Weil [4] in Saccharomyces cerevisiae, it was proved that in C. albicans, cultured in the presence of 5-FC, 5-fluorouracil becomes incorporated in place of uracil in both rRNA and tRNA, without any 5-FC being detectable.

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