FIG. 1.

Pyrimidine salvage pathway in S. cerevisiae. In the strains used, de novo biosynthesis of pyrimidines is conditionally blocked by a ura3 mutation before UMP. Reactions leading to the conversion of UTP into CTP and further to DNA and RNA synthesis are indicated merely by dashed lines. External uridine is taken up by uridine permease (encoded by FUI1). Uridine can be either directly converted into UMP by uridine kinase (URK1) or first converted into uracil by uridine ribohydrolase (encoded by URH1 = YDR400w) (this study) and then into UMP by uracil phosphoribosyltransferase (encoded by FUR1). Cytidine can be converted into uridine by cytidine deaminase (encoded by CDD1), and cytosine can be converted into uracil by cytosine deaminase (encoded by FCY1), respectively. Uptake of cytosine (and cytidine) is mediated by a cytosine-purine permease (encoded by FCY2).