New insights into the pyrimidine salvage pathway of Saccharomyces cerevisiae: requirement of six genes for cytidine metabolism

Cytidine metabolism in the yeast Saccharomyces cerevisiae was analyzed by g enetic and biochemical approaches. Disruption of a unique ORF (Genbank acce ssion No. U 20865) bearing homology with eucaryotic or bacterial cytidine d eaminases abolished cytidine deaminase activity and resulted in 5-fluorocyt idine resistance. The gene encoding cytidine deaminase will be referred to as CDD1 (Genbank accession number AF080089). The ability to isolate mutants resistant to 5-fluorocytidine which mapped to five other loci demonstrated the existence of a complex cytidine metabolic network. Deciphering this ne twork revealed several original features: (1) cytidine entry is mediated by the purine-cytosine transporter (Fcy2p), (2) cytidine is cleaved into cytosine by the uridine nucleosidase (Urh1p), (3) cytidine is phosphorylated into CMP by the uridine kinase (Urk1p), (4) a block in cytosine deaminase (Fcy1p), but not in cytidine deaminase (C dd1p), constitutes a limiting step in cytidine utilisation as a UMP precurs or.