Involved in the pyrimidine salvage pathway. diphosphate + UMP = 5-phospho-alpha-D-ribose 1-diphosphate + uracil ATP + cytidine = ADP + CMP + H(+) ATP + uridine = ADP + H(+) + UMP Binds 1 Mg(2+) ion per subunit. The magnesium is bound as Mg-PRPP. Allosterically activated by GTP. Pyrimidine metabolism; UMP biosynthesis via salvage pathway; UMP from uracil: step 1/1. Pyrimidine metabolism; CTP biosynthesis via salvage pathway; CTP from cytidine: step 1/3. Pyrimidine metabolism; UMP biosynthesis via salvage pathway; UMP from uridine: step 1/1. In the N-terminal section; belongs to the uridine kinase family. In the C-terminal section; belongs to the UPRTase family.