Phosphorylates uridine and cytidine to uridine monophosphate and cytidine monophosphate (PubMed:11306702, PubMed:11494055). Does not phosphorylate deoxyribonucleosides or purine ribonucleosides (PubMed:11306702). Can use ATP or GTP as a phosphate donor (PubMed:11306702). Can also phosphorylate cytidine and uridine nucleoside analogs such as 6-azauridine, 5-fluorouridine, 4-thiouridine, 5-bromouridine, N(4)-acetylcytidine, N(4)-benzoylcytidine, 5-fluorocytidine, 2-thiocytidine, 5-methylcytidine, and N(4)-anisoylcytidine (PubMed:11306702). ATP + uridine = ADP + H(+) + UMP ATP + cytidine = ADP + CMP + H(+) 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. Homotetramer. According to PubMed:8812458; testis-specific. According to PubMed:11306702, placenta-specific. Belongs to the uridine kinase family.