Catalyzes the reversible phosphorylation of UMP to UDP, with ATP as the most efficient phosphate donor. ATP + UMP = ADP + UDP Inhibited by UTP. Pyrimidine metabolism; CTP biosynthesis via de novo pathway; UDP from UMP (UMPK route): step 1/1. Homohexamer; trimer of dimers. Belongs to the UMP kinase family.