Catalyzes the reversible phosphorylation of UMP to UDP. ATP + UMP = ADP + UDP Allosterically activated by GTP. Probably inhibited by UTP. Positive cooperativity is observed with ATP as variable substrate, but it is strongly reduced in the presence of GTP. GTP enhances the affinity for ATP. Pyrimidine metabolism; CTP biosynthesis via de novo pathway; UDP from UMP (UMPK route): step 1/1. Homohexamer. Belongs to the UMP kinase family.