Catalyzes the conversion of uracil and 5-phospho-alpha-D-ribose 1-diphosphate (PRPP) to UMP and diphosphate. diphosphate + UMP = 5-phospho-alpha-D-ribose 1-diphosphate + uracil 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. Belongs to the UPRTase family. Truncated N-terminus.