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. Binding of GTP leads to 5-time activation of the enzyme. Pyrimidine metabolism; UMP biosynthesis via salvage pathway; UMP from uracil: step 1/1. Monomer. Forms homodimers in presence of substrates and homotetramers in the presence of GTP. Belongs to the UPRTase family.