Catalyzes the transfer of a ribosyl phosphate group from 5-phosphoribose 1-diphosphate to orotate, leading to the formation of orotidine monophosphate (OMP). diphosphate + orotidine 5'-phosphate = 5-phospho-alpha-D-ribose 1-diphosphate + orotate Pyrimidine metabolism; UMP biosynthesis via de novo pathway; UMP from orotate: step 1/2. Homodimer. Repressed by uracil. Belongs to the purine/pyrimidine phosphoribosyltransferase family. PyrE subfamily.