Orotate phosphoribosyltransferase (OPRTase) or Orotic acid phosphoribosyltransferase is an enzyme involved in pyrimidine biosynthesis. It catalyzes the formation of orotidine 5'-monophosphate (OMP) from orotate and phosphoribosyl pyrophosphate. In yeast and bacteria, orotate phosphoribosyltransferase is an independent enzyme with a unique gene coding for the protein, whereas in mammals and other multicellular organisms, the catalytic function is carried out by a domain of the bifunctional enzyme UMP synthase.http://www.creative-enzymes.com/product/Native-Brewers-Bottom-Yeast-Orotidine5monophosphate-Pyrophosphorylase_1332.html