H(+) + orotidine 5'-phosphate = CO2 + UMP Pyrimidine metabolism; UMP biosynthesis via de novo pathway; UMP from orotate: step 2/2. Belongs to the OMP decarboxylase family. Strain CLIB 122 / E 150 has a defective URA3 sequence (ura3-302) which is disrupted (positions 8 to 249) by S.cerevisiae SUC2.