Protein involved in the biosynthesis of pyrimidine, a nitrogenous heterocyclic base, e.g. uracil, thymine, cytosine and orotic acid. Pyrimidines are synthesized from carbamoyl phosphate and aspartate. Ribose-5-phosphate is then attached to yield pyrimidine ribonucleotides. Cytosine is found in both DNA and RNA. Uracil is found only in RNA. Thymine is normally found in DNA. Sometimes tRNA contains some thymine as well as uracil.