The 2 known types of pyrimidine nucleoside phosphorylases, uridine phosphorylase (UP; EC 2.4.2.3) and thymidine phosphorylase (TP; EC 2.4.2.4), in the presence of orthophosphate, catalyze the reversible phosphorolysis of uridine and thymidine or deoxyuridine, respectively, to free bases and ribose-1-phosphate or deoxyribose-1-phosphate. Pyrimidine nucleoside phosphorylases can add ribose or deoxyribose to pyrimidine bases to form nucleosides that can be incorporated into RNA or DNA