Monofunctional pyrimidine deaminase involved in the riboflavin biosynthesis pathway. Has also a reductase domain that lacks catalytically essential substrate-binding residues (By similarity). 2,5-diamino-6-hydroxy-4-(5-phosphoribosylamino)-pyrimidine + H(+) + H2O = 5-amino-6-(5-phospho-D-ribosylamino)uracil + NH4(+) Binds 1 zinc ion. Cofactor biosynthesis; riboflavin biosynthesis; 5-amino-6-(D-ribitylamino)uracil from GTP: step 2/4. Unlike bacteria that have a bifunctional, two-domain RibD enzyme, plants have a monofunctional reductase and a monofunctional deaminase, each having an enzymatically inactive domain.