Pyrimidine reductase involved in the riboflavin biosynthesis pathway. Has also a non-functional N-terminal deaminase domain that lacks the catalytically essential zinc-binding residues. Catalyzes the hydrolysis of the N-glycosidic bond in the first two intermediates of riboflavin biosynthesis, which are highly reactive metabolites, yielding relatively innocuous products. Thus, can divert a surplus of harmful intermediates into relatively harmless products and pre-empt the damage these intermediates would otherwise do. Helps maintain flavin levels. Has no activity against GTP, nucleoside monophosphates or ADP-ribose. 5-amino-6-(5-phospho-D-ribitylamino)uracil + NADP(+) = 5-amino-6-(5-phospho-D-ribosylamino)uracil + H(+) + NADPH 2,5-diamino-6-hydroxy-4-(5-phosphoribosylamino)-pyrimidine + H2O = 2,5,6-triamino-4-hydroxypyrimidine + D-ribose 5-phosphate 5-amino-6-(5-phospho-D-ribosylamino)uracil + H2O = 5,6-diaminouracil + D-ribose 5-phosphate Cofactor biosynthesis; riboflavin biosynthesis; 5-amino-6-(D-ribitylamino)uracil from GTP: step 3/4. A number of isoforms are produced. According to EST sequences. The C-terminal domain (416-599) is not required for the reductase activity while the non-functional deaminase domain (21-150) is necessary. 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. In the C-terminal section; belongs to the YbiA family.