SMM1 (DUS2) catalyzes the reduction of the 5,6 double bond in uridine residues in the D-loop of tRNA(Leu) (Xing et al. 2002, Xing et al. 2004), yielding 5,6-dihydrouridine. NADH or NADPH is the reducing agent. The subcellular location of the reaction is unknown.
tRNA dihydrouridine synthase activity of SMM1 [cytoplasm]