Catalyzes the synthesis of 5,6-dihydrouridine (D), a modified base found in the D-loop of most tRNAs, via the reduction of the C5-C6 double bond in target uridines. a 5,6-dihydrouridine in tRNA + NAD(+) = a uridine in tRNA + H(+) + NADH a 5,6-dihydrouridine in tRNA + NADP(+) = a uridine in tRNA + H(+) + NADPH A dusA dusB dusC triple mutant exhibits a complete lack of 5,6-dihydrouridine modification in cellular tRNA, whereas each single mutant exhibits a partial reduction, compared to wild type. DusB and DusC together account for about half of the 5,6-dihydrouridine modification observed in wild-type cellular tRNA, and DusA accounts for the other half. These three enzymes seem to act site-specifically on the tRNA D-loop and contain nonredundant catalytic functions in vivo. Belongs to the Dus family. DusB subfamily.