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. Specifically modifies U20 and U20a in tRNAs. 5,6-dihydrouridine(20) in tRNA + NADP(+) = H(+) + NADPH + uridine(20) in tRNA 5,6-dihydrouridine(20) in tRNA + NAD(+) = H(+) + NADH + uridine(20) in tRNA 5,6-dihydrouridine(20a) in tRNA + NADP(+) = H(+) + NADPH + uridine(20a) in tRNA 5,6-dihydrouridine(20a) in tRNA + NAD(+) = H(+) + NADH + uridine(20a) in tRNA 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 (PubMed:11983710). Cells lacking this gene can introduce D modification at neither 20 or 20a in tRNA (PubMed:22123979). 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. DusA subfamily. The U21 position mentioned in PubMed:11983710 corresponds in fact to U20 with the conventional numbering.