<p>This family represents one branch of <db_xref db="COG" dbkey="COG0042"/> (Predicted TIM-barrel enzymes, possibly dehydrogenases, nifR3 family) and includes NifR3 itself, from <taxon tax_id="1061">Rhodobacter capsulatus</taxon>. Members of this family catalyse the reduction of the 5,6-double bond of a uridine residue on tRNA. Dihydrouridine modification of tRNA is widely observed in prokaryotes and eukaryotes [<cite idref="PUB00014304"/>], and also in some archaea. Most dihydrouridines are found in the D loop of t-RNAs. The role of dihydrouridine in tRNA is currently unknown, but may increase conformational flexibility of the tRNA. </p>