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The D arm is a conserved structural element of transfer RNA (tRNA) composed of a short D stem and a D-loop that characteristically contains the modified nucleotide dihydrouridine. It plays important roles in stabilizing tRNA tertiary structure and in molecular recognition, particularly by aminoacyl-tRNA synthetases during protein synthesis. Although broadly conserved across Bacteria, Archaea, and Eukaryota, the D arm exhibits structural variation among different tRNA types and organisms, and is absent from certain mitochondrial tRNAs.

The D arm is a feature in the tertiary structure of transfer RNA (tRNA). It is composed of two short D stems around 5bp in length leading up to the D-loop.^[1] The D-loop contains the base dihydrouridine (D), for which the arm is named.^[2] Dihydrouridine is formed by addition of two hydrogens to a uracil (U) base which remove its planar structure and aromaticity.^[3] This D modification provides more flexibility to the D-loop structure of the D arm.^[3] This appears to play a large role in the stabilization of the tRNA's tertiary structure as demonstrated in a detailed study comparing the structure of the D arm of the Schizosaccharomyces pombe tRNA_i^Met with an unmodified uracil (U) to with the modified D nucleotide.^[4] The D-loop is a highly variable region and is notable for its unusual conformation due to the over-crowding on one of the guanosine residues.

The D loop's main function is that of recognition. It is widely believed that it acts as a recognition site for aminoacyl-tRNA synthetase, an enzyme involved in the aminoacylation of the tRNA molecule.^[2][8] The D stem is also believed to have a recognition role although this has yet to be verified.