Enzymatic Formation of 5‑Aminomethyl‑Uridine Derivatives in tRNA: Functional and Evolutionary Implications
Yoshitaka Bessho and Shigeyuki Yokoyama
Posttranscriptional modification of the wobble uridine at position 34 in the anticodon of tRNA allows accurate and efficient decoding of the genetic code. In particular, decoding of the synonymous two‑codon sets specific for Leu, Gln, Lys, Glu and Arg primarily depends on the presence of a methylene carbon on the C‑5 atom of U34 (xm5U), combined with the thiolation S‑2 (xm5s2U) in the cases of Gln, Lys and Glu, or methylation of the 2′‑hydroxyl of ribose‑34 (xm5Um) in the case of Leu. Together with other structural parameters of the anticodon arm, including the type of modification of the purine nucleotide at position 37, adjacent to the anticodon xm5UNN, these xm5U34‑containing tRNAs are efficiently and accurately able to decode only the purine‑ending codons in the correct reading frame (no frameshift). The various enzymes in Bacteria (MnmE, GidA and MnmC) involved in the formation of these wobble xm5U34 derivatives have been identified. In this chapter, we will summarize in structural terms what is known about these enzymes. Their relationships with other modification enzymes that also act on carbon‑5 of uridine in other positions of tRNA (mainly position 54) and their evolutionary interrelationships will also be discussed.