The Biology of tRNA t⁶A Modification and Hypermodifications-Biogenesis and Disease Relevance

The structure and function of transfer RNAs (tRNAs) are highly dependent on post-transcriptional chemical modifications that attach distinct chemical groups to various nucleobase atoms at selected tRNA positions via enzymatic reactions. In all three domains of life, the greatest diversity of chemical modifications is concentrated at positions 34 and 37 of the tRNA anticodon loops. N⁶-threonylcarbamoyladenosine (t⁶A) is an essential and universal modification occurring at position 37 of tRNAs that decode codons beginning with an adenine. In a subset of tRNAs from specific organisms, t⁶A is converted into a variety of hypermodified forms, including cyclic N⁶-threonylcarbamoyladenosine (ct⁶A), hydroxy-N⁶-threonylcarbamoyladenosine (ht⁶A), N⁶-methyl-N⁶-threonylcarbamoyladenosine (m⁶t⁶A), 2-methylthio-N⁶-threonylcarbamoyladenosine (ms²t⁶A) and 2-methylthio-cyclic N⁶-threonylcarbamoyladenosine (ms²ct⁶A). The tRNAs carrying t⁶A or one of its hypermodified derivatives are dubbed as the t⁶A family. The t⁶A family modifications pre-organize the anticodon loop in a conformation that enhances binding to the cognate mRNA codons, thereby promoting translational fidelity. The dysfunctional installation of modifications in the tRNA t⁶A family leads to translation errors, compromises proteostasis and cell viability, interferes with the growth and development of higher eukaryotes and is implicated in several human diseases, such as neurological disorders, mitochondrial encephalomyopathies, type 2 diabetes and cancers. In addition, loss-of-function mutations in KEOPS complex-the tRNA t⁶A-modifying enzyme-are associated with shortened telomeres, defects in DNA damage response and transcriptional dysregulation in eukaryotes. The chemical structures, the molecular functions, the known cellular roles and the biosynthetic pathways of the t⁶A tRNA family are described by integrating and linking biochemical and structural data on these modifications to their biological functions.