Figure 1.

The Elongator-dependent tRNA modification pathway. (A) The anticodon uridine at position 34 (U₃₄) is modified initially to cm⁵ by the Elongator complex and further modified either by an unknown enzyme to ncm⁵, which is found in the indicated six tRNAs (purple), or by Trm9/Trm112 to mcm⁵ that is found in the two tRNAs shown. Ctu1/Ctu2 finally are thiolating the C2 of the pyrimidine ring in mcm5 resulting in mcm⁵s²U, which is found in the three tRNAs displayed. An alternative order for these steps has also been described (4). (B) Schematic drawing of U₃₄’s position in tRNAs during decoding of an mRNA with codon and anticodon indicated. (C) Formation of a G•U wobble base pair during decoding depends on the absence (left) or presence (right) of chemical modifications (X, R) on the nucleobase. Panel (C) was modified from (72).