Figure 3. Additional chemical complexity of tRNA modifications is observed in selected organisms.

(A) The Wybutosine (yW) modification found at position 37 of eukaryotic tRNA^Phe is the result of a multi-step biosynthetic process; the proposed biosynthetic pathway in yeast is indicated by the various shaded groups, each of which represents a distinct step of yW production. In higher eukaryotes and many Archaea, the yW modification is further elaborated into several derivatives, including the OHyW that is the product of TYW5 action in humans. (B) The carboxymethyluridine modification and its derivatives found at position 34 of several tRNAs in eukaryotes. Functional groups added to yield the indicated nucleotide species in parentheses are shown in colored circles. All of these species are related by the common presence of the cm⁵U modification, synthesized by a common set of enzymes.