Figure 4.
Cell biology of tRNA modifications. Solid black circle indicates a modification known to occur on initial pre-tRNAs. Several modifications occur in the nucleus; magenta circles indicate those modifications that require the substrate to contain an intron, whereas orange circles indicate modifications that do not appear to require intron-containing tRNAs as substrate. Numerous other modifications occur in the cytoplasm; those that require that the intron first be spliced are brown, whereas those with no known substrate specificity or are restricted to tRNAs encoded by intronless genes are colored khaki. Open circles are catalyzed by enzymes whose subcellular locations are unknown. Different tRNA species possess different subsets of modifications; particular nucleosides that can possess numerous different modifications are indicated; half-colored circles indicate that the modifying enzymes have varying substrate specificity and/or subcellular location. Note that modification of G37 by Trm5 that requires tRNAs to be spliced occurs in the nucleoplasm after retrograde nuclear import.