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. 2023 Jul;29(7):898–957. doi: 10.1261/rna.079620.123

FIGURE 8.

FIGURE 8.

Bidirectional tRNA trafficking between the nucleus and cytoplasm and generation of tRNAPhe yW37 in S. cerevisiae. Step 1. Upon 5′ and 3′ processing and addition of several nucleoside modifications to newly transcribed intron-containing tRNAs, Los1, Mex67–Mtr2, and Crm1 escort the end-processed, partially modified intron-containing tRNAs to the cytoplasm via the primary tRNA nuclear export step. The tRNAs are then spliced on the mitochondrial outer membrane. Numerous additional nucleoside modifications also occur in the cytoplasm after splicing. Cm32 and Gm34 (orange circles) modifications added in the cytoplasm are important for yW biogenesis. Step 2. Spliced, modified tRNAs are returned to the nucleus via the tRNA retrograde nuclear import step. Mtr10 functions indirectly in tRNA nuclear import both constitutively and upon amino acid deprivation (red symbol), whereas Ssa2 functions only upon amino acid deprivation. tRNAPhe imported into the nucleus is further modified at G37 (yellow circle) to m1G37 (empty colored circle). Step 3. Msn5, Los1, Mex67–Mtr2, and pehaps also Crm1, escort the imported tRNAs back to the cytoplasm via the tRNA reexport step. Once reexported to the cytoplasm, tRNAPhe m1G37 is further modified to yW (black circle). Red circles indicate anticodon nucleotides 34, 35, and 36.